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Africa, and in particular East Africa, plays a prominent role in human evolution (Foley, Martin, Mirazon Lahr & Stringer, 2016). Trauth, et al. (2010) proposed that rift lakes in East Africa amplified by climate change may have heavily influenced early human evolution. Discuss the role that the environment, such as climate change or tectonic processes, may have had in human evolution. Provide specific examples.

Trauth, M. H., Maslin, M. A., Deino, A. L., Junginger, A., Lesoloyia, M., Odada, E. O., Tiedemann, R. (2010). Human evolution in a variable environment: the amplifier lakes of Eastern Africa. Quaternary Science Reviews, 29(23), 2981–2988. https://doi.org/10.1016/j.quascirev.2010.07.007

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The environment has influenced many facets of human evolution and will continue to impact future human evolution. Specific examples of impactful environments include  climatic variability, the forest to grassland conversion, geographic isolation, and pathogens in environment. These have been attributed to human evolutionary phenomenon such as bipedalism, and brain development. How will the current changes in our environment due to human-induced climate change impact future humans?

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Foley, Martin, Lahr and Stringer (2016) briefly discuss four directions of research currently being used to study human evolution: Multi-proxy approaches, new technologies and analytical methods, the limits of concepts, and the nature of hominin variation. Choose one of these topics and describe them in substantial detail using outside sources.

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The direction that current research on human evolution is taking has a variety of common themes. With regards to the discussion from this week, multi-proxy approaches, new analytical methods, and new technology were at the forefront of dialogue. The main topics highlighted this week were the multi-proxy approach of combining cross disciplines of study (such as paleolimnology and climatology) with the study of human evolution, use of CT scans for new insights, and new analytic approaches to help gain a deeper understanding of human evolution.

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Computed tomography (CT) technology has been used for scientists to piece together ancient fossils with minimally invasive or destructive approaches.  Some of the benefits of CT is that is also gives a more detailed, three dimensional image of a given fossil, and allows for models of the fossil to be made (either physically or virtually) which provide more scientists access and ability to study the fossils without moving the specimens themselves.  While this was a common topic of interest throughout the discussion, it was also mentioned that positron emission tomography (PET) scans have also been used in similar ways with hominin fossils.

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Microbiome data can reveal much in the ways of cultural practices with diet and lifestyle throughout hominin evolution. It was mentioned that the study of microbiome evolution and human evolution is meeting together to see what the future of our microbiome might be, and where it came from. Here is a fun video to explain the importance and origins of our microbiome:

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The Invisible Universe Of The Human Microbiome

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The next time you look in a mirror, think about this: In many ways you're more microbe than human. There are 10 times more cells from microorganisms like bacteria and fungi in and on our bodies than there are human cells. But these tiny compatriots are invisible to the naked eye.

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Scientists who have studied fertility and mortality rates in relatively recent human evolution have observed a possible shift in behavior.  Where Homo sapiens originally had high fertility rates and high mortality rates, the shift has now occurred to low fertility and low mortality. Below is an image from Our World in Data, projecting world population by level of fertility over time to the year 2100.

Roser, M. (2017, December 2). Our World in Data: Fertility Rate. Retrieved December 5, 2018, from https://ourworldindata.org/fertility-rate

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Are humans still evolving? Discuss one of the following topics that provides evidence using outside sources:

How and why infectious diseases transform and transmit to humans

Examples of cultural evolution in the past, present, and future

Evolutionary developmental biology and future prospects for humans

Phenotypic plasticity

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Based on the examples reviewed in this discussion, there was an overall affirmation that we are still evolving. This included exploring the changes in infant size due to modern birth practices; delving into recent gene-editing tools that may change the future course of human evolutionary development; investigating sexual selection that have influenced physical characteristics among distinct human groups; analyzing human tolerance against chemical compounds and substances due to thousands of years of innovation in agriculture and cooking; examining disease-resistant adaptations in geographically isolated communities; and researching slow changes in human evolutionary development due to modern technology. Although these evolutionary trends were reviewed, argued conflicts and controversies associated with this prompt were also highlighted in this discussion (e.g. lack of biological change historically, evolutionary theory of social change, and difficulties in studying current trends).

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The climate has exhibited extreme natural shifts in climate many times during earth's history. For example, the Pleistocene period contained dramatic climate shifts that also happened to correspond to significant developments in human evolution.

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The Smithsonian Institution's Human Origins Program

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This article explores the hypothesis that key human adaptations evolved in response to environmental instability. This idea was developed during research conducted by Dr. Rick Potts of the Smithsonian's Human Origins Program. Natural selection was not always a matter of 'survival of the fittest' but also survival of those most adaptable to changing surroundings.

Climate Fluctuation

Paleoanthropologists - scientists who study human - have proposed a variety of ideas about how environmental conditions may have stimulated important developments in human origins. Diverse have emerged over the course of human evolution, and a suite of adaptations have accumulated over time, including upright walking, the capacity to make tools, enlargement of the brain, prolonged maturation, the emergence of complex mental and social behavior, and dependence on technology to alter the surroundings.

The period of human evolution has coincided with environmental change, including cooling, drying, and wider climate fluctuations over time. How did environmental change shape the evolution of new adaptations, the origin and extinction of early species, and the emergence of our species, Homo sapiens? ('Hominin' refers to any species closely related to humans - that is, on the human divide of the evolutionary tree since human and chimpanzee ancestors branched off from a common ancestor sometime between 6 and 8 million years ago.)

How do we know Earth's climate has changed? How quickly and how much has climate changed? One important line of evidence is the record of oxygen isotopes through time. This record of δ ¹⁸O, or oxygen stable isotopes, comes from measuring oxygen in the microscopic skeletons of foraminifera (forams, for short) that lived on the sea floor. This measure can be used as an indicator of changing temperature and glacial ice over time. There are two main trends: an overall decrease in temperature and a larger degree of climate fluctuation over time. The amount of variability in environmental conditions was greater in the later stages of human evolution than in the earlier stages.

