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This proves that human and apes have common ancestors.

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Main traits of human are that they stand upright(bipedalism), a large complex brain, the ability to use and make tools, ability to create an advanced language, complex symbolic expression, art, and elaborate cultural diversity.

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The Smithsonian Institution's Human Origins Program

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Human evolution

Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.

One of the earliest defining human traits, bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago. Other important human characteristics -- such as a large and complex brain, the ability to make and use tools, and the capacity for language -- developed more recently. Many advanced traits -- including complex symbolic expression, art, and elaborate cultural diversity -- emerged mainly during the past 100,000 years.

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called "pygmy chimpanzees") and gorillas -- share a common ancestor that lived between 8 and 6 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 and 2 million years ago come entirely from Africa.

Most scientists currently recognize some 15 to 20 different species of early humans. Scientists do not all agree, however, about how these species are related or which ones simply died out. Many early human species -- certainly the majority of them - left no living descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.

Early humans first migrated out of Africa into Asia probably between 2 million and 1.8 million years ago. They entered Europe somewhat later, between 1.5 million and 1 million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years and to the Americas within the past 30,000 years or so. The beginnings of agriculture and the rise of the first civilizations occurred within the past 12,000 years.

Paleoanthropology

Paleoanthropology is the scientific study of human evolution. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. The field involves an understanding of the similarities and differences between humans and other species in their genes, body form, physiology, and behavior. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it investigates the origin, over millions of years, of the universal and defining traits of our species. However, some people find the concept of human evolution troubling because it can seem not to fit with religious and other traditional beliefs about how people, other living things, and the world came to be. Nevertheless, many people have come to reconcile their beliefs with the scientific evidence.

Early human fossils and archeological remains offer the most important clues about this ancient past. These remains include bones, tools and any other evidence (such as footprints, evidence of hearths, or butchery marks on animal bones) left by earlier people. Usually, the remains were buried and preserved naturally. They are then found either on the surface (exposed by rain, rivers, and wind erosion) or by digging in the ground. By studying fossilized bones, scientists learn about the physical appearance of earlier humans and how it changed. Bone size, shape, and markings left by muscles tell us how those predecessors moved around, held tools, and how the size of their brains changed over a long time. Archeological evidence refers to the things earlier people made and the places where scientists find them. By studying this type of evidence, archeologists can understand how early humans made and used tools and lived in their environments.

The process of evolution

The process of evolution involves a series of natural changes that cause species (populations of different organisms) to arise, adapt to the environment, and become extinct. All species or organisms have originated through the process of biological evolution. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring -- that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens.

Evolution occurs when there is change in the genetic material -- the chemical molecule, DNA -- which is inherited from the parents, and especially in the proportions of different genes in a population. Genes represent the segments of DNA that provide the chemical code for producing proteins. Information contained in the DNA can change by a process known as mutation. The way particular genes are expressed - that is, how they influence the body or behavior of an organism -- can also change. Genes affect how the body and behavior of an organism develop during its life, and this is why genetically inherited characteristics can influence the likelihood of an organism's survival and reproduction.

Evolution does not change any single individual. Instead, it changes the inherited means of growth and development that typify a population (a group of individuals of the same species living in a particular habitat). Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population. As a result, the offspring inherit those genetic characteristics that enhance their chances of survival and ability to give birth, which may work well until the environment changes. Over time, genetic change can alter a species' overall way of life, such as what it eats, how it grows, and where it can live. Human evolution took place as new genetic variations in early ancestor populations favored new abilities to adapt to environmental change and so altered the human way of life.

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Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.

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Sapiens by Yuval Noah Harari describes the three major revolutions of prehistoric human, the Cognitive Revolution, the Agricultural Revolution, and the Scientific Revolution.

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The Book in Three Sentences

Human history has been shaped by three major revolutions: the Cognitive Revolution (70,000 years ago), the Agricultural Revolution (10,000 years ago), and the Scientific Revolution (500 years ago). These revolutions have empowered humans to do something no other form of life has done, which is to create and connect around ideas that do not physically exist (think religion, capitalism, and politics). These shared "myths" have enabled humans to take over the globe and have put humankind on the verge of overcoming the forces of natural selection.

Sapiens summary

This is my book summary of Sapiens by Yuval Noah Harari. My notes are informal and often contain quotes from the book as well as my own thoughts. This summary also includes key lessons and important passages from the book.

Human cultures began to take shape about 70,000 years ago.

There have been three major revolutions in human history: the cognitive revolution, the agricultural revolution, and the scientific revolution.

Prehistoric humans (2 million years old or so) were no more important and impressive than other mammals.

Homo Sapiens means "wise man."

Humans first evolved in Africa about 2.5 million years ago.

The author believes it is unlikely Homo sapiens will survive for another 1,000 years.

From about 2 million years ago until 10,000 years ago, multiple human species roamed the earth together. The depiction of man evolving from hunched over to upright incorrectly displays human evolution as a linear trajectory. In fact, the species lived simultaneously.

Humans have huge brains for their body size.

Human brains account for 2-3 percent of body size, but use 25 percent of energy.

Human kind was very much in the middle of the food chain until 400,000 years ago and didn't leap to the top of the food chain until 100,000 years ago.

Most animals at the top of the food chain made it there gradually over millions of years. Humans, however, jumped to the top relatively rapidly. This means that the rest of the food chain wasn't ready and neither were we. Hence we feel anxious and stressed because we aren't used to being at the top.

The advent of fire and cooking food may have opened the way for the evolution of a smaller intestinal track and a larger brain.

There are two theories of how Homo sapiens evolved: Interbreeding theory and Replacement theory. The reality is probably a combination of both theories.

Perhaps this is why Homo sapiens wiped out the Neanderthals: "They were too familiar to ignore, but too different to tolerate."

The last dwarf species of humans died out 12,000 years ago.

Homo sapiens conquered the world because of its unique language.

The Cognitive Revolution occurred between 70,000 to 30,000 years ago. It allowed Homo sapiens to communicate at a level never seen before in language.

As far as we know, only Homo sapiens can talk about things we have never seen, touched, or smelled. Think religions, myths, legends, and fantasies.

The telling of myths and stories allow Homo sapiens to collaborate in large numbers in extremely flexible ways. This separates us from all other animals.

Chimps can't form groups of more than 50 or so. For humans, the group size is usually 150 or so. Beyond that, you can't rely on gossip and personal communication. You need something more to get large numbers of people working together.

Large numbers of people can collaborate by sharing common myths and beliefs.

In academic circles, stories are known as fictions, social constructs, or imagined realities.

An imagined reality is not a lie because the entire group believes it.

Ever since the Cognitive Revolution, humans have been living in a dual reality: the physical reality and the imagined reality.

The way people cooperate can be changed by changing the stories as myths we tell.

Because Homo sapiens shared myths were not genetically based, they could adapt and change their behavior as soon as they adapted their new belief. They didn't have to wait millions of years for a genetic change.

Homo sapiens are the only animals that conduct trade.

As far as we know, the humans of 30,000 years ago had the same physical, emotional, and intellectual capabilities that we have today.

Evolutionary psychology claims that most of our psychology was developed during the period before the Agricultural Revolution about 10,000 years ago.

The instinct to gorge on high calorie food is wired into our DNA.

Ever since the Agricultural Revolution, there hasn't been one predominant way of life for all humans. There have only been options from a variety of cultures.

The dog was the first animal domesticated by humans around 15,000 years ago.

In ancient human groups (over 10,000 years ago) there was very little privacy, but also very little loneliness.

Most of our ancient ancestors had much wider and deeper knowledge of their physical surroundings than we do. They were not unintelligent at all.

The human collective today knows far more overall than the whole population of 15,000 years ago. However, at the individual level we are much more specialized today. Ancient foragers were the most knowledgable and skillful people in history.

It is far easier to pass "unremarkable" genes along today than it was 10,000 years ago.

Our lack of knowledge about prehistoric religions and beliefs is one of the biggest holes in our understanding of human history.

Humans traveling across the sea and landing in Australia was one of the most important expeditions in history. It marked the moment humans cemented themselves at the top of the food chain.

Homo sapiens first made it to America about 16,000 years ago.

The settling of America - across the Siberian peninsula through Alaska into Canada and the United States down through Mexico and Central America into the Andes and the Amazon and all the way to the tip of South America - was one of the most rapid and incredible invasions by a single species the world had ever seen.

Incredibly, the Agricultural Revolution sprang up independently in many different parts of the world.

There is no evidence modern humans have become more intelligent with time.

The Agricultural Revolution actually didn't make the life of the average human better at first. It did, however, allow humans to collect more food per unit area and thus the overall population multiplied exponentially.

Fascinatingly, the first few thousand years of the Agricultural Revolution actually made life harder for humans by creating more work, less leisure, and a ballooning population that created more mouths to feed. Each individual generation didn't see how their life was becoming worse because the small changes were so tiny.

One of history's few iron laws is that luxuries tend to become necessities and to spawn new obligations. Once people begin to enjoy new luxuries they tend to become expected and then count on them.

The evolutionary success of the Agricultural Revolution (greater population) was actually cause for much suffering on the individual level. Not just for humans, but for domesticated animals like cows, sheep, and chickens as well.

The advent of the Agricultural Revolution marked the time when worries of the future became prevalent: the weather, the crop yield this year, etc.

The myths that surround us and make up our lives dictate so much of what we believe and what we do.

Like the ancient Egyptians, most people dedicate their lives to building pyramids. It's just that the names, shapes, and sizes of the pyramids change from one culture to another.

In order to change the imagined order, you must first find a group that believes in a current imagined order. New myths must build upon or evolve from previous myths.

The main purpose of writing is to record numbers, which our brains did not evolve to manage well. Our brains are much better at remembering biological, zoological, and social information.

There is an ancient writing system used by the Incas known as a quipu. They are not written words at all, but a series of knots of different colors and strings that represent words and numbers.

Writing has actually changed the way humans think. We can use writing and record keeping to think far more categorically than ever before.

Numbers are the world's most prevalent language.

Social hierarchies, inequality, and so on are human inventions.

Most rich people are rich because they were born into rich families. Most poor people are poor because they were born into poor families.

Unjust discrimination often gets worse, not better, with time.

As of 2006, there were still 53 countries where a husband could not be legally prosecuted for raping his wife.

When it comes to gender inequality: biology enables, culture forbids. The idea of "unnatural" behaviors is actually a result of Christian theology, not biology.

If it is possible biologically, then it is natural. From a scientific perspective, two men having sex is natural. Traveling at the speed of light is not natural.

Why are men valued in many cultures more than women?

All human cultures are filled with inconsistencies. For example, America currently values individual freedom and equality. But these two ideals don't always play nicely. It is part of the human experience to reconcile them. These inconsistencies aren't necessarily bad. They force us to think critically. Consistency is the playground of dull minds.

History is moving relentlessly toward unity. The whole planet is moving toward one world culture.

The creation of money was purely an intellectual revolution. It doesn't exist except in our minds.

More than 90 percent of all money is just electronic data, not physical money.

Everyone always wants money precisely because everyone else always wants money.

Empires have been the world's most common form of political organization for the last 2,500 years.

In general, empires do not fall because of uprisings. They almost always succumb to outside invasion or splits from within the empower class.

Most of what we firmly believe is part of "our culture" was actually forced upon us by other empires who conquered our ancestors.

Despite the obvious negatives of empires taking over a culture, there are many benefits too. Art, music, governance, and more are the result of empires forming. Often, they blended new together with the conquered people to create a new culture.

It seems obvious that we are moving fast toward a singe global empire. Global markets, global warming, and commonly accepted concepts like human rights make it clear we all need one collective entity, not man states and countries.

Religion is the third great unifier of humankind, alongside money and empires.

The Agricultural Revolution was accompanied by a Religious Revolution.