Ten-million-year record of oxygen stable isotopes, measured in foraminifera recovered from deep-sea sediment cores, illustrates that global ocean temperature and glacial ice varied widely over the past 6 million years, the period of human evolution. The δ¹⁸O measurement is the ratio of the heavier ¹⁸O isotope and the lighter ¹⁶O, which is more easily evaporated from the ocean and sequestered in glacial ice on land. During the course of human evolution, the overall δ¹⁸O trend has been toward a cooler, glaciated world. However, the amplitude of oscillation also increased beginning around 6 Ma, and became even larger over the past 2.5 Ma. Evolution of the genus Homo and of the adaptations that typify H. sapiens were associated with the largest oscillations in global climate. Icons: (a) hominin origins, (b) habitual bipedality, (c) first stone toolmaking and eating meat/marrow from large animals, (d) onset of long-endurance mobility, (e) onset of rapid brain enlargement, (f) expansion of symbolic expression, innovation, and cultural diversity.
Organisms and Environmental Change

All organisms encounter some amount of environmental change. Some changes occur over a short time, and may be cyclical, such as daily or seasonal variations in the amount of temperature, light, and precipitation. On longer time scales, hominins experienced large-scale shifts in temperature and precipitation that, in turn, caused vast changes in vegetation - shifts from grasslands and shrub lands to woodlands and forests, and also from cold to warm climates. Hominin environments were also altered by tectonics - earthquakes and uplift, such as the rise in elevation of the Tibetan Plateau, which changed rainfall patterns in northern China and altered the topography of a wide region. Tectonic activity can change the location and size of lakes and rivers. Volcanic eruptions and forest fires also altered the availability of food, water, shelter, and other resources. Unlike seasonal or daily shifts, the effects of many of these changes lasted for many years, and were unexpected to hominins and other organisms, raising the level of instability and uncertainty in their survival conditions.

Many organisms have habitat preferences, such as particular types of vegetation (grassland versus forests), or preferred temperature and precipitation ranges. When there's a change in an animal's preferred habitat, they can either move and track their favored habitat or adapt by genetic change to the new habitat. Otherwise, they become extinct. Another possibility, though, is for the adaptability of a population to increase - that is, the potential to adjust to new and changing environments. The ability to adjust to a variety of different habitats and environments is a characteristic of humans.

Three possible outcomes of population evolution in environmental dynamics typical of the Plio-Pleistocene (left). The ability to move and track habitat change geographically (narrow lines) or to expand the degree of adaptive versatility is important for any lineage to persist. Extinction occurs if species populations have specific dietary/habitat adaptations (i.e., a narrow band of 'adaptive versatility'; highlighted bands) and cannot relocate to a favored habitat. In the hypothetical situation (right band) where adaptive versatility expands, migration and dispersal may occur independently of the timing and direction of environmental change. The evolution of adaptive versatility is the impetus behind the variability selection idea, which is explored later in this article.

Adaptation to Change

There are many ideas about the role of the environment in human evolution. Some views assume that certain adaptations, such as upright walking or tool-making, were associated with drier habitat and the spread of grasslands, an idea often known as the savanna hypothesis. According to this long-held view, many important human adaptations arose in the African savanna or were influenced by the environmental pressure of an expanding dry grassland.

If key human adaptations evolved in response to selection pressure by a specific environment, we would expect those adaptations to be especially suited to that habitat. Hominin fossils would be found in those environments and not present in diverse types of habitat.

The Variability Selection Hypothesis

A different hypothesis is that the key events in human evolution were shaped not by any single type of habitat (e.g., grassland) or environmental trend (e.g., drying) but rather by environmental instability. This idea, developed by Dr. Rick Potts of the Human Origins Program, is called variability selection. This hypothesis calls attention to the variability observed in all environmental records and to the fact that the genus Homo was not limited to a single type of environment. Over the course of human evolution, human ancestors increased their ability to cope with changing habitats rather than specializing on a single type of environment. How did hominins evolve the ability to respond to shifting surroundings and new environmental conditions?
Over time (from left to right), new adaptations may evolve during periods of (A) relatively stable environment; (B) directional or progressive change, such as from wet to dry; or (C) highly variable habitat, as predicted by the variability selection hypothesis.
One way organisms can cope with environmental fluctuation is through genetic adaptation, where several alleles, or different versions of genes, are present in the population at different frequencies. As conditions change, natural selection favors one allele or genetic variant over another. Genes that can facilitate a range of different forms under different environments (phenotypic plasticity) can also help an organism adapt to changing conditions.

Another response to environmental change is to evolve structures and behaviors that can be used to cope with different environments. The selection of these structures and behaviors as a result of environmental instability is known as variability selection. This hypothesis differs from those based on consistent environmental trends. Environmental change in an overall direction leads to specializations for those specific conditions. But if the environment becomes highly variable, specializations for particular environments would be less advantageous than structures and behaviors that enable coping with changing and unpredictable conditions. Variability selection refers to the benefits conferred by variations in behavior that help organisms survive change. To test the variability selection hypothesis, and to compare it with habitat-specific hypotheses, Potts examined the hominin fossil record and the records of environmental change during the time of human evolution.

If environmental instability was the key factor favoring human adaptations, new adaptations would be expected to occur during periods of increased environmental variability, and these adaptations would have improved the ability of early human ancestors to deal with habitat change and environmental diversity.

Overall, the hominin fossil record and the environmental record show that hominins evolved during an environmentally variable time. Higher variability occurred as changes in seasonality produced large-scale environmental fluctuations over periods that often lasted tens of thousands of years. The variability selection hypothesis implies that human traits evolved over time because they enabled human ancestors to adjust to environmental uncertainty and change. The hypothesis addresses the matter of how, exactly, adaptability can evolve over time.

Ancient Hominins Were Found in Diverse Habitats

Ancient hominin remains have been found in a variety of different habitats. While some hominins, such as Orrorin tugenensis and Ardipithecus ramidus have been found in wooded habitats, others such as Sahelanthropus tchadensis were found associated with diverse types of vegetation within a small geographic area. Reconstructions of the ancient habitat of Ardipithecus ramidus at two different Ethiopian sites suggest that this species occupied both wooded areas (the Aramis site) and wooded grasslands in which grazing animals predominated (the Gona site). Australopithecus anamensis has been found at Kanapoi and Allia Bay, Kenya, in association with another type of mosaic - an open savanna with low trees and shrubs, but with both grasslands and gallery forests nearby.

At Kanapoi, research by Dr. Jonathan Wynn on paleosols and pedogenic carbonates demonstrates the presence of these varied habitats at the time when Australopithecus anamensis inhabited the area. Other members of Au. anamensis at Allia Bay encountered a different environment. The fossil animals represent several different habitats including open floodplains, gallery forests, and dry bushlands. Isotopic studies done by Dr. Margaret Schoeninger and her colleagues indicate that most of the Allia Bay vegetation consisted of woody plants such as trees and shrubs (known as C ₃ vegetation). Australopithecus anamensis at Allia Bay was thus associated with a mosaic environment, including woodlands near the ancestral Omo River and open savanna further away.
Two different types of environment - dense woodlands and open bushland - occurred in the same areas of East Africa during the period of human evolution. Climate fluctuation altered the proportion of these habitats, and thus led to repeated changes only in population density and variable conditions of natural selection.
Two Legs, Long Arms; Moving Around in Diverse Habitats

By about 4 million years ago, the genus Australopithecus had evolved a skeletal form that enabled adjustment to changes in moisture and vegetation. The best current example of adaptability in Australopithecus is apparent in the skeleton known as Lucy, which represents Au. afarensis. Lucy's 3.18-million-year-old skeleton has a humanlike hip bone and knee joints coupled with long apelike arms, longer grasping fingers than in humans, and flexible feet for walking or climbing. This combination of features, which appears to have characterized Australopithecus for nearly 2 million years and possibly older hominins, afforded an ability to move around in diverse habitats by changing the degree of reliance on terrestrial walking and arboreal climbing. This flexibility may also have characterized earlier hominins such as Ardipithecus ramidus.

Stone Toolmaking: Gaining Access to Diverse Foods

The first known stone tools date to around 2.6 million years ago. Making and using stone tools also conferred versatility in how hominin toolmakers interacted with and adjusted to their surroundings.

Simple toolmaking by stone-on-stone fracturing of rock conferred a selective advantage in that these hominin toolmakers possessed sharp flakes for cutting and hammerstones that were useful in pounding and crushing foods. Basic stone tools thus greatly enhanced the functions of teeth in a way that allowed access to an enormous variety of foods. These foods included meat from large animals, which was sliced from carcasses using sharp edges of flakes. Bones were broken open using stones to access the marrow inside. Other tools could be used to grind plants or to sharpen sticks to dig for tubers. Tool use would have made it easier for hominins to obtain food from a variety of different sources. Tool use would have widened the diet of hominins. Meat, in particular, is a food that was obtainable in equivalent ways, with similar nutritional value, in virtually any type of habitat that early humans encountered.

Although making simple toolmaking may have developed originally in one type of environment, the carrying of stone tools over considerable distances - and becoming reliant on stone technology - may have arisen due to the benefits of altering the diet as environments changed. The oldest known stone technology - called Oldowan toolmaking - involved carrying rock over several kilometers and is found associated with a variety of ancient habitats. Redistributing stone and other resources, such as parts of animal carcasses, by transporting them may have helped hominins cope with variable habitats.

The Expanding World of Early Homo

As predicted by the variability selection hypothesis, hominins were not found solely in one kind of habitat, but rather in a variety. A major signal of the ability to tolerate different environments was the dispersal of the genus early Homo beyond Africa into Asian environments. After 1.9 million years ago, the genus Homo is found in a variety of locations in Asia, including some that are relatively far north.

Early evidence of the diversity of Homo erectus environments in Asia includes the following sites:

Dmanisi, Republic of Georgia, 1.85 to 1.78 Million years ago. This site has grasslands surrounded by mountains with forests. Hominins had access to lava as a raw material for tools.

Yuanmou, China, 1.7 Million years ago. This site, located near an ancient lake, had a mixture of habitats with grasslands, bushlands and forests.

Nihewan Basin, China, 1.66 Million years ago. The Nihewan sites were also near a lake. Hominin toolmakers experienced many changes in vegetation over time, with habitats ranging from forests to grasslands. This region may have been much more arid than others, and temperatures changed seasonally between warm and cold.

Java, 1.66 Ma: Hominins here encountered grasslands, rivers and marine coastal environments in a tropical latitude setting.

In these locations, hominin groups encountered distinctly different environments, different plants and animals and foods, and different climatic conditions - a very wide range of temperature and strong variations in aridity and monsoonal rains.

Hominins Persisted Through Environmental Change

Environmental instability may have been a factor not only in shaping adaptations but also in contributing to the extinction of some lineages. Environmental variability associated with the extinction of large mammal species has been proposed for the southern Kenya region. Sediments, stone artifacts, and animal faunal at the site of Olorgesailie span most of the past 1.2 million years. Numerous environmental shifts are recorded in the Olorgesailie deposits. The ancient lake level and its chemistry, for example, changed frequently, and sometimes the lake dried up, leaving small wetlands and streams as the main source of water in the basin. Volcanic eruptions also blanketed the landscape in ash, killing off grass and reshaping the properties of the ecosystem.
An example of a hillside of sediments in the Olorgesailie region. The hillside, which represents about 10,000 years of time with a volcanic ash at its base dated around 1 million years ago, shows evidence of strong environmental shifts. Inset: Layers of sediments show the fluctuation between dry and wet environments and a time when volcanic ash covered the ancient landscape.
Dr. Rick Potts studied the pattern of climatic turnover in the fauna and the occurrence of archeological sites at Olorgesailie and another site in southern Kenya, and found that several large mammal species that had previously dominated the fauna of this region went extinct between about 700,000 and 300,000 years ago, during a period of repeated environmental instability. These species were replaced by modern relatives, which tended to be smaller in body size and not as specialized in diet or habitat.