Interestingly, polytheism is more open and accepting of multiple beliefs even though we often look at it as more barbarian and uneducated than our current beliefs.

Monotheism seems to push away polytheism, but actually is very similar to polytheistic gods with the use of patron saints. Praying to the patron saints of farmers isn't much different than praying to the god of rain.

The central tension with monotheism is how to deal with the fact that there is evil in the world while the omnipoten God is believed to be good and caring. If God is good why would he allow evil things to happen?

Even the rich and famous are rarely satisfied.

According to Buddhist tradition: the mind naturally craves more in all situations. And all suffering arrives from craving.

There are a variety of "natural law religions" that are popular today like communism, capitalism, and liberalism.

Over the last 200 years, science has increasingly revealed that human behavior is determined by hormones, genes, and neurological synapses. If this is true, then for how much longer will we ignore that biology does not agree with the concept of free will?

To describe how something happened means to reconstruct the series of specific events that led from one point to another.

To describe why something happened means to find causal connections that led to this particular series of events to the exclusion of all others.

The deeper your knowledge of a particular area of history, the harder it becomes to explain why one particular outcome occurred and not another.

It is an inevitable rule of history that what seems obvious in hindsight is impossible to predict beforehand.

The are level one and level two Chaotic Systems. Level one does not respond to predictions about it, like the weather and weather forecasts. Level two does respond to predictions about it, like the stock market and analyst reports about rising oil prices.

There is no proof that history is working for the benefit of humans or that human well being increases overtime. It's good for the victors, but is it good for us all?

The Scientific Revolution started in Europe around 500 years ago. The last 500 years have witnessed an unprecedented growth of human impact.

One difference between religion and science is that science assumes humankind does not know the answers to many of life's biggest questions. Religion, however, assumes that the important stuff is already known. Science assumes human ignorance.

Modern culture has been able to admit ignorance more than any previous culture.

Previous cultures and belief systems compiled their theories using stories. Science compiles its theories using mathematics.

The story of how Scottish Widows was founded is an awesome example of the power of probability.

Scientists generally agree that no theory is 100 percent correct. Thus, the real test of knowledge is not truth, but utility. Science gives us power. The more useful that power, the better the science.

The military arms race drives science forward in rapid fashion. The truth is war prompts many scientific discoveries.

In the past, the best minds of the day worked on finding ways to give meaning to death. Today, our best minds work on preventing death through biological, hormonal, and genetic means. Science does not take death as an inevitability.

The economic, religious, and political interests that impact the flow of money into scientific and technological research have a huge impact on the output of science.

It is not enough to consider science in a vacuum. Economic and capitalistic interests, for example, determine what we research and what to do with the research findings.

Why did Europeans discover and conquer the Americas? Why not the Chinese or those from India or the Middle East who possessed just as much knowledge and technology as the Europeans? The European ideology to explore the world was the primary difference.

For most of human history, per capita production remained the same. Since the launch of capitalism, however, per capita production has skyrocketed.

Modern capitalism has exploded the growth of humankind thanks to the creation of credit, which allows you to borrow money now because we collectively trust that the future will be better than the present.

Adam Smith's brilliant insight about capitalism in The Wealth of Nations was that increasing private profits is the basis for increasing collective wealth and prosperity. In other words, by becoming richer you benefit everyone, not just yourself. Both parties get a bigger slice of pie. (Note: this only works if profits get reinvested, not hoarded.)

For capitalism to work, profits must be reinvested in new production.

The "religion" of capitalism says economic growth is the supreme because justice, freedom, and happiness requires economic growth.

All credit is based on the idea that science and technology will advance. Scientists ultimately foot the bill of capitalism.

The annual sugar intake of the average Englishman rose from nearly zero in the early 17th century to 18 pounds in the early 19th century.

The life expectancy, child mortality, and calorie intake are significantly improved for the average person in 2014 compared to 1914, despite exponential population growth.

Until the industrial revolution, human behavior was largely dictated by solar energy and plant growth. Day and night. Summer and winter. Everything was determined by man power and animal power, which were determined by food, which is determined by photosynthesis.

"This is the basic lesson of evolutionary psychology: a need shaped in the wild continues to be felt subjectively even if it is no longer really necessary for survival and reproduction."

Harlow's infant monkey studies from the 1950s (and a variety of followup studies) have shown that animals have strong psychological needs as well as purgative physical needs. Note to self: never disregard your psychological needs.

Each year the United States population spends more money on diets than the amount needed to feed all the hungry in the rest of the world.

Most people don't realize just how peaceful of the times are we live in.

In recent years, more people die from suicide each year than from war and violent crime. The same can said for car accidents.

Live a safe community, drive as little as possible, and love yourself. Violent local crime, car accidents, and suicide are some of the biggest killers of humans.

War is at an all time low because the costs of war have increased because of nuclear weapons, the benefits of war have decreased because physical resources drive less of the economy and international trade is more lucrative than conquest, and the tightening of international connections because a worldwide culture is less likely to battle itself.

Our view of the past is heavily influenced by recent events.

Researchers have investigated nearly all aspects of history, but have rarely have asked whether historical changes have made humans happier.

Nietzsche: "He who has a why to live can bear almost any how."

If happiness is based on pleasurable feelings, then increasing our happiness is a matter of increases biochemical release. If happiness is based on meaning, then increasing our happiness is a matter of deluding ourselves about the meaning of our lives.

One uncommonly cited benefit of religion: belief in the afterlife gives meaning to your life in the present.

Buddhism has studied happiness for over 2,000 years. Interestingly, Buddhism shares many viewpoints on happiness with science. Most notably, that happiness results from processes within the body and not from the outside world.

The Buddhist philosophy of happiness centers around the idea that you are not the events that happen to you, but you are also not the feelings you have. You are not your feelings. They are just feelings. Thus, if you understand this, you can release the needs to keep chasing the need to feel happy or to not feel angry or to not feel sad. In other words, you have to understand yourself.

For close to 4 billion years, every organism developed according to evolution. But in recent decades, humans have begun to evolve according to intelligent design. In other words, there are people who would have been selected out of the gene pool millennia ago, but not today.

Genetic engineering is allowing humans to break the laws of natural selection.

The next stage of human history will not only involve biological and technological changes, but also changes in human consciousness and identity. Changes that are this fundamental will call the very term "human" into question.

Many people think the question we should ask to guide our scientific pursuits is, "What do we want to become?" However, because we seem to be on the path to genetically engineering and programming nearly every facets of our wants, desires, and consciousness, the real question we should ask is, "What do we want to want?"

In the past 1000 years, humans have evolved to take over the world and are on the verge of overcoming natural selection and becoming gods. Yet, we still seem unhappy in many ways and we are unsure of what we want. Is there anything more dangerous that dissatisfied and irresponsible gods who don't know what they want?

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This is a book summary of Sapiens by Yuval Noah Harari. Read this Sapiens summary to review key ideas and lessons from the book.

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Archaeologist Jean Jacques Hublin led an archaeological dig in Morocco, unearthing a 300,000 years old Homo Sapiens fossil. “The face is the face of somebody you could cross in the metro,”  said Hublin while describing the skull of the fossil. This is the discovery of the oldest fossil of a Homo Sapiens.

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Archaeologists have unearthed 300,000-year-old fossilized bones of early humans - the oldest remains of Homo sapiens yet discovered, two new studies report. The ancient bones contain a mix of modern and primitive features that hint at an early, and previously unknown, phase of our species's evolution.

On the family tree of human relatives, collectively known as hominins, our ancestors split from the Neanderthal branch more than 500,000 years ago. Fossils that looked like modern humans started showing up in East Africa about 200,000 years ago. But when exactly modern humans evolved from our most recent ancestor, probably Homo heidelbergensis, is a mystery. Now, two new studies published today in the journal Nature fill in some of those missing millennia, and suggest that hominins were well on their way to looking like modern-day humans about 300,000 years ago.

The fossils come from an archaeological site in Morocco, called Jebel Irhoud. Hominin bones were first pulled out of the ground there in the 1960s, and were dated to about 40,000 years ago. But the archaeologists were sloppy, and experts began to suspect that date was very wrong. So, in 2004, the site reopened and over the next seven years, archaeologists unearthed more hominin remains, including parts of skulls and jaw bones. They also found animal bones that had been butchered and possibly cooked, and stone tools that had been burned at some point - all in the same layer of dirt. That's key: being in the same layer means these finds were all approximately the same age.

The stone tools were a lucky find, because they gave the researchers - led by Jean-Jacques Hublin and Shannon McPherron at the Max Planck Institute in Germany - a chance to date that layer precisely. By measuring how many electrons accumulated in the tools over millennia, the researchers dated the layer and the hominin bones within it to about 315,000 years ago.

From the skull fragments, the researchers virtually reconstructed the faces of the ancient humans who had once lived at the site. Their features were small, and their faces were tucked underneath the brain case, rather than jutting out in front. "The face is the face of somebody you could cross in the metro," Hublin told Michael Greshko in National Geographic. But the brain case was more primitive - flatter and longer instead of high and round, like ours. That's why Hublin and his colleagues think the ancient denizens of Jebel Irhoud were an early evolutionary phase of Homo sapiens - somewhere on the developmental path between Homo heidelbergensis and us.

The two authors of an analysis published alongside the study agree with this assessment. But, the findings haven't been met with universal approval. Paleoanthropologist John Hawks at the University of Wisconsin-Madison told Greshko that these new studies "really aren't adding anything new except the date." Hawks added that he doesn't think the findings warrant creating a new category of "early modern humans" to describe the mix of primitive and modern traits. But regardless of what we call these ancient relatives of ours, these new findings give us a better idea of how we became who we are today.

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Archaeologists unearth the oldest Homo sapiens ever discovered

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Archaeologists have unearthed 300,000-year-old fossilized bones of early humans - the oldest remains of Homo sapiens yet discovered, two new studies report. The ancient bones contain a mix of modern and primitive features that hint at an early, and previously unknown, phase of our species's evolution.

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Human evolution is the evolutionary process that eventually lead to the existence of modern human.

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Human evolution is the evolutionary process within the history of primates that led to the emergence of Homo sapiens as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development of traits such as human bipedalism and language, as well as interbreeding with other hominins, which indicate that human evolution was not linear but a web.

The study of human evolution involves several scientific disciplines, including physical anthropology, primatology, archaeology, paleontology, neurobiology, ethology, linguistics, evolutionary psychology, embryology and genetics. Genetic studies show that primates diverged from other mammals about 85 million years ago, in the Late Cretaceous period, and the earliest fossils appear in the Paleocene, around 55 million years ago.

Within the superfamily Hominoidea, the family Hominidae (great apes) diverged from the family Hylobatidae (gibbons) some 15-20 million years ago; subfamily Homininae (African apes) diverged from Ponginae ( orangutans) about 14 million years ago; the tribe Hominini (including humans, Australopithecus, and chimpanzees) parted from the tribe Gorillini (gorillas) between 8-9 million years ago; and, in turn, the subtribes Hominina (humans and extinct biped ancestors) and Panina ( chimpanzees) separated 4-7 million years ago.

Human evolution from its first separation from the last common ancestor of humans and chimpanzees is characterized by a number of morphological, developmental, physiological, and behavioral changes. The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny (gestation and infancy), and decreased sexual dimorphism. The relationship between these changes is the subject of ongoing debate.[page needed ] Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in .

Bipedalism is the basic adaptation of the hominid and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominids. The earliest hominin, of presumably primitive bipedalism, is considered to be either Sahelanthropus or , both of which arose some 6 to 7 million years ago. The non-bipedal knuckle-walkers, the gorillas and chimpanzees, diverged from the hominin line over a period covering the same time, so either Sahelanthropus or Orrorin may be our last shared ancestor. Ardipithecus, a full biped, arose approximately 5.6 million years ago.

The early bipeds eventually evolved into the australopithecines and still later into the genus Homo. There are several theories of the adaptation value of bipedalism. It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion, enabled long-distance running and hunting, provided an enhanced field of vision, and helped avoid hyperthermia by reducing the surface area exposed to direct sun; features all advantageous for thriving in the new savanna and woodland environment created as a result of the East African Rift Valley uplift versus the previous closed forest habitat. A 2007 study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knuckle-walking. However, recent studies suggest that bipedality without the ability to use fire would not have allowed global dispersal. This change in gait saw a lengthening of the legs proportionately when compared to the length of the arms, which were shortened through the removal of the need for brachiation. Another change is the shape of the big toe. Recent studies suggest that australopithecines still lived part of the time in trees as a result of maintaining a grasping big toe. This was progressively lost in habilines.

Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the vertebral column, feet and ankles, and skull. The femur evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became increasingly robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the lumbar vertebrae became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion. The arms and forearms shortened relative to the legs making it easier to run. The foramen magnum migrated under the skull and more anterior.

The most significant changes occurred in the pelvic region, where the long downward facing iliac blade was shortened and widened as a requirement for keeping the center of gravity stable while walking; bipedal hominids have a shorter but broader, bowl-like pelvis due to this. A drawback is that the birth canal of bipedal apes is smaller than in knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, permitting the passage of newborns due to the increase in cranial size but this is limited to the upper portion, since further increase can hinder normal bipedal movement.

The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth which is much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position (compared to the mother) during entry into the birth canal and rotate about 90 degrees upon exit. The smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period leading to the relative immaturity of human offspring, who are unable to walk much before 12 months and have greater neoteny, compared to other primates, who are mobile at a much earlier age. The increased brain growth after birth and the increased dependency of children on mothers had a major effect upon the female reproductive cycle, and the more frequent appearance of alloparenting in humans when compared with other hominids. Delayed human sexual maturity also led to the evolution of menopause with one explanation providing that elderly women could better pass on their genes by taking care of their daughter's offspring, as compared to having more children of their own.

The human species eventually developed a much larger brain than that of other primates-typically 1,330 cm ³ (81 cu in) in modern humans, nearly three times the size of a chimpanzee or gorilla brain. After a period of stasis with Australopithecus anamensis and Ardipithecus, species which had smaller brains as a result of their bipedal locomotion, the pattern of encephalization started with Homo habilis, whose 600 cm ³ (37 cu in) brain was slightly larger than that of chimpanzees. This evolution continued in Homo erectus with 800-1,100 cm ³ (49-67 cu in), and reached a maximum in Neanderthals with 1,200-1,900 cm ³ (73-116 cu in), larger even than modern Homo sapiens. This brain increase manifested during postnatal brain growth, far exceeding that of other apes ( heterochrony). It also allowed for extended periods of social learning and language acquisition in juvenile humans, beginning as much as 2 million years ago.

Furthermore, the changes in the structure of human brains may be even more significant than the increase in size.

The temporal lobes, which contain centers for language processing, have increased disproportionately, as has the prefrontal cortex, which has been related to complex decision-making and moderating social behavior. Encephalization has been tied to increased meat and starches in the diet, and the development of cooking, and it has been proposed that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex. Changes in skull morphology, such as smaller mandibles and mandible muscle attachments, allowed more room for the brain to grow.

The increase in volume of the neocortex also included a rapid increase in size of the cerebellum. Its function has traditionally been associated with balance and fine motor control, but more recently with speech and cognition. The great apes, including hominids, had a more pronounced cerebellum relative to the neocortex than other primates. It has been suggested that because of its function of sensory-motor control and learning complex muscular actions, the cerebellum may have underpinned human technological adaptations, including the preconditions of speech.

The immediate survival advantage of encephalization is difficult to discern, as the major brain changes from Homo erectus to Homo heidelbergensis were not accompanied by major changes in technology. It has been suggested that the changes were mainly social and behavioural, including increased empathic abilities, increases in size of social groups, and increased behavioural plasticity. Encephalization may be due to a dependency on calorie-dense, difficult-to-acquire food.

The reduced degree of sexual dimorphism in humans is visible primarily in the reduction of the male canine tooth relative to other ape species (except gibbons) and reduced brow ridges and general robustness of males. Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans are the only hominoids in which the female is fertile year round and in which no special signals of fertility are produced by the body (such as genital swelling or overt changes in proceptivity during estrus).

Nonetheless, humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 15% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on pair bonding as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.

The ulnar opposition-the contact between the thumb and the tip of the little finger of the same hand-is unique to the genus Homo, including Neanderthals, the Sima de los Huesos hominins and anatomically modern humans. In other primates, the thumb is short and unable to touch the little finger. The ulnar opposition facilitates the precision grip and power grip of the human hand, underlying all the skilled manipulations.

A number of other changes have also characterized the evolution of humans, among them an increased importance on vision rather than smell; a longer juvenile developmental period and higher infant dependency; a smaller gut; faster basal metabolism; loss of body hair; evolution of sweat glands; a change in the shape of the dental arcade from being u-shaped to being parabolic; development of a chin (found in Homo sapiens alone); development of styloid processes; and the development of a descended larynx.

The word homo, the name of the biological genus to which humans belong, is Latin for "human". It was chosen originally by Carl Linnaeus in his classification system. The word "human" is from the Latin humanus, the adjectival form of homo. The Latin "homo" derives from the Indo-European root * dhghem, or "earth". Linnaeus and other scientists of his time also considered the great apes to be the closest relatives of humans based on morphological and anatomical similarities.

The possibility of linking humans with earlier apes by descent became clear only after 1859 with the publication of Charles Darwin's On the Origin of Species, in which he argued for the idea of the evolution of new species from earlier ones. Darwin's book did not address the question of human evolution, saying only that "Light will be thrown on the origin of man and his history."

The first debates about the nature of human evolution arose between Thomas Henry Huxley and Richard Owen. Huxley argued for human evolution from apes by illustrating many of the similarities and differences between humans and other apes, and did so particularly in his 1863 book Evidence as to Man's Place in Nature. Many of Darwin's early supporters (such as Alfred Russel Wallace and Charles Lyell) did not initially agree that the origin of the mental capacities and the moral sensibilities of humans could be explained by natural selection, though this later changed. Darwin applied the theory of evolution and sexual selection to humans in his 1871 book The Descent of Man, and Selection in Relation to Sex.

A major problem in the 19th century was the lack of fossil intermediaries. Neanderthal remains were discovered in a limestone quarry in 1856, three years before the publication of On the Origin of Species, and Neanderthal fossils had been discovered in Gibraltar even earlier, but it was originally claimed that these were the remains of a modern human who had suffered some kind of illness. Despite the 1891 discovery by Eugène Dubois of what is now called Homo erectus at Trinil, Java, it was only in the 1920s when such fossils were discovered in Africa, that intermediate species began to accumulate. In 1925, Raymond Dart described Australopithecus africanus. The type specimen was the Taung Child, an australopithecine infant which was discovered in a cave. The child's remains were a remarkably well-preserved tiny skull and an endocast of the brain.

Although the brain was small (410 cm ³), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen showed short canine teeth, and the position of the foramen magnum (the hole in the skull where the spine enters) was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung Child was a bipedal human ancestor, a transitional form between apes and humans.

During the 1960s and 1970s, hundreds of fossils were found in East Africa in the regions of the Olduvai Gorge and Lake Turkana. These searches were carried out by the Leakey family, with Louis Leakey and his wife Mary Leakey, and later their son Richard and daughter-in-law Meave, fossil hunters and paleoanthropologists. From the fossil beds of Olduvai and Lake Turkana they amassed specimens of the early hominins: the australopithecines and Homo species, and even Homo erectus.

These finds cemented Africa as the cradle of humankind. In the late 1970s and the 1980s, Ethiopia emerged as the new hot spot of paleoanthropology after "Lucy", the most complete fossil member of the species Australopithecus afarensis, was found in 1974 by Donald Johanson near Hadar in the desertic Afar Triangle region of northern Ethiopia. Although the specimen had a small brain, the pelvis and leg bones were almost identical in function to those of modern humans, showing with certainty that these hominins had walked erect. Lucy was classified as a new species, Australopithecus afarensis, which is thought to be more closely related to the genus as a direct ancestor, or as a close relative of an unknown ancestor, than any other known hominid or hominin from this early time range; see terms "hominid" and "hominin". (The specimen was nicknamed "Lucy" after the Beatles' song " Lucy in the Sky with Diamonds", which was played loudly and repeatedly in the camp during the excavations.) The Afar Triangle area would later yield discovery of many more hominin fossils, particularly those uncovered or described by teams headed by Tim D. White in the 1990s, including Ardipithecus ramidus and Ardipithecus kadabba.

In 2013, fossil skeletons of Homo naledi, an extinct species of hominin assigned (provisionally) to the genus , were found in the Rising Star Cave system, a site in South Africa's Cradle of Humankind region in Gauteng province near Johannesburg. As of September 2015, fossils of at least fifteen individuals, amounting to 1,550 specimens, have been excavated from the cave. The species is characterized by a body mass and stature similar to small-bodied human populations, a smaller endocranial volume similar to Australopithecus, and a cranial morphology (skull shape) similar to early Homo species. The skeletal anatomy combines primitive features known from australopithecines with features known from early hominins. The individuals show signs of having been deliberately disposed of within the cave near the time of death. The fossils were dated close to 250,000 years ago, and thus are not a direct ancestor but a contemporary with the first appearance of larger-brained anatomically modern humans.

The genetic revolution in studies of human evolution started when Vincent Sarich and Allan Wilson measured the strength of immunological cross-reactions of blood serum albumin between pairs of creatures, including humans and African apes (chimpanzees and gorillas). The strength of the reaction could be expressed numerically as an immunological distance, which was in turn proportional to the number of amino acid differences between homologous proteins in different species. By constructing a calibration curve of the ID of species' pairs with known divergence times in the fossil record, the data could be used as a molecular clock to estimate the times of divergence of pairs with poorer or unknown fossil records.

In their seminal 1967 paper in , Sarich and Wilson estimated the divergence time of humans and apes as four to five million years ago, at a time when standard interpretations of the fossil record gave this divergence as at least 10 to as much as 30 million years. Subsequent fossil discoveries, notably "Lucy", and reinterpretation of older fossil materials, notably Ramapithecus, showed the younger estimates to be correct and validated the albumin method.

Progress in DNA sequencing, specifically mitochondrial DNA (mtDNA) and then Y-chromosome DNA (Y-DNA) advanced the understanding of human origins. Application of the molecular clock principle revolutionized the study of molecular evolution.

On the basis of a separation from the orangutan between 10 and 20 million years ago, earlier studies of the molecular clock suggested that there were about 76 mutations per generation that were not inherited by human children from their parents; this evidence supported the divergence time between hominins and chimpanzees noted above. However, a 2012 study in Iceland of 78 children and their parents suggests a mutation rate of only 36 mutations per generation; this datum extends the separation between humans and chimpanzees to an earlier period greater than 7 million years ago ( Ma). Additional research with 226 offspring of wild chimpanzee populations in eight locations suggests that chimpanzees reproduce at age 26.5 years on average; which suggests the human divergence from chimpanzees occurred between 7 and 13 million years ago. And these data suggest that Ardipithecus (4.5 Ma), Orrorin (6 Ma) and Sahelanthropus (7 Ma) all may be on the hominid lineage, and even that the separation may have occurred outside the East African Rift region.