For example, the zebra Equus oldowayensis had large and tall teeth specialized for eating grass. Its last known appearance in the fossil record of southern Kenya is between 780,000 and 600,000 years ago; it was replaced by Equus grevyi, which can graze (feed on grass) as well as browse (feed on leaves and other high-growing vegetation). The fossil baboon Theropithecus oswaldi, which weighed over 58 kg (over 127.6 pounds), lived on the ground exclusively; it had very large teeth and consumed grass. It also went extinct between 780,000 and 600,000 years ago. Its extant relative, Papio anubis, is omnivorous and moves easily on the ground and in trees. Two other large-bodied animals that specialized in eating grass, the elephant Elephas recki and the ancient pig Metridiochoerus, were also replaced by related species that were smaller and had more versatile diets ( Loxodonta africana and Phacochoerus aethiopicus). The aquatic specialist Hippopotamus gorgops was replaced by the living hippopotamus, which is capable of traversing long distances between water bodies.

The replacement of the specialized species by closely related animals that possessed more flexible adaptations during a time of wide fluctuation in climate was a key piece of initial evidence that led to the variability selection hypothesis. Although Acheulean toolmaking hominins were able to cope with changing habitats throughout much of the Olorgesailie record, the Acheulean way of life disappeared from the region sometime between 500,000 and 300,000 years ago, perhaps also a casualty of strong environmental uncertainty and changing circumstances.

Encephalization and Adaptability
Brain enlargement during human evolution has been dramatic. During the first four million years of human evolution, brain size increased very slowly. Encephalization, or the evolutionary enlargement of the brain relative to body size, was especially pronounced over the past 800,000 years, coinciding with the period of strongest climate fluctuation worldwide. Larger brains allowed hominins to process and store information, to plan ahead, and to solve abstract problems. A large brain able to produce versatile solutions to new and diverse survival challenges was, according to the variability selection hypothesis, favored with an increase in the range of environments hominins confronted over time and space.
New Tools for Many Different Purposes

After 400,000 years ago, hominins found new ways of coping with the environment by creating a variety of different tools. In some parts of Africa, a shift occurred in which a technology dominated by large cutting tools was replaced by smaller, more diverse toolkits. Technological innovations began to appear in the Middle Stone Age in Africa, with some early examples dating prior to 280,000 years ago. Some of the new tools provided ways for hominins to access food in new ways. Points were hafted, or attached to handles such as spear or arrow shafts, and were later used as part of projectile weapons, which allowed hominins to hunt fast and dangerous prey without approaching as closely. Barbed points were used to spear fish. Barbed points made from bone were found at the site of Katanda, in the Democratic Republic of the Congo, along with the remains of huge catfish. Grindstones were used to process plant foods. Other tools were used to make clothing which would have been important for hominins in cold environments.

Regional Exchange and Social Networks

Over the past 300,000 years or so, the direct ancestors of living humans developed the capacity to create new and diverse tools. Archeological discoveries show that wider social networks began to arise, enabling the transfer of stone material over long distances. Symbolic artifacts connoting complex language and the ability to plan are also evident in the archeological record of the Middle Stone Age of Africa. These findings indicate an improved capacity to adjust to new environments. Most of the past 350,000 years in East Africa was a time of strong climate oscillation. The timeline at the bottom of the image is 280,000 to 40,000 years ago (right to left). This figure is based on an analysis by archeologists Sally McBrearty and Alison Brooks.
Trading between groups to obtain materials and to cement alliances is a hallmark of modern human behavior. Larger brains and symbolic ability facilitated more complex social interactions. By 130,000 years ago, hominins were exchanging materials over distances of over 300 km. The social bonds that were forged by exchanging materials between groups may have been critical for survival during times of environmental change when one group relied on the resources or territories of a distant group. Modern foragers use social ties to mitigate the effects of famines and droughts. The exchange of gifts maintains relationships between groups, which may be called upon when one group needs to live at another's camp or waterhole, a capacity that proved especially beneficial during times of environmental change and resource uncertainty.

Communication and Symbols

Engraved ocher plaque from Blombos Cave, Republic of South Africa; about 77,000-75,000 years old

Evidence of the human capacity for communication using symbols is apparent in the archeological record back to at least 250,000 years old, and probably older. The use of color, incised symbols, decorative objects, and language are part of this capacity for communication. Symbolic communication may be linked with information storage. Language is an essential part of modern human communication. Language makes it possible to convey complex ideas to others. Communication of ideas and circumstances via language would have made survival in a changing world much easier. However, there is no fossil evidence for words and grammar that are the direct hallmarks of human language.

Preserved pieces of pigment are one of the earliest forms of symbolic communication. Ocher and manganese can be used to color objects and skin. Other symbolic objects such as jewelry, personal adornments, and art convey information about the owner's social status, group membership, age or sex. Paintings and drawings were also used to represent the natural world. Use of symbols is ultimately connected to the human ability to plan, record information, and imagine.

Neanderthals Endured Climatic Oscillations, Too!

Neanderthal populations ( Homo neanderthalensis) in Europe endured many environmental changes, including large shifts in climate between glacial and interglacial conditions, while living in a habitat that was colder overall than settings where most other hominin species lived. Some of the environmental shifts they endured involved rapid swings between cold and warm climate.

The Neanderthals were able to adjust their behavior to fit the circumstances. During cold, glacial periods, they focused on hunting reindeer, which are cold-adapted animals. During warmer, interglacial periods, they hunted red deer. During extreme cold periods, they shifted their range southwards toward warmer environments.

Neanderthals and modern humans had different ways of dealing with environmental fluctuation and the survival challenges it posed. Modern humans, Homo sapiens, had specialized tools to extract a variety of dietary resources. They also had broad social networks as shown by the exchange of goods over a long distance. They used symbols as a means of communicating and storing information. Neanderthals did not make tools that were as specialized as those of modern humans who moved from Africa into Europe sometime around 46,000 years ago. The Neanderthals usually did not exchange materials over so wide a distance as Homo sapiens. They occasionally produced symbolic artifacts. Despite many climatic fluctuations, modern humans were able to expand their range over Europe and Asia, and into new areas such as Australia and the Americas. Neanderthals went extinct. This evidence suggests that adaptability to varying environments was one of the key differences between these two evolutionary cousins.

During the time when Neanderthals evolved in Europe, global climate fluctuated dramatically between warm and cold. The highlighted area on the right side of the graph represents the last 200,000 years.
Conculsions

Overall, the evidence shows that hominins were able to adapt to changing environments to different degrees. The genus Homo, to which our species belongs, had the capacity to adjust to a variety of environmental conditions, and Homo sapiens is especially able to cope with a broad range of climatic conditions, hot and cold environments, arid and moist ones, and with all kinds of varying vegetation. We use resources from a vast variety of plants and animals and use many specialized tools. We have many social contacts and means of exchanging resources and information to help us survive in a constantly changing world.
The idea that the major adaptations in our evolutionary history arose in response to environmental variability and shifting selection pressures (variability selection) leads to a new understanding of human evolution. The figure above illustrates how the emergence of human characteristics from 6 million years ago to present conferred benefits that improved the ability of our ancestors to survive unpredictable and novel environments. Ma = million years ago; ka = thousand year ago.
Humans today represent the one species that has survived from the diversity of hominin species. Despite their very close relationship with our species, and despite the fact that all of them possessed some combination of features that characterize humans today, these earlier species and their ways of life are now extinct. The question ahead is how well our sources of resilience as a species will succeed as our alterations of the landscape, atmosphere, and water interact with the tendency of Earth's environment to shift all on its own. This is an 'experiment' just now unfolding, one that has never occurred before. The intensity of environmental change seems likely to create entirely new survival challenges for the lone hominin species on the planet, and many other organisms as well.

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This article explores the hypothesis that key human adaptations evolved in response to environmental instability. This idea was developed during research conducted by Dr. Rick Potts of the Smithsonian's Human Origins Program. Natural selection was not always a matter of 'survival of the fittest' but also survival of those most adaptable to changing surroundings.

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Pollen markers indicate a shift from forests to grasslands around 15 million years ago, perhaps after a period of global cooling. Grasslands presented new challenges to humans, including the need for faster movement and collaborative efforts to avoid predators.

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Two of the earliest rodents, including the earliest chinchilla, have been discovered in Chile.

The finds add weight to the suggestion that grasslands appeared in South America 15 million years before anywhere else on Earth, after a period of global cooling.

The rodents have been described from two fragmentary fossils of lower jaws.

Both lived near a volcanic chain 32.5 million years ago that was buried in volcanic debris.

The structure of the animals' cheek teeth have been interpreted as being adapted to a diet of plants found in open, dry environments.

The work appears in American Museum Novitates .

Chewing in Chile

"You don't always realise the importance of what you have when you find these fossils" Dr John Flynn of the American Museum of Natural History, who has for 26 years led expeditions in the Tinguiririca River Valley, told BBC news.

You don't always realise the importance of what you have when you find these fossils
Dr John Flynn, Frick Curator of Fossil Mammals, AMNH

Dr Flynn and his colleagues were surprised to find rodents, as out of the hundreds of specimens collected from their field site so far, these were the first.

"It's often not until we get them back to the lab, but if you split open a rock and you find a jaw there, you get the 'a-ha, eureka!' moment," he said.

However, what really got him and his co-authors excited was the presence of "hypsodont" teeth - a specific type of extremely tall cheek teeth where the enamel extends far down past the gum line.

The fossils were found in volcanic sediment, which is extremely hard to work with as it encases the fossils. But although this limited the range of analyses the team could perform, the presence of hypsodonty meant they could extrapolate the habitat in which it lived.

Hypsodonty is interpreted to be an adaptation to eating grassy plants and is found in modern day grazing animals. As grass contains silicious, glassy particles, the higher crowns enable tooth wear to be less severe. "It's basically an adaptation to eating food that functions like sandpaper".

Dr Flynn explained how the rodents had "convergently" evolved this tooth type at the same time as other animals during this period. "We have a whole series of extinct groups that become hypsodont at that time as well. We even see it in tiny marsupials," he explained.

Gnawing doubt

"Here's another case where we find its predecessors had low crowned teeth, but these animals have become high crowned, so it seems to be compelling evidence for an ecosystem change... We know today that hypsodonty is correlated primarily with grassland habitats, so it's a reasonable inference that it was grassland then," he said.

Other palaeontologists, however have remained sceptical about the claim.

"Palaeontologists have long been interpreting the vegetation context in which the oldest hypsodonts lived by means of indirect evidence," commented Dr Luis Palazzesi of the Museo Argentino de Ciencias Naturales, whose forthcoming research into pollen records of plants counters Dr Flynn's conclusions.

Whether the first hypsodonts were feeding on grasslands, or actually evolved within rainforests, remains to be debated by palaeobiologists.