Furthermore, analysis of the two species' genes in 2006 provides evidence that after human ancestors had started to diverge from chimpanzees, interspecies mating between "proto-human" and "proto-chimpanzees" nonetheless occurred regularly enough to change certain genes in the new gene pool:

A new comparison of the human and chimpanzee genomes suggests that after the two lineages separated, they may have begun interbreeding... A principal finding is that the X chromosomes of humans and chimpanzees appear to have diverged about 1.2 million years more recently than the other chromosomes.

The research suggests:

There were in fact two splits between the human and chimpanzee lineages, with the first being followed by interbreeding between the two populations and then a second split. The suggestion of a hybridization has startled paleoanthropologists, who nonetheless are treating the new genetic data seriously.

In the 1990s, several teams of paleoanthropologists were working throughout Africa looking for evidence of the earliest divergence of the hominin lineage from the great apes. In 1994, Meave Leakey discovered Australopithecus anamensis. The find was overshadowed by Tim D. White's 1995 discovery of Ardipithecus ramidus, which pushed back the fossil record to 4.2 million years ago.

In 2000, Martin Pickford and Brigitte Senut discovered, in the Tugen Hills of Kenya, a 6-million-year-old bipedal hominin which they named Orrorin tugenensis. And in 2001, a team led by Michel Brunet discovered the skull of Sahelanthropus tchadensis which was dated as 7.2 million years ago, and which Brunet argued was a bipedal, and therefore a hominid-that is, a hominin ( Hominidae; terms "hominids" and hominins).

Anthropologists in the 1980s were divided regarding some details of reproductive barriers and migratory dispersals of the genus Homo. Subsequently, genetics has been used to investigate and resolve these issues. According to the Sahara pump theory evidence suggests that the genus Homo have migrated out of Africa at least three and possibly four times (e.g. Homo erectus, Homo heidelbergensis and two or three times for Homo sapiens). Recent evidence suggests these dispersals are closely related to fluctuating periods of climate change.

Recent evidence suggests that humans may have left Africa half a million years earlier than previously thought. A joint Franco-Indian team has found human artifacts in the Siwalk Hills north of New Delhi dating back at least 2.6 million years. This is earlier than the previous earliest finding of genus Homo at Dmanisi, in Georgia, dating to 1.85 million years. Although controversial, tools found at a Chinese cave strengthen the case that humans used tools as far back as 2.48 million years ago. This suggests that the Asian "Chopper" tool tradition, found in Java and northern China may have left Africa before the appearance of the Acheulian hand axe.

Dispersal of modern Homo sapiens

Up until the genetic evidence became available, there were two dominant models for the dispersal of modern humans. The multiregional hypothesis proposed that the genus Homo contained only a single interconnected population as it does today (not separate species), and that its evolution took place worldwide continuously over the last couple of million years. This model was proposed in 1988 by Milford H. Wolpoff. In contrast, the "out of Africa" model proposed that modern H. sapiens speciated in Africa recently (that is, approximately 200,000 years ago) and the subsequent migration through Eurasia resulted in the nearly complete replacement of other Homo species. This model has been developed by Chris B. Stringer and Peter Andrews.

Sequencing mtDNA and Y-DNA sampled from a wide range of indigenous populations revealed ancestral information relating to both male and female genetic heritage, and strengthened the "out of Africa" theory and weakened the views of multiregional evolutionism. Aligned in genetic tree differences were interpreted as supportive of a recent single origin.

"Out of Africa" has thus gained much support from research using female mitochondrial DNA and the male Y chromosome. After analysing genealogy trees constructed using 133 types of mtDNA, researchers concluded that all were descended from a female African progenitor, dubbed Mitochondrial Eve. "Out of Africa" is also supported by the fact that mitochondrial genetic diversity is highest among African populations.

A broad study of African genetic diversity, headed by Sarah Tishkoff, found the San people had the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located a possible origin of modern human migration in southwestern Africa, near the coastal border of Namibia and Angola. The fossil evidence was insufficient for archaeologist Richard Leakey to resolve the debate about exactly where in Africa modern humans first appeared. Studies of haplogroups in Y-chromosomal DNA and mitochondrial DNA have largely supported a recent African origin. All the evidence from autosomal DNA also predominantly supports a Recent African origin. However, evidence for archaic admixture in modern humans, both in Africa and later, throughout Eurasia has recently been suggested by a number of studies.

Recent sequencing of Neanderthal and Denisovan genomes shows that some admixture with these populations has occurred. All modern human groups outside Africa have 1-4% or (according to more recent research) about 1.5-2.6% Neanderthal alleles in their genome, and some Melanesians have an additional 4-6% of Denisovan alleles. These new results do not contradict the "out of Africa" model, except in its strictest interpretation, although they make the situation more complex. After recovery from a genetic bottleneck that some researchers speculate might be linked to the Toba supervolcano catastrophe, a fairly small group left Africa and interbred with Neanderthals, probably in the Middle East, on the Eurasian steppe or even in North Africa before their departure. Their still predominantly African descendants spread to populate the world. A fraction in turn interbred with Denisovans, probably in southeastern Asia, before populating Melanesia. HLA haplotypes of Neanderthal and Denisova origin have been identified in modern Eurasian and Oceanian populations. The Denisovan EPAS1 gene has also been found in Tibetan populations. Studies of the human genome using machine learning have identified additional genetic contributions in Eurasians from an "unknown" ancestral population potentially related to the Neanderthal-Denisovan lineage.

There are still differing theories on whether there was a single exodus from Africa or several. A multiple dispersal model involves the Southern Dispersal theory, which has gained support in recent years from genetic, linguistic and archaeological evidence. In this theory, there was a coastal dispersal of modern humans from the Horn of Africa crossing the Bab el Mandib to Yemen at a lower sea level around 70,000 years ago. This group helped to populate Southeast Asia and Oceania, explaining the discovery of early human sites in these areas much earlier than those in the Levant. This group seems to have been dependent upon marine resources for their survival.

Stephen Oppenheimer has proposed a second wave of humans may have later dispersed through the Persian Gulf oases, and the Zagros mountains into the Middle East. Alternatively it may have come across the Sinai Peninsula into Asia, from shortly after 50,000 yrs BP, resulting in the bulk of the human populations of Eurasia. It has been suggested that this second group possibly possessed a more sophisticated "big game hunting" tool technology and was less dependent on coastal food sources than the original group. Much of the evidence for the first group's expansion would have been destroyed by the rising sea levels at the end of each glacial maximum. The multiple dispersal model is contradicted by studies indicating that the populations of Eurasia and the populations of Southeast Asia and Oceania are all descended from the same mitochondrial DNA L3 lineages, which support a single migration out of Africa that gave rise to all non-African populations.

On the basis of the early date of Badoshan Iranian Aurignacian, Oppenheimer suggests that this second dispersal may have occurred with a pluvial period about 50,000 years before the present, with modern human big-game hunting cultures spreading up the Zagros Mountains, carrying modern human genomes from Oman, throughout the Persian Gulf, northward into Armenia and Anatolia, with a variant travelling south into Israel and to Cyrenicia.

Recent genetic evidence suggests that all modern non-African populations, including those of Eurasia and Oceania, are descended from a single wave that left Africa between 65,000 and 50,000 years ago.

The evidence on which scientific accounts of human evolution are based comes from many fields of natural science. The main source of knowledge about the evolutionary process has traditionally been the fossil record, but since the development of genetics beginning in the 1970s, DNA analysis has come to occupy a place of comparable importance. The studies of ontogeny, phylogeny and especially evolutionary developmental biology of both vertebrates and invertebrates offer considerable insight into the evolution of all life, including how humans evolved. The specific study of the origin and life of humans is anthropology, particularly paleoanthropology which focuses on the study of human prehistory.

The closest living relatives of humans are bonobos and chimpanzees (both genus Pan) and gorillas (genus Gorilla). With the sequencing of both the human and chimpanzee genome, as of 2012 estimates of the similarity between their DNA sequences range between 95% and 99%. By using the technique called the molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated.

The gibbons (family Hylobatidae) and then the orangutans (genus Pongo) were the first groups to split from the line leading to the hominins, including humans-followed by gorillas (genus Gorilla), and, ultimately, by the chimpanzees (genus Pan). The splitting date between hominin and chimpanzee lineages is placed by some between 4 to 8 million years ago, that is, during the Late Miocene. 7 to 13 5 to 6 million years ago. million years ago, but ongoing hybridization blurred the separation and delayed complete separation during several millions of years. Patterson (2006) dated the final divergence at Speciation, however, appears to have been unusually drawn out. Initial divergence occurred sometime between

Genetic evidence has also been employed to resolve the question of whether there was any gene flow between early modern humans and Neanderthals, and to enhance the understanding of the early human migration patterns and splitting dates. By comparing the parts of the genome that are not under natural selection and which therefore accumulate mutations at a fairly steady rate, it is possible to reconstruct a genetic tree incorporating the entire human species since the last shared ancestor.

Each time a certain mutation ( single-nucleotide polymorphism) appears in an individual and is passed on to his or her descendants, a haplogroup is formed including all of the descendants of the individual who will also carry that mutation. By comparing mitochondrial DNA which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 200,000 years ago.

Human evolutionary genetics studies how one human genome differs from the other, the evolutionary past that gave rise to it, and its current effects. Differences between genomes have anthropological, medical and forensic implications and applications. Genetic data can provide important insight into human evolution.

There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages. The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis dating from 7 million years ago, Orrorin tugenensis dating from 5.7 million years ago, and Ardipithecus kadabba dating to 5.6 million years ago. Each of these have been argued to be a bipedal ancestor of later hominins but, in each case, the claims have been contested. It is also possible that one or more of these species are ancestors of another branch of African apes, or that they represent a shared ancestor between hominins and other apes.

The question then of the relationship between these early fossil species and the hominin lineage is still to be resolved. From these early species, the australopithecines arose around 4 million years ago and diverged into robust (also called Paranthropus) and gracile branches, one of which (possibly ) probably went on to become ancestors of the genus Homo. The australopithecine species that is best represented in the fossil record is Australopithecus afarensis with more than 100 fossil individuals represented, found from Northern Ethiopia (such as the famous "Lucy"), to Kenya, and South Africa. Fossils of robust australopithecines such as Au. robustus (or alternatively Paranthropus robustus) and Au./P. boisei are particularly abundant in South Africa at sites such as Kromdraai and Swartkrans, and around Lake Turkana in Kenya.

The earliest member of the genus Homo is Homo habilis which evolved around 2.8 million years ago. Homo habilis is the first species for which we have positive evidence of the use of stone tools. They developed the Oldowan lithic technology, named after the Olduvai Gorge in which the first specimens were found. Some scientists consider Homo rudolfensis, a larger bodied group of fossils with similar morphology to the original H. habilis fossils, to be a separate species, while others consider them to be part of H. habilis-simply representing intraspecies variation, or perhaps even sexual dimorphism. The brains of these early hominins were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living.

During the next million years, a process of encephalization began and, by the arrival (about 1.9 million years ago) of Homo erectus in the fossil record, cranial capacity had doubled. Homo erectus were the first of the hominins to emigrate from Africa, and, from 1.8 to 1.3 million years ago, this species spread through Africa, Asia, and Europe. One population of H. erectus, also sometimes classified as a separate species Homo ergaster, remained in Africa and evolved into Homo sapiens. It is believed that these species, H. erectus and H. ergaster, were the first to use fire and complex tools.

The earliest transitional fossils between H. ergaster/erectus and archaic H. sapiens are from Africa, such as Homo rhodesiensis. These descendants of African H. erectus spread through Eurasia from ca. 500,000 years ago, evolving into H. antecessor, H. heidelbergensis and H. neanderthalensis. The earliest fossils of anatomically modern humans are from the Middle Paleolithic, about 300-200,000 years ago such as the Herto and Omo remains of Ethiopia, Jebel Irhoud remains of Morocco, and Florisbad remains of South Africa; later fossils from Es Skhul cave in Israel and Southern Europe begin around 90,000 years ago ( 0.09 million years ago).

As modern humans spread out from Africa, they encountered other hominins such as Homo neanderthalensis and the Denisovans, who may have evolved from populations of Homo erectus that had left Africa around 2 million years ago. The nature of interaction between early humans and these sister species has been a long-standing source of controversy, the question being whether humans replaced these earlier species or whether they were in fact similar enough to interbreed, in which case these earlier populations may have contributed genetic material to modern humans.

This migration out of Africa is estimated to have begun about 70-50,000 years BP and modern humans subsequently spread globally, replacing earlier hominins either through competition or hybridization. They inhabited Eurasia and Oceania by 40,000 years BP, and the Americas by at least 14,500 years BP.

The hypothesis of interbreeding, also known as hybridization, admixture or hybrid-origin theory, has been discussed ever since the discovery of Neanderthal remains in the 19th century. The linear view of human evolution began to be abandoned in the 1970s as different species of humans were discovered that made the linear concept increasingly unlikely. In the 21st century with the advent of molecular biology techniques and computerization, whole-genome sequencing of Neanderthal and human genome were performed, confirming recent admixture between different human species. In 2010, evidence based on molecular biology was published, revealing unambiguous examples of interbreeding between archaic and modern humans during the Middle Paleolithic and early Upper Paleolithic. It has been demonstrated that interbreeding happened in several independent events that included Neanderthals and Denisovans, as well as several unidentified hominins. Today, approximately 2% of DNA from all non-African populations (including Europeans, Asians, and Oceanians) is Neanderthal, with traces of Denisovan heritage. Also, 4-6% of modern Melanesian genetics are Denisovan. Comparisons of the human genome to the genomes of Neandertals, Denisovans and apes can help identify features that set modern humans apart from other hominin species. In a 2016 comparative genomics study, a Harvard Medical School/UCLA research team made a world map on the distribution and made some predictions about where Denisovan and Neanderthal genes may be impacting modern human biology.

For example, comparative studies in the mid-2010s found several traits related to neurological, immunological, developmental, and metabolic phenotypes, that were developed by archaic humans to European and Asian environments and inherited to modern humans through admixture with local hominins.

Although the narratives of human evolution are often contentious, several discoveries since 2010 show that human evolution should not be seen as a simple linear or branched progression, but a mix of related species. In fact, genomic research has shown that hybridization between substantially diverged lineages is the rule, not the exception, in human evolution. Furthermore, it is argued that hybridization was an essential creative force in the emergence of modern humans.

The evolutionary history of the primates can be traced back 65 million years. One of the oldest known primate-like mammal species, the Plesiadapis, came from North America; another, Archicebus, came from China. Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene.

David R. Begun concluded that early primates flourished in Eurasia and that a lineage leading to the African apes and humans, including to Dryopithecus, migrated south from Europe or Western Asia into Africa. The surviving tropical population of primates-which is seen most completely in the Upper Eocene and lowermost Oligocene fossil beds of the Faiyum depression southwest of Cairo-gave rise to all extant primate species, including the lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and to the anthropoids, which are the Platyrrhines or New World monkeys, the Catarrhines or Old World monkeys, and the great apes, including humans and other hominids.

The earliest known catarrhine is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Great Rift Valley in Kenya, dated to 24 million years ago. Its ancestry is thought to be species related to Aegyptopithecus, Propliopithecus, and Parapithecus from the Faiyum, at around 35 million years ago. In 2010, was described as a close relative of the last common ancestor of the crown catarrhines, and tentatively dated to 29-28 million years ago, helping to fill an 11-million-year gap in the fossil record.

In the Early Miocene, about 22 million years ago, the many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Fossils at 20 million years ago include fragments attributed to Victoriapithecus, the earliest Old World monkey. Among the genera thought to be in the ape lineage leading up to 13 million years ago are , Rangwapithecus, Dendropithecus, Limnopithecus, Nacholapithecus, Equatorius, Nyanzapithecus, Afropithecus, Heliopithecus, and Kenyapithecus, all from East Africa.

The presence of other generalized non-cercopithecids of Middle Miocene from sites far distant-Otavipithecus from cave deposits in Namibia, and Pierolapithecus and Dryopithecus from France, Spain and Austria-is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the Early and Middle Miocene. The youngest of the Miocene hominoids, Oreopithecus, is from coal beds in Italy that have been dated to 9 million years ago.

Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) diverged from the line of great apes some 18-12 million years ago, and that of orangutans (subfamily Ponginae) diverged from the other great apes at about 12 million years; there are no fossils that clearly document the ancestry of gibbons, which may have originated in a so-far-unknown Southeast Asian hominoid population, but fossil proto-orangutans may be represented by Sivapithecus from India and Griphopithecus from Turkey, dated to around 10 million years ago.

Species close to the last common ancestor of gorillas, chimpanzees and humans may be represented by Nakalipithecus fossils found in Kenya and Ouranopithecus found in Greece. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas, and then the chimpanzees (genus Pan) split off from the line leading to the humans. Human DNA is approximately 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms (see human evolutionary genetics). The fossil record, however, of gorillas and chimpanzees is limited; both poor preservation - rain forest soils tend to be acidic and dissolve bone - and sampling bias probably contribute to this problem.

Other hominins probably adapted to the drier environments outside the equatorial belt; and there they encountered antelope, hyenas, dogs, pigs, elephants, horses, and others. The equatorial belt contracted after about 8 million years ago, and there is very little fossil evidence for the split-thought to have occurred around that time-of the hominin lineage from the lineages of gorillas and chimpanzees. The earliest fossils argued by some to belong to the human lineage are Sahelanthropus tchadensis (7 Ma) and Orrorin tugenensis (6 Ma), followed by Ardipithecus (5.5-4.4 Ma), with species Ar. kadabba and .

It has been argued in a study of the life history of Ar. ramidus that the species provides evidence for a suite of anatomical and behavioral adaptations in very early hominins unlike any species of extant great ape. This study demonstrated affinities between the skull morphology of Ar. ramidus and that of infant and juvenile chimpanzees, suggesting the species evolved a juvenalised or paedomorphic craniofacial morphology via heterochronic dissociation of growth trajectories. It was also argued that the species provides support for the notion that very early hominins, akin to bonobos (Pan paniscus) the less aggressive species of the genus Pan, may have evolved via the process of self-domestication. Consequently, arguing against the so-called "chimpanzee referential model" the authors suggest it is no longer tenable to use chimpanzee ( Pan troglodytes) social and mating behaviors in models of early hominin social evolution. When commenting on the absence of aggressive canine morphology in Ar. ramidus and the implications this has for the evolution of hominin social psychology, they wrote:

Of course Ar. ramidus differs significantly from bonobos, bonobos having retained a functional canine honing complex. However, the fact that Ar. ramidus shares with bonobos reduced sexual dimorphism, and a more paedomorphic form relative to chimpanzees, suggests that the developmental and social adaptations evident in bonobos may be of assistance in future reconstructions of early hominin social and sexual psychology. In fact the trend towards increased maternal care, female mate selection and self-domestication may have been stronger and more refined in Ar. ramidus than what we see in bonobos.

The authors argue that many of the basic human adaptations evolved in the ancient forest and woodland ecosystems of late Miocene and early Pliocene Africa. Consequently, they argue that humans may not represent evolution from a chimpanzee-like ancestor as has traditionally been supposed. This suggests many modern human adaptations represent phylogenetically deep traits and that the behavior and morphology of chimpanzees may have evolved subsequent to the split with the common ancestor they share with humans.

The genus Australopithecus evolved in eastern Africa around 4 million years ago before spreading throughout the continent and eventually becoming extinct 2 million years ago. During this time period various forms of australopiths existed, including Australopithecus anamensis, Au. afarensis, , and Au. africanus. There is still some debate among academics whether certain African hominid species of this time, such as Au. robustus and , constitute members of the same genus; if so, they would be considered to be Au. robust australopiths whilst the others would be considered Au. gracile australopiths. However, if these species do indeed constitute their own genus, then they may be given their own name, Paranthropus.

A new proposed species Australopithecus deyiremeda is claimed to have been discovered living at the same time period of Au. afarensis. There is debate if Au. deyiremeda is a new species or is Au. afarensis. Australopithecus prometheus, otherwise known as Little Foot has recently been dated at 3.67 million years old through a new dating technique, making the genus Australopithecus as old as afarensis. Given the opposable big toe found on Little Foot, it seems that he was a good climber, and it is thought given the night predators of the region, he probably, like gorillas and chimpanzees, built a nesting platform at night, in the trees.

Evolution of genus Homo

The earliest documented representative of the genus Homo is Homo habilis, which evolved around 2.8 million years ago, and is arguably the earliest species for which there is positive evidence of the use of stone tools. The brains of these early hominins were about the same size as that of a chimpanzee, although it has been suggested that this was the time in which the human SRGAP2 gene doubled, producing a more rapid wiring of the frontal cortex. During the next million years a process of rapid encephalization occurred, and with the arrival of Homo erectus and Homo ergaster in the fossil record, cranial capacity had doubled to 850 cm ³. (Such an increase in human brain size is equivalent to each generation having 125,000 more neurons than their parents.) It is believed that Homo erectus and Homo ergaster were the first to use fire and complex tools, and were the first of the hominin line to leave Africa, spreading throughout Africa, Asia, and Europe between 1.3 to 1.8 million years ago.

According to the recent African origin of modern humans theory, modern humans evolved in Africa possibly from Homo heidelbergensis, Homo rhodesiensis or Homo antecessor and migrated out of the continent some 50,000 to 100,000 years ago, gradually replacing local populations of Homo erectus, Denisova hominins, Homo floresiensis, Homo luzonensis and Homo neanderthalensis. Archaic Homo sapiens, the forerunner of anatomically modern humans, evolved in the Middle Paleolithic between 400,000 and 250,000 years ago. Recent DNA evidence suggests that several haplotypes of Neanderthal origin are present among all non-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day humans, suggestive of a limited interbreeding between these species. The transition to behavioral modernity with the development of symbolic culture, language, and specialized lithic technology happened around 50,000 years ago, according to some anthropologists, although others point to evidence that suggests that a gradual change in behavior took place over a longer time span.

Homo sapiens is the only extant species of its genus, Homo. While some (extinct) Homo species might have been ancestors of Homo sapiens, many, perhaps most, were likely "cousins", having speciated away from the ancestral hominin line. There is yet no consensus as to which of these groups should be considered a separate species and which should be a subspecies; this may be due to the dearth of fossils or to the slight differences used to classify species in the genus Homo. The Sahara pump theory (describing an occasionally passable "wet" Sahara desert) provides one possible explanation of the early variation in the genus Homo.

Based on archaeological and paleontological evidence, it has been possible to infer, to some extent, the ancient dietary practices of various Homo species and to study the role of diet in physical and behavioral evolution within Homo.

Some anthropologists and archaeologists subscribe to the Toba catastrophe theory, which posits that the supereruption of Lake Toba on Sumatran island in Indonesia some 70,000 years ago caused global consequences, killing the majority of humans and creating a population bottleneck that affected the genetic inheritance of all humans today. The genetic and archaeological evidence for this remains in question however.

Homo habilis lived from about 2.8 to 1.4 Ma. The species evolved in South and East Africa in the Late Pliocene or Early Pleistocene, 2.5-2 Ma, when it diverged from the australopithecines. Homo habilis had smaller molars and larger brains than the australopithecines, and made tools from stone and perhaps animal bones. One of the first known hominins was nicknamed 'handy man' by discoverer Louis Leakey due to its association with stone tools. Some scientists have proposed moving this species out of Homo and into Australopithecus due to the morphology of its skeleton being more adapted to living on trees rather than to moving on two legs like Homo sapiens.