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Two of the earliest rodents, including the earliest chinchilla, have been discovered in Chile. The finds add weight to the suggestion that grasslands appeared in South America 15 million years before anywhere else on Earth, after a period of global cooling. The rodents have been described from two fragmentary fossils of lower jaws.

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Mountain ranges, rift valleys and lakes in East Africa and refugiums during the Ice Ages may all have isolation early human populations, creating population bottlenecks that could lead to genetic change and speciation.

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A team of Genographic researchers and their collaborators have published the most extensive survey to date of African mitochondrial DNA (mtDNA). Over 600 complete mtDNA genomes from indigenous populations across the continent were analyzed by the scientists, led by Doron Behar, Genographic Associate Researcher, based at Rambam Medical Center, Haifa, and Saharon Rosset of IBM T.J. Watson Research Center, NY and Tel Aviv University. Analyses of the extensive data presented in this study provide surprising insights into the early demographic history of human populations before they moved out of Africa, illustrating that these early human populations were small and isolated from each other for many tens of thousands of years.

MtDNA, inherited down the maternal line, was used to discover the age of the famous 'mitochondrial Eve' in 1987. This work has since been extended to show unequivocally that the most recent common female ancestor of everyone alive today was an African woman who lived in the past 200,000 years. Paleontology provides corroborating evidence that our species originated on this continent approximately 200,000 years ago.

The migrations after 60,000 years ago that led modern humans on their epic journeys to populate the world have been the primary focus of anthropological genetic research, but relatively little is known about the demographic history of our species over the previous 140,000 years in Africa. The current study returns the focus to Africa and in doing so refines our understanding of early modern Homo sapiens history.

Doron Behar, Rambam Medical Center, Haifa, said: "We see strong evidence of ancient population splits beginning as early as 150,000 years ago, probably giving rise to separate populations localized to Eastern and Southern Africa. It was only around 40,000 years ago that they became part of a single pan-African population, reunited after as much as 100,000 years apart."

Recent paleoclimatological data suggests that Eastern Africa went through a series of massive droughts between 135,000-90,000 years ago. It is possible that this climatological shift contributed to the population splits. What is surprising is the length of time the populations were separate - as much as half of our entire history as a species.

Saharon Rosset, IBM T.J. Watson Research Center, NY and Tel Aviv University, said: "The analysis of such a massive dataset presents statistical and computational challenges as well as great opportunities for discovery of the events that shaped our history and genetic landscape. For example, we can see evidence of a population expansion period starting around 70,000 years ago, perhaps leading to the out of Africa dispersal shortly afterward."

The timing of these events coincides with the onset of the Late Stone Age in Africa, a change in material culture that many archaeologists believe heralds the beginning of fully modern human behavior, including abstract thought and complex spoken language.

Previous studies have shown that while human populations had been quite small prior to the Late Stone Age, perhaps numbering fewer than 2,000 around 70,000 years ago, the expansion after this time led to the occupation of many previously uninhabited areas, including the world beyond Africa.

Dr. Spencer Wells, National Geographic Explorer-in-Residence and Director of the Genographic Project, said: "This new study released today illustrates the extraordinary power of genetics to reveal insights into some of the key events in our species' history. Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world. Truly an epic drama, written in our DNA."

Paleontologist Meave Leakey, Genographic Advisory Board member, National Geographic Explorer in Residence and Research Professor, Stony Brook University, added: "Who would have thought that as recently as 70,000 years ago, extremes of climate had reduced our population to such small numbers that we were on the very edge of extinction."

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origin of life; early humans; fossils; human evolution; anthropology; cultures; evolution; early climate

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Early Human Populations Evolved Separately For 100,000 Years

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Over 600 complete mtDNA genomes from indigenous populations across the continent were analyzed and the data provided surprising insights into the early demographic history of human populations before they moved out of Africa. The extensive data analysis revealed that early human populations were small and isolated from each other for many tens of thousands of years.

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The existence of particular pathogens in the environment may have influenced human evolution. For example, in tropical regions with high rates of malaria, the allele for sickle cell anemia has increased in frequency, leaving the population more protected from malaria.

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Survival: Evolutionary Forces in Humans and Pathogens | Pardis Sabeti

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The ability to walk on two feet provided several adaptive advantages to early humans, including the ability to see predators and move more quickly.

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Welcome to Hominid Hunting's new series "Becoming Human," which will periodically examine the evolution of the major traits and behaviors that define humans, such as big brains, language, technology and art. Today, we look at the most fundamental human characteristic: walking upright.
Walking upright on two legs is the trait that defines the hominid lineage: Bipedalism separated the first hominids from the rest of the four-legged apes. It took a while for anthropologists to realize this. At the turn of the 20th century, scientists thought that big brains made hominids unique. This was a reasonable conclusion since the only known hominid fossils were of brainy species-Neanderthals and Homo erectus.

That thinking began to change in the 1920s when anatomist Raymond Dart discovered the skull known as the Taung Child in South Africa. Taung Child had a small brain, and many researchers thought the approximately three-million-year-old Taung was merely an ape. But one feature stood out as being human-like. The foramen magnum, the hole through which the spinal cord leaves the head, was positioned further forward under the skull than an ape's, indicating that Taung held its head erect and therefore likely walked upright. In the 1930s and 1940s, further fossil discoveries of bipedal apes that predated Neanderthals and H. erectus(collectively called australopithecines) helped convince anthropologists that walking upright came before big brains in the evolution of humans. This was demonstrated most impressively in 1974 with the finding of Lucy, a nearly complete australopithecine skeleton. Although Lucy was small, she had the anatomy of a biped, including a broad pelvis and thigh bones that angled in toward the knees, which brings the feet in line with the body's center of gravity and creates stability while walking.