In May 2010, a new species, Homo gautengensis, was discovered in South Africa.

These are proposed species names for fossils from about 1.9-1.6 Ma, whose relation to Homo habilis is not yet clear.

Homo rudolfensis refers to a single, incomplete skull from Kenya. Scientists have suggested that this was another Homo habilis, but this has not been confirmed.

Homo georgicus, from Georgia, may be an intermediate form between Homo habilis and Homo erectus, or a subspecies of Homo erectus.

The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He originally named the material Anthropopithecus erectus (1892-1893, considered at this point as a chimpanzee-like fossil primate) and Pithecanthropus erectus (1893-1894, changing his mind as of based on its morphology, which he considered to be intermediate between that of humans and apes). Years later, in the 20th century, the German physician and paleoanthropologist Franz Weidenreich (1873-1948) compared in detail the characters of Dubois' Java Man, then named Pithecanthropus erectus, with the characters of the Peking Man, then named Sinanthropus pekinensis. Weidenreich concluded in 1940 that because of their anatomical similarity with modern humans it was necessary to gather all these specimens of Java and China in a single species of the genus , the species Homo erectus. Homo erectus lived from about 1.8 Ma to about 70,000 years ago - which would indicate that they were probably wiped out by the Toba catastrophe; however, nearby Homo floresiensis survived it. The early phase of Homo erectus, from 1.8 to 1.25 Ma, is considered by some to be a separate species, Homo ergaster, or as Homo erectus ergaster, a subspecies of Homo erectus.

In Africa in the Early Pleistocene, 1.5-1 Ma, some populations of Homo habilis are thought to have evolved larger brains and to have made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, Homo erectus-in Africa. The evolution of locking knees and the movement of the foramen magnum are thought to be likely drivers of the larger population changes. This species also may have used fire to cook meat. Richard Wrangham suggests that the fact that Homo seems to have been ground dwelling, with reduced intestinal length, smaller dentition, "and swelled our brains to their current, horrendously fuel-inefficient size", suggest that control of fire and releasing increased nutritional value through cooking was the key adaptation that separated Homo from tree-sleeping Australopithecines.

A famous example of Homo erectus is Peking Man; others were found in Asia (notably in Indonesia), Africa, and Europe. Many paleoanthropologists now use the term Homo ergaster for the non-Asian forms of this group, and reserve Homo erectus only for those fossils that are found in Asia and meet certain skeletal and dental requirements which differ slightly from H. ergaster.

These are proposed as species that may be intermediate between H. erectus and H. heidelbergensis.

H. antecessor is known from fossils from Spain and England that are dated 1.2 Ma-500 ka.

H. cepranensis refers to a single skull cap from Italy, estimated to be about 800,000 years old.

H. heidelbergensis ("Heidelberg Man") lived from about 800,000 to about 300,000 years ago. Also proposed as Homo sapiens heidelbergensis or Homo sapiens paleohungaricus.

H. rhodesiensis, and the Gawis cranium

H. rhodesiensis, estimated to be 300,000-125,000 years old. Most current researchers place Rhodesian Man within the group of Homo heidelbergensis, though other designations such as archaic Homo sapiens and Homo sapiens rhodesiensis have been proposed.

In February 2006 a fossil, the Gawis cranium, was found which might possibly be a species intermediate between H. erectus and H. sapiens or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000-250,000 years old. Only summary details are known, and the finders have not yet released a peer-reviewed study. Gawis man's facial features suggest its being either an intermediate species or an example of a "Bodo man" female.

Homo neanderthalensis, alternatively designated as Homo sapiens neanderthalensis, lived in Europe and Asia from 400,000 to about 28,000 years ago. There are a number of clear anatomical differences between anatomically modern humans (AMH) and Neanderthal populations. Many of these relate to the superior adaptation to cold environments possessed by the Neanderthal populations. Their surface to volume ratio is an extreme version of that found amongst Inuit populations, indicating that they were less inclined to lose body heat than were AMH. From brain Endocasts, Neanderthals also had significantly larger brains. This would seem to indicate that the intellectual superiority of AMH populations may be questionable. More recent research by Eiluned Pearce, Chris Stringer, R.I.M. Dunbar, however, have shown important differences in brain architecture. For example, in both the orbital chamber size and in the size of the occipital lobe, the larger size suggests that the Neanderthal had a better visual acuity than modern humans. This would give a superior vision in the inferior light conditions found in Glacial Europe. It also seems that the higher body mass of Neanderthals had a correspondingly larger brain mass required for body care and control.

The Neanderthal populations seem to have been physically superior to AMH populations. These differences may have been sufficient to give Neanderthal populations an environmental superiority to AMH populations from 75,000 to 45,000 years BP. With these differences, Neanderthal brains show a smaller area was available for social functioning. Plotting group size possible from endocranial volume, suggests that AMH populations (minus occipital lobe size), had a Dunbars number of 144 possible relationships. Neanderthal populations seem to have been limited to about 120 individuals. This would show up in a larger number of possible mates for AMH humans, with increased risks of inbreeding amongst Neanderthal populations. It also suggests that humans had larger trade catchment areas than Neanderthals (confirmed in the distribution of stone tools). With larger populations, social and technological innovations were easier to fix in human populations, which may have all contributed to the fact that modern Homo sapiens replaced the Neanderthal populations by 28,000 BP.

Earlier evidence from sequencing mitochondrial DNA suggested that no significant gene flow occurred between H. neanderthalensis and H. sapiens, and that the two were separate species that shared a common ancestor about 660,000 years ago. However, a sequencing of the Neanderthal genome in 2010 indicated that Neanderthals did indeed interbreed with anatomically modern humans circa 45,000 to 80,000 years ago (at the approximate time that modern humans migrated out from Africa, but before they dispersed into Europe, Asia and elsewhere). The genetic sequencing of a 40,000-year-old human skeleton from Romania showed that 11% of its genome was Neanderthal, and it was estimated that the individual had a Neanderthal ancestor 4-6 generations previously, in addition to a contribution from earlier interbreeding in the Middle East. Though this interbred Romanian population seems not to have been ancestral to modern humans, the finding indicates that interbreeding happened repeatedly.

All modern non-African humans have about 1% to 4% or, according to more recent data, about 1.5% to 2.6% of their DNA derived from Neanderthal DNA, and this finding is consistent with recent studies indicating that the divergence of some human alleles dates to one Ma, although the interpretation of these studies has been questioned. Neanderthals and Homo sapiens could have co-existed in Europe for as long as 10,000 years, during which human populations exploded vastly outnumbering Neanderthals, possibly outcompeting them by sheer numerical strength.

In 2008, archaeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile member of Denisovans. Artifacts, including a bracelet, excavated in the cave at the same level were carbon dated to around 40,000 BP. As DNA had survived in the fossil fragment due to the cool climate of the Denisova Cave, both mtDNA and nuclear DNA were sequenced.

While the divergence point of the mtDNA was unexpectedly deep in time, the full genomic sequence suggested the Denisovans belonged to the same lineage as Neanderthals, with the two diverging shortly after their line split from the lineage that gave rise to modern humans. Modern humans are known to have overlapped with Neanderthals in Europe and the Near East for possibly more than 40,000 years, and the discovery raises the possibility that Neanderthals, Denisovans, and modern humans may have co-existed and interbred. The existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene than previously thought. Evidence has also been found that as much as 6% of the DNA of some modern Melanesians derive from Denisovans, indicating limited interbreeding in Southeast Asia.

Alleles thought to have originated in Neanderthals and Denisovans have been identified at several genetic loci in the genomes of modern humans outside of Africa. HLA haplotypes from Denisovans and Neanderthal represent more than half the HLA alleles of modern Eurasians, indicating strong positive selection for these introgressed alleles. Corinne Simoneti at Vanderbilt University, in Nashville and her team have found from medical records of 28,000 people of European descent that the presence of Neanderthal DNA segments may be associated with a likelihood to suffer depression more frequently.

The flow of genes from Neanderthal populations to modern humans was not all one way. Sergi Castellano of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has in 2016 reported that while Denisovan and Neanderthal genomes are more related to each other than they are to us, Siberian Neanderthal genomes show similarity to the modern human gene pool, more so than to European Neanderthal populations. The evidence suggests that the Neanderthal populations interbred with modern humans possibly 100,000 years ago, probably somewhere in the Near East.

Studies of a Neanderthal child at Gibraltar show from brain development and teeth eruption that Neanderthal children may have matured more rapidly than is the case for Homo sapiens.

H. floresiensis, which lived from approximately 190,000 to 50,000 years before present (BP), has been nicknamed the for its small size, possibly a result of insular dwarfism. H. floresiensis is intriguing both for its size and its age, being an example of a recent species of the genus Homo that exhibits derived traits not shared with modern humans. In other words, H. floresiensis shares a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path. The main find was a skeleton believed to be a woman of about 30 years of age. Found in 2003, it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380 cm ³ (considered small for a chimpanzee and less than a third of the H. sapiens average of 1400 cm ³).

However, there is an ongoing debate over whether H. floresiensis is indeed a separate species. Some scientists hold that H. floresiensis was a modern H. sapiens with pathological dwarfism. This hypothesis is supported in part, because some modern humans who live on Flores, the Indonesian island where the skeleton was found, are pygmies. This, coupled with pathological dwarfism, could have resulted in a significantly diminutive human. The other major attack on H. floresiensis as a separate species is that it was found with tools only associated with H. sapiens.

The hypothesis of pathological dwarfism, however, fails to explain additional anatomical features that are unlike those of modern humans (diseased or not) but much like those of ancient members of our genus. Aside from cranial features, these features include the form of bones in the wrist, forearm, shoulder, knees, and feet. Additionally, this hypothesis fails to explain the find of multiple examples of individuals with these same characteristics, indicating they were common to a large population, and not limited to one individual.

A small number of specimens from the island of Luzon, dated 50,000 to 67,000 years ago, have recently been assigned by their discoverers, based on dental characteristics, to a novel human species, H. luzonensis.

H. sapiens (the adjective is Latin for "wise" or "intelligent") emerged in Africa around 300,000 years ago, likely derived from Homo heidelbergensis or a related lineage. In September 2019, scientists reported the computerized determination, based on 260 CT scans, of a virtual skull shape of the last common human ancestor to modern humans/ H. sapiens, representative of the earliest modern humans, and suggested that modern humans arose between 260,000 and 350,000 years ago through a merging of populations in East and South Africa.

Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in intra-cranial volume expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. The direct evidence suggests there was a migration of H. erectus out of Africa, then a further speciation of H. sapiens from H. erectus in Africa. A subsequent migration (both within and out of Africa) eventually replaced the earlier dispersed H. erectus. This migration and origin theory is usually referred to as the "recent single-origin hypothesis" or "out of Africa" theory. H. sapiens interbred with archaic humans both in Africa and in Eurasia, in Eurasia notably with Neanderthals and Denisovans.

The Toba catastrophe theory, which postulates a population bottleneck for H. sapiens about 70,000 years ago, was controversial from its first proposal in the 1990s and by the 2010s had very little support. Distinctive human genetic variability has arisen as the result of the founder effect, by archaic admixture and by recent evolutionary pressures.

The use of tools has been interpreted as a sign of intelligence, and it has been theorized that tool use may have stimulated certain aspects of human evolution, especially the continued expansion of the human brain. Paleontology has yet to explain the expansion of this organ over millions of years despite being extremely demanding in terms of energy consumption. The brain of a modern human consumes about 13 watts (260 kilocalories per day), a fifth of the body's resting power consumption. Increased tool use would allow hunting for energy-rich meat products, and would enable processing more energy-rich plant products. Researchers have suggested that early hominins were thus under evolutionary pressure to increase their capacity to create and use tools.

Precisely when early humans started to use tools is difficult to determine, because the more primitive these tools are (for example, sharp-edged stones) the more difficult it is to decide whether they are natural objects or human artifacts. There is some evidence that the australopithecines (4 Ma) may have used broken bones as tools, but this is debated.

Many species make and use tools, but it is the human genus that dominates the areas of making and using more complex tools. The oldest known tools are flakes from West Turkana, Kenya, which date to 3.3 million years ago. The next oldest stone tools are from Gona, Ethiopia, and are considered the beginning of the Oldowan technology. These tools date to about 2.6 million years ago. A Homo fossil was found near some Oldowan tools, and its age was noted at 2.3 million years old, suggesting that maybe the Homo species did indeed create and use these tools. It is a possibility but does not yet represent solid evidence. The third metacarpal styloid process enables the hand bone to lock into the wrist bones, allowing for greater amounts of pressure to be applied to the wrist and hand from a grasping thumb and fingers. It allows humans the dexterity and strength to make and use complex tools. This unique anatomical feature separates humans from apes and other nonhuman primates, and is not seen in human fossils older than 1.8 million years.

Bernard Wood noted that Paranthropus co-existed with the early Homo species in the area of the "Oldowan Industrial Complex" over roughly the same span of time. Although there is no direct evidence which identifies Paranthropus as the tool makers, their anatomy lends to indirect evidence of their capabilities in this area. Most paleoanthropologists agree that the early Homo species were indeed responsible for most of the Oldowan tools found. They argue that when most of the Oldowan tools were found in association with human fossils, Homo was always present, but Paranthropus was not.

In 1994, Randall Susman used the anatomy of opposable thumbs as the basis for his argument that both the Homo and Paranthropus species were toolmakers. He compared bones and muscles of human and chimpanzee thumbs, finding that humans have 3 muscles which are lacking in chimpanzees. Humans also have thicker metacarpals with broader heads, allowing more precise grasping than the chimpanzee hand can perform. Susman posited that modern anatomy of the human opposable thumb is an evolutionary response to the requirements associated with making and handling tools and that both species were indeed toolmakers.

Stone tools are first attested around 2.6 million years ago, when hominins in Eastern Africa used so-called core tools, choppers made out of round cores that had been split by simple strikes. This marks the beginning of the Paleolithic, or Old Stone Age; its end is taken to be the end of the last Ice Age, around 10,000 years ago. The Paleolithic is subdivided into the Lower Paleolithic (Early Stone Age), ending around 350,000-300,000 years ago, the Middle Paleolithic (Middle Stone Age), until 50,000-30,000 years ago, and the Upper Paleolithic, (Late Stone Age), 50,000-10,000 years ago.

Archaeologists working in the Great Rift Valley in Kenya have discovered the oldest known stone tools in the world. Dated to around 3.3 million years ago, the implements are some 700,000 years older than stone tools from Ethiopia that previously held this distinction.

The period from 700,000 to 300,000 years ago is also known as the Acheulean, when H. ergaster (or erectus) made large stone hand axes out of flint and quartzite, at first quite rough (Early Acheulian), later " retouched" by additional, more-subtle strikes at the sides of the flakes. After 350,000 BP the more refined so-called Levallois technique was developed, a series of consecutive strikes, by which scrapers, slicers ("racloirs"), needles, and flattened needles were made. Finally, after about 50,000 BP, ever more refined and specialized flint tools were made by the Neanderthals and the immigrant Cro-Magnons (knives, blades, skimmers). Bone tools were also made by H. sapiens in Africa by 90-70,000 years ago and are also known from early H. sapiens sites in Eurasia by about 50,000 years ago.

Transition to behavioral modernity

Until about 50,000-40,000 years ago, the use of stone tools seems to have progressed stepwise. Each phase ( H. habilis, H. ergaster, H. neanderthalensis) started at a higher level than the previous one, but after each phase started, further development was slow. Currently paleoanthropologists are debating whether these Homo species possessed some or many of the cultural and behavioral traits associated with modern humans such as language, complex symbolic thinking, technological creativity etc. It seems that they were culturally conservative maintaining simple technologies and foraging patterns over very long periods.

Around 50,000 BP, modern human culture started to evolve more rapidly. The transition to behavioral modernity has been characterized by some as a "Great Leap Forward", or as the "Upper Palaeolithic Revolution", due to the sudden appearance of distinctive signs of modern behavior and big game hunting in the archaeological record. Evidence of behavioral modernity significantly earlier also exists from Africa, with older evidence of abstract imagery, widened subsistence strategies, more sophisticated tools and weapons, and other "modern" behaviors, and many scholars have recently argued that the transition to modernity occurred sooner than previously believed. Some other scholars consider the transition to have been more gradual, noting that some features had already appeared among archaic African Homo sapiens since 300-200,000 years ago. Recent evidence suggests that the Australian Aboriginal population separated from the African population 75,000 years ago, and that they made a sea journey of up to 160 km 60,000 years ago, which may diminish the evidence of the Upper Paleolithic Revolution.

Modern humans started burying their dead, using animal hides to make clothing, hunting with more sophisticated techniques (such as using trapping pits or driving animals off cliffs), and engaging in cave painting. As human culture advanced, different populations of humans introduced novelty to existing technologies: artifacts such as fish hooks, buttons, and bone needles show signs of variation among different populations of humans, something that had not been seen in human cultures prior to 50,000 BP. Typically, H. neanderthalensis populations do not vary in their technologies, although the Chatelperronian assemblages have been found to be Neanderthal innovations produced as a result of exposure to the Homo sapiens Aurignacian technologies.

Among concrete examples of modern human behavior, anthropologists include specialization of tools, use of jewellery and images (such as cave drawings), organization of living space, rituals (for example, burials with grave gifts), specialized hunting techniques, exploration of less hospitable geographical areas, and barter trade networks. Debate continues as to whether a "revolution" led to modern humans ("the big bang of human consciousness"), or whether the evolution was more "gradual".

Anatomically modern human populations continue to evolve, as they are affected by both natural selection and genetic drift. Although selection pressure on some traits, such as resistance to smallpox, has decreased in the modern age, humans are still undergoing natural selection for many other traits. Some of these are due to specific environmental pressures, while others are related to lifestyle changes since the development of agriculture (10,000 years ago), urbanization (5,000), and industrialization (250 years ago). It has been argued that human evolution has accelerated since the development of agriculture 10,000 years ago and civilization some 5,000 years ago, resulting, it is claimed, in substantial genetic differences between different current human populations, and more recent research indicates that for some traits, the developments and innovations of human culture have driven a new form of selection that coexists with, and in some cases has largely replaced, natural selection.

Particularly conspicuous is variation in superficial characteristics, such as Afro-textured hair, or the recent evolution of light skin and blond hair in some populations, which are attributed to differences in climate. Particularly strong selective pressures have resulted in high-altitude adaptation in humans, with different ones in different isolated populations. Studies of the genetic basis show that some developed very recently, with Tibetans evolving over 3,000 years to have high proportions of an allele of EPAS1 that is adaptive to high altitudes.

Other evolution is related to endemic diseases: the presence of malaria selects for sickle cell trait (the heterozygous form of sickle cell gene), while in the absence of malaria, the health effects of sickle-cell anemia select against this trait. For another example, the population at risk of the severe debilitating disease kuru has significant over-representation of an immune variant of the prion protein gene G127V versus non-immune alleles. The frequency of this genetic variant is due to the survival of immune persons. Some reported trends remain unexplained and the subject of ongoing research in the novel field of evolutionary medicine: polycystic ovary syndrome (PCOS) reduces fertility and thus is expected to be subject to extremely strong negative selection, but its relative commonality in human populations suggests a counteracting selection pressure. The identity of that pressure remains the subject of some debate.

Recent human evolution related to agriculture includes genetic resistance to infectious disease that has appeared in human populations by crossing the species barrier from domesticated animals, as well as changes in metabolism due to changes in diet, such as lactase persistence.

Culturally-driven evolution can defy the expectations of natural selection: while human populations experience some pressure that drives a selection for producing children at younger ages, the advent of effective contraception, higher education, and changing social norms have driven the observed selection in the opposite direction. However, culturally-driven selection need not necessarily work counter or in opposition to natural selection: some proposals to explain the high rate of recent human brain expansion indicate a kind of feedback whereupon the brain's increased social learning efficiency encourages cultural developments that in turn encourage more efficiency, which drive more complex cultural developments that demand still-greater efficiency, and so forth. Culturally-driven evolution has an advantage in that in addition to the genetic effects, it can be observed also in the archaeological record: the development of stone tools across the Palaeolithic period connects to culturally-driven cognitive development in the form of skill acquisition supported by the culture and the development of increasingly complex technologies and the cognitive ability to elaborate them.

In contemporary times, since industrialization, some trends have been observed: for instance, menopause is evolving to occur later. Other reported trends appear to include lengthening of the human reproductive period and reduction in cholesterol levels, blood glucose and blood pressure in some populations.

This list is in chronological order across the table by genus. Some species/subspecies names are well-established, and some are less established - especially in genus Homo. Please see articles for more information.

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Human evolution is the evolutionary process within the history of primates that led to the emergence of Homo sapiens as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development of traits such as human bipedalism and language, as well as interbreeding with other hominins, which indicate that human evolution was not linear but a web.

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''Europe is a little peninsula that happens to have a large amount of spectacular archaeology,'' said Dr. Clive Gamble, director of the Center for the Archaeology of Human Origins at the University of Southampton in England. ''But the European grip of having all the evidence is beginning to slip. We're finding wonderful new evidence in Africa and other places. And in the last two or three years, this has changed and widened the debate over modern human behavior.''

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Scientists are certain about evolution because 99% of our genetic sequence matches the ones of chimpanzees and bonobos.

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The origins of humankind have long fascinated scientists and the public alike-but the topic can get confusing. Here are answers to a few common questions.

Why are scientists certain that human evolution happened?

For a number of reasons. We share nearly 99 percent of our genetic sequence with chimpanzees and bonobos, which strongly suggests we share a common ancestor. And there are thousands of fossils documenting progressively more human-like species in the evolution of our lineage after it split from the other great apes and later from chimps and bonobos.

Biologists have actually observed evolution happening in other species, both in the field and in the lab-the recent emergence of antibiotic-resistant microbes is a form of evolution. And animal breeders make evolution happen all the time-think of the tremendous variety of dog breeds that have been created from wolves.

Is evolution at odds with the Bible?

Yes, if you think the Bible has to be interpreted literally. Evolution contradicts the Genesis story that God created all organisms in their present form. But you can believe in God without believing that the Bible is literally true.

So how does evolution work?

The DNA that makes up our genes, and those of every organism on Earth except some viruses, is subject to random mutation. Every now and then one of those mutations affects an important trait, such as an animal's coat color or a particular behavior. Animal breeders selectively breed animals that have the traits they want; that's artificial selection. In nature, the selection is done by the animal's environment-and by the opposite sex.

If an animal is born with a coat color that offers more protection from predators, for instance, it could survive longer and produce more offspring. If a longer courtship display is more appealing to mates, that too could lead to greater reproductive success. Over time, such favorable mutations spread through a population and change how it looks. Over enough time, the process can even produce new species.

What are the major milestones in human evolution?

The human lineage diverged from that of apes at least seven million years ago and maybe as long as 13 million years ago. The earliest undisputed members of our lineage to regularly walk upright were the australopithecines, of which the most famous is Lucy's species, Australopithecus afarensis. (Lucy herself is dated to 3.2 million years ago.) The oldest known fossil attributed to our genus, Homo, dates to about 2.8 million years ago and was reported just this year. The ability to make stone tools was once thought to be the hallmark of our genus. But the oldest stone tools are now thought to be 3.3 million years old. Either they were made by australopithecines like Lucy-or scientists simply haven't yet found the early form of Homo that made them. Like australopithecines, early Homo species, such as H. erectus and H. habilis, walked on two legs.