In more recent decades, anthropologists have determined that bipedalism has very ancient roots. In 2001, a group of French paleoanthropologists unearthed the seven-million-year-old Sahelanthropus tchadensis in Chad. Known only from a skull and teeth, Sahelanthropus' status as an upright walker is based solely on the placement of its foramen magnum, and many anthropologists remain skeptical about the species' form of locomotion. In 2000, paleoanthropologists working in Kenya found the teeth and two thigh bones of the six-million-year-old Orrorin tugenensis. The shape of the thigh bones confirms Orrorin was bipedal. The earliest hominid with the most extensive evidence for bipedalism is the 4.4-million-year-old Ardipithecus ramidus. In 2009, researchers announced the results of more than 15 years of analysis of the species and introduced the world to a nearly complete skeleton called Ardi.

Although the earliest hominids were capable of upright walking, they probably didn't get around exactly as we do today. They retained primitive features-such as long, curved fingers and toes as well as longer arms and shorter legs-that indicate they spent time in trees. It's not until the emergence of H. erectus 1.89 million years ago that hominids grew tall, evolved long legs and became completely terrestrial creatures.

While the timeline of the evolution of upright walking is well understood, why hominids took their first bipedal steps is not. In 1871, Charles Darwin offered an explanation in his book The Descent of Man: Hominids needed to walk on two legs to free up their hands. He wrote that "...the hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with a true aim, as long as they were habitually used for locomotion." One problem with this idea is that the earliest stone tools don't show up in the archaeological record until roughly 2.5 million years ago, about 4.5 million years after bipedalism's origin.

But after the unveiling of Ardi in 2009, anthropologist C. Owen Lovejoy of Kent State University revived Darwin's explanation by tying bipedalism to the origin of monogamy. I wrote about Lovejoy's hypothesis for EARTH magazine in 2010. Lovejoy begins by noting that Ardi's discoverers say the species lived in a forest. As climatic changes made African forests more seasonal and variable environments, it would have become harder and more time-consuming for individuals to find food. This would have been especially difficult for females raising offspring. At this point, Lovejoy suggests, a mutually beneficial arrangement evolved: Males gathered food for females and their young and in return females mated exclusively with their providers. To be successful providers, males needed their arms and hands free to carry food, and thus bipedalism evolved. This scenario, as with all bipedalism hypotheses, is really hard to test. But earlier this year, researchers offered some support when they found that chimpanzees tend to walk bipedally when carrying rare or valuable foods.

Another theory considers the efficiency of upright walking. In the 1980s, Peter Rodman and Henry McHenry, both at the University of California, Davis, suggested that hominids evolved to walk upright in response to climate change. As forests shrank, hominid ancestors found themselves descending from the trees to walk across stretches of grassland that separated forest patches. The most energetically efficient way to walk on the ground was bipedally, Rodman and McHenry argued. (Full disclosure: Rodman was my graduate school advisor.) In 2007, researchers studying chimpanzees on treadmills determined that the chimps required 75 percent more energy while walking than two-legged humans, providing some evidence that bipedalism has advantages.

Numerous other explanations for bipedalism have been outright rejected, such as the idea that our ancestors needed to stand up to see over tall grass or to minimize the amount of the body exposed to the sun in a treeless savannah. Both ideas were debunked by the fact that the first hominids lived in at least partially wooded habitats.

Although difficult to study, the question of why bipedalism evolved might come closer to an answer if paleoanthropologists dig up more fossils of the earliest hominids that lived seven million to six million years ago. Who knows how many species of bipedal apes they'll find. But each new discovery has the potential to fundamentally change how we understand the origins of one of our most distinctive traits.

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Becoming Human: The Evolution of Walking Upright

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Welcome to Hominid Hunting's new series "Becoming Human," which will periodically examine the evolution of the major traits and behaviors that define humans, such as big brains, language, technology and art. Today, we look at the most fundamental human characteristic: walking upright.

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Significant brain development occurred during the Pleistocene period. The "social brain" that developed was adaptable, flexible and allowed for collaboration to better withstand climatic and other challenges. Additionally, brain plasticity may have enabled early humans to better survive in the environment. What does it mean that our brains are getting smaller?

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A recent study he conducted with a colleague, Drew Bailey, led Geary to this epiphany. The aim of their investigation was to explore how cranial size changed as our species adapted to an increasingly complex social environment between 1.9 million and 10,000 years ago. Since that period predates the first alphabets, the researchers had no written record with which to gauge the social milieu of our predecessors. Consequently, the Missouri team used population density as a proxy for social complexity, reasoning that when more people are concentrated in a geographic region, trade springs up between groups, there is greater division of labor, the gathering of food becomes more efficient, and interactions among individuals become richer and more varied.

Bailey and Geary found population density did indeed track closely with brain size, but in a surprising way. When population numbers were low, as was the case for most of our evolution, the cranium kept getting bigger. But as population went from sparse to dense in a given area, cranial size declined, highlighted by a sudden 3 to 4 percent drop in EQ starting around 15,000 to 10,000 years ago. "We saw that trend in Europe, China, Africa, Malaysia-everywhere we looked," Geary says.

The observation led the researchers to a radical conclusion: As complex societies emerged, the brain became smaller because people did not have to be as smart to stay alive. As Geary explains, individuals who would not have been able to survive by their wits alone could scrape by with the help of others-supported, as it were, by the first social safety nets.

Geary is not implying that our beetle-browed forebears would have towered over us intellectually. But if Cro-Magnons had been raised with techno-toys and the benefits of a modern education, he ventures, "I'm sure we would get good results. Don't forget, these guys were responsible for the 'cultural explosion'"-a revolution in thinking that led to such startling new forms of expression as cave paintings, specialized tools, and bones carved into the first flutes. In terms of raw innate smarts, he believes, they probably were as "bright as today's brightest" and might even have surpassed us.