There's considerable debate over when our ancestors first harnessed fire-it may have been anywhere from 1.8 million to around 800,000 years ago. According to one theory, the invention of cooking allowed us to gain more energy from meat, which fueled the dramatic evolution of the human brain. Bigger brains and more dextrous hands in turn were the prerequisite for the developments that set humans apart, including complex language, art, and agriculture, all of which emerged in the past 100,000 years.

Where did all this happen, and why does it matter where?

Both genetic and fossil evidence show that until relatively recently, human evolution happened in Africa. Whether the genus Homo first emerged in southern or in eastern Africa remains unclear. Knowing where our species evolved matters because the environment it adapted to helped shape the genetic makeup we still carry with us today. Where we came from is the first chapter in the long story of how we got to where we are now.

Around 60,000 years ago-again according to both genetic and fossil evidence-modern humans migrated out of Africa and began colonizing the world. Genetic evidence suggests that soon after leaving Africa, they interbred to some extent with the Neanderthals and a mysterious population in Asia called the Denisovans. Homo sapiens is now the only species of human on Earth. But that's been true for less than 30,000 years.

Why haven't scientists found a "missing link" between apes and us?

Because there isn't one. Chimpanzees (or other apes) didn't evolve into humans. Both lineages descended from a common ancestor and went their separate ways. The real question here is, who was that last common ancestor, the missing progenitor of both chimps and humans? We don't know yet.

Is evolution over, for us or for the other apes?

Certainly not. Humans are continuing to evolve, but now the evolution is driven as much by our culture and the technology we've invented as by our biology. And other animals, including the apes, are also continuing to evolve-especially now, in response to the huge changes in their environment that humans have made.

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The origins of humankind have long fascinated scientists and the public alike-but the topic can get confusing. Here are answers to a few common questions. Why are scientists certain that human evolution happened? For a number of reasons. We share nearly 99 percent of our genetic sequence with chimpanzees and bonobos, which strongly suggests we share a common ancestor.

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Archaeological discoveries points out that mankind originated out of Africa.

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The question of where we humans come from is one many people ask, and the answer is getting more complicated as new evidence is emerging all the time.

For most of recorded history humankind has been placed on a metaphorical, and sometimes literal, pedestal. Sure, modern humans were flesh and blood like other animals.

But they were regarded as being so special that in the Linnaean taxonomy that prevailed well into the second half of the 20th century they were given their own family, the Hominidae.

Read more: Ancient DNA sheds light on the origin of Europeans

This distinguished them from the Pongidae, the separate family used for the three African great apes - the common chimpanzee, bonobo and gorilla - plus the orangutan from Southeast Asia.

We now realise that modern humans are just one of the African great apes.

So when and how did this radically changed perception come about?

Early observations

In the 19th century the only evidence available for determining the closeness of the relationship between any two living animals was how similar they were in terms of what the naked eye could tell from their bones, teeth, muscles and organs.

Wikimedia/Carte de Visite Fotografie

The first person to undertake a systematic comparative review of these differences between modern humans and the apes was English biologist Thomas Henry Huxley.

In the central section of a small book he published in 1863, called Evidence as to Man's Place in Nature, Huxley concluded that the differences between modern humans and African apes were less than those between African apes and orangutans.

This was the evidence the English naturalist Charles Darwin referred to in The Descent of Man in 1871.

He speculated that because African apes were morphologically closer to modern humans than the apes from Asia, then the ancestors of modern humans were more likely to be found in Africa than elsewhere.

A closer inspection

Developments in biochemistry and immunology during the first half of the 20th century enabled the search for evidence of the relationships between modern humans and the apes to shift from macroscopic morphology to the morphology of molecules.

The results of applying a new generation of analytical methods to proteins were reported by the Austrian-born French biologist Emile Zuckerkandl and American biologist Morris Goodman in the early 1960s.

Zuckerkandl used enzymes to break up the protein component of haemoglobin into its peptide components. He showed that the patterns of the peptides from modern humans, gorilla and chimpanzee were indistinguishable.

Goodman used a different method, immunodiffusion, to study albumin, a serum protein. He showed that the patterns produced by the albumins of modern humans and the chimpanzee were identical. He concluded that this was because the albumin molecules were, to all intents and purposes, identical.

Apes and humans: related

Proteins are made up of a string of amino acids and in many instances one amino acid can be substituted for another without changing the function of the protein.

In the late 1960s, the American anthropologist Vince Sarich and New Zealand biologist Allan Wilson exploited these minor differences in protein structure and concluded that modern humans and the African apes were very closely related.

They also provided the first molecular clock estimate of modern human-African ape divergence, dating the split to only around five million years ago. This date was less than half of contemporary estimates based on fossil evidence.

In 1975 the American human geneticist Mary-Claire King and Allan Wilson showed that 99% of the amino-acid sequences of chimpanzee and modern human blood proteins were identical.

Enter DNA

The discovery by James Watson and Francis Crick, with unwitting help from Rosalind Franklin, of the basic structure of DNA, and the subsequent discovery by Crick and others of the nature of the genetic code, meant that the relationships among organisms could be pursued at the level of the genome.

Nowadays technological advances mean that whole genomes can be sequenced. Over the past decade researchers have published good draft sequences of the nuclear genomes of the chimpanzee, orangutan, gorilla and the bonobo.

Shutterstock/Sergey Uryadnikov/Petr Masek/Sergey Uryadnikov/Eric Gevaert

More and better data are steadily being accumulated, and in 2013 a review of ape DNA based on the genomes of 79 great apes was published.

These new ape genome sequences support the results of earlier analyses of both nuclear and mitochondrial DNA that suggested modern humans and chimpanzees are more closely related to each other than either is to the gorilla.

When DNA differences among modern humans and the great apes are calibrated using the best palaeontological evidence for the split between the apes and the old world monkeys, those differences predict that the hypothetical common ancestor of modern humans, chimpanzees and bonobos lived about 8 million years ago.

The rise of the hominins

Most researchers now recognise the modern human as hominins.

Still, the question "where do we come from" can from a scientific perspective be difficult for someone outside of the discipline to come to grips with. In part this is because the fossil record for human evolution seems to grow exponentially, with the author of each new discovery often claiming that the textbooks need to be rewritten.

The interdisciplinary nature of palaeoanthropology also means that new evidence that helps us make sense of our ancestry does not always come in the form of new fossils.

It comes from advances in a range of disciplines that include archaeology, comparative anatomy, earth sciences, evolutionary biology, genomics and primatology.

A further complicating factor is that the human fossil record does not just consist of the fossil evidence of our direct ancestors.

Many of the fossils belong to lineages that do not make it to the surface of the Tree of Life. They belong to extinct close relatives, and the task of sorting close relatives from ancestors is one with which we are only just now beginning to grapple.

There is a lineage that leads to today's Homo sapiens, but there are also a host of side experiments that are equally important to understand. They represent some of the most interesting chapters in human evolution.

Origins of the genus Homo

Understanding the origins of our own genus Homo means establishing what fossils we recognise as being the first early humans.

Sometime before 4 million years ago we see the first evidence of the genus Australopithecus. These fossils sample the kind of creature that was most likely the ancestor to the genus Homo.

Around 2.5 million years ago we see the first fossil evidence of species in Africa that many argue belong to our own lineage. One of these, Homo habilis, almost certainly made stone tools, had a slightly larger brain than Australopithecus, stood upright and regularly walked on two legs.

Some recognise a second species, Homo rudolfensis, about which we know even less.

These possible human ancestors lived alongside close relatives that were almost certainly not our ancestors. These species are called Paranthropus or robust australopiths - they had small brains, big jaw bones, large flat faces, and huge chewing teeth.

They lasted for at least a million years, so whatever they were eating (which is still a mystery) they were successful in the sense that they lasted as long in the fossil record as the average mammal.

But some researchers think that Homo habilis and Homo rudolfensis are not different enough from the australopiths that preceded them to justify being included in the genus Homo.

They claim that the size and shape of their body and the size of their teeth and jaws was little different from that of the australopiths. This means that their locomotion and diet had not shifted far enough in the direction of pre-modern Homo species such as Homo erectus to justify inclusion in Homo.

Tool making is not enough

Also, because it is becoming evident that australopiths may have been making tools earlier than Homo habilis it means that tool-making can no longer be seen as the sole prerogative of Homo.

There is a developing consensus that the relaxation of the criteria more than 50 years ago that saw the inclusion of Homo habilis into the genus Homo needs to be reconsidered.

Species that emerge slightly later from Africa, such as Homo ergaster, fit much more clearly within what we understand by the genus Homo. That species probably left Africa around 2 million years ago and migrated ultimately as far east as China and Indonesia where it evolved, eventually, into Homo erectus.

A number of further migrations out of Africa probably occurred after the initial Homo ergaster migration, one of which, Homo heidelbergensis, is considered by many palaeoanthropologists to be the ancestor of both Neanderthals (Homo neanderthalensis) and modern humans (Homo sapiens).

As far as we know, Neanderthals evolved outside of Africa, perhaps in response to the ice ages of Europe. Our ancestors remained in Africa where perhaps as early as 300,000 years ago, as revealed from recent redating of the Moroccan site of Jebel Irhoud, were well along in the process of evolving into modern humans.

So the origins of 'us'

Once we get to the origins of our own species Homo sapiens we have the added advantage that we are able to now use next generation sequencing methods to recover ancient DNA (aDNA).

As geneticists recover ancient genomes from different extinct hominin species, they are generating insights that are not possible from comparing the anatomy of the fossils alone.

There is now fossil evidence from teeth to suggest that Homo sapiens may have been in China by 120,000 years ago and in South East Asia by 67,000 years.

The discovery of some distinctive modern human DNA within the DNA recovered from a Neanderthal fossil suggests that modest interbreeding was occurring between Neanderthals and modern humans in Central Asia by 100,000 years ago.

Modern humans have not shared the planet with another hominin species for several tens of thousands of years. But before that, in the past 300,000 years or so, there is fossil and DNA evidence of several hominin species, including the recently reported archaic hominin Homo naledi

First and foremost there was Homo neanderthalensis, whose range overlapped with modern humans in the Near East. Neanderthals most likely became extinct as a result of direct competition with the more technologically sophisticated Homo sapiens.

The evidence from DNA shows that there was interbreeding between our species and pre-modern humans, including the Neanderthals and the other enigmatic hominin referred to as the Denisovans.

We do not yet know how and when Homo erectus became extinct. It would appear that another unexpected side experiment in hominin evolution, known from the island of Flores and called Homo floresiensis most likely became extinct sometime after 60,000 years ago.

Indeed this hominin may represent something far more significant than simply an interesting side experiment, with many leading palaeoanthropologists arguing that the Hobbit may represent a pre-ergaster migration out of Africa.

Read more: A 700,000-year-old fossil find shows the Hobbits' ancestors were even smaller

What next?

Even though thousands of hominin fossils have now been recovered and described there is still much work to be done.

Was there a hominin that successfully migrated out of Africa prior to Homo ergaster? Did most of human evolution occur in Africa? Did some important transitions occur outside of Africa?

When did Homo erectus become extinct, and was there genetic exchange between erectus, sapiens and perhaps other hominin species?

As is often the case in science, with the recovery of additional data, in this case fossils and DNA extracted from fossils, we generate more questions than answers.

But ultimately all of this new evidence will result in a far more sophisticated appreciation of not only our evolution, but also the evolution of our extinct fossil cousins.

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The origin of 'us': what we know so far about where we humans come from

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The question of where we humans come from is one many people ask, and the answer is getting more complicated as new evidence is emerging all the time. For most of recorded history humankind has been placed on a metaphorical, and sometimes literal, pedestal. Sure, modern humans were flesh and blood like other animals.