Still, Geary hesitates to use words like genius or brilliant in describing them. "Practically speaking," he explains, "our ancestors were not our intellectual or creative equals because they lacked the same kind of cultural support. The rise of agriculture and modern cities based on economic specialization has allowed the very brightest people to focus their efforts in the sciences, the arts, and other fields. Their ancient counterparts didn't have that infrastructure to support them. It took all their efforts just to get through life."

SMALLER BUT SMARTER
When I follow up with Hawks, the anthropologist who first tipped me off about our missing gray matter, I assume that his interpretation of the trend will be like Geary's. But even though Hawks does not doubt the findings of the Missouri team, he puts a completely different (and, in his view, more uplifting) spin on the data.

Hawks spent last summer measuring skulls of Europeans dating from the Bronze Age, 4,000 years ago, to medieval times. Over that period the land became even more densely packed with people and, just as the Missouri team's model predicts, the brain shrank more quickly than did overall body size, causing EQ values to fall. In short, Hawks documented the same trend as Geary and Bailey did in their older sample of fossils; in fact, the pattern he detected is even more pronounced. "Since the Bronze Age, the brain shrank a lot more than you would expect based on the decrease in body size," Hawks reports. "For a brain as small as that found in the average European male today, the body would have to shrink to the size of a pygmy" to maintain proportional scaling.

Hawks chose to focus on Europe in the relatively recent past, he explains, because there is an exceptionally large number of complete remains from that era. That allowed him to reconstruct a detailed picture of what was happening during our downsizing. The process, he discovered, occurred in fits and starts. There were times when the brain stayed the same size and the body shrank-most notably, he says, from the Roman era until medieval times. But more frequently, the brain got smaller while the body remained the same. Indeed, Hawks says, that is the overarching trend for the thousands of years he studied.

The image of a brain dwarfed by its body conjures up dinosaurs, a group not exactly known for their intellectual prowess. But Hawks sees nothing alarming in the trend. Quite the contrary, he believes the startling decrease in our brain volume-both in absolute terms and relative to our stature-may be a sign that we are actually getting smarter.

As complex societies emerged, brains shrank because those previously unable to survive by wits alone could now scrape by with the help of others.

This upbeat perspective is shaped by Hawks's focus on the energy demands of the brain. The organ is such a glutton for fuel, he says, that it gobbles up 20 percent of all the calories we consume. "So although a bigger brain can presumably carry out more functions, it takes longer to develop and it uses more energy." Brain size probably depends on how those opposing forces play out.

The optimal solution to the problem, he suggests, "is a brain that yields the most intelligence for the least energy." For evolution to deliver up such a product, Hawks admits, would probably require several rare beneficial mutations-a seeming long shot. But a boom in the human population between 20,000 and 10,000 years ago greatly improved the odds of such a fortuitous development. He cites a central tenet of population genetics: The more individuals, the bigger the gene pool, and the greater the chance for an unusual advantageous mutation to happen. "Even Darwin knew this," he says. "That's why he recommended that animal breeders maintain large herds. You don't have to wait so long for desirable traits to arise."

Hawks notes that such changes would be consistent with the many brain-related DNA mutations seen over the past 20 millennia. He speculates that the organ's wiring pattern became more streamlined, the neurochemistry shifted, or perhaps both happened in tandem to boost our cognitive ability.

A TAMER BREED
Other researchers think many of their colleagues are barking up the wrong tree with their focus on intelligence as the key to the riddle of our disappearing gray matter. What may have caused the trend instead, they argue, is selection against aggression. In essence, we domesticated ourselves, according to Richard Wrangham, a primatologist at Harvard University and a leading proponent of this view.

Some 30 animals have been domesticated, he notes, and in the process every one of them has lost brain volume-typically a 10 to 15 percent reduction compared with their wild progenitors. Domesticated animals also have more gracile builds, smaller teeth, flatter faces, a more striking range of coloration and hair types-and, in many breeds, floppy ears and curly tails. Except for those last two traits, the domesticated breeds sound a lot like us.

"When you select against aggression, you get some surprising traits that come along with it," Wrangham says. "My suspicion is that the easiest way for natural selection to reduce aggressiveness is to favor those individuals whose brains develop relatively slowly in relation to their bodies." When fully grown, such an animal does not display as much aggression because it has a more juvenile brain, which tends to be less aggressive than that of an adult. "This is a very easy target for natural selection," Wrangham argues, because it probably does not depend on numerous mutations but rather on the tweaking of one or two regulatory genes that determine the timing of a whole cascade of developmental events. For that reason, he says, "it happens consistently." The result, he believes, is an adult possessing a suite of juvenile characteristics, including a very different temperament.

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If Modern Humans Are So Smart, Why Are Our Brains Shrinking?

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A recent study he conducted with a colleague, Drew Bailey, led Geary to this epiphany. The aim of their investigation was to explore how cranial size changed as our species adapted to an increasingly complex social environment between 1.9 million and 10,000 years ago.

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Human-Induced Climate Change and Future Humans

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Human-Induced Climate Change and Future Humans

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What does the future look like for humans given the significant, human-induced changes in climate that are occurring? Research is pointing to expanded ranges for disease, decreased air quality, more respiratory issues and other health implications. Additionally, climate change may have a role in increased geopolitical conflicts.

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As climate change brings rising temperatures, droughts, shifting patterns of precipitation and longer growing seasons, plants and animals are evolving to keep pace.

Biologists have observed squirrels and salmon developing at an accelerated pace, causing them to reproduce at a younger age. Earlier summers have caused some flowers to bloom earlier in the year. And corals are forging new relationships with microscopic algae to survive in warmer, more acidic seas.

As the planet continues to warm, evolutionary changes are expected in other species as well - including Homo sapiens. Climate change will alter the internal workings of our bodies in subtle but significant ways and will likely cause a noticeable shift in our appearance.

A warmer climate means malaria, West Nile virus and other diseases long confined primarily to the tropics will spread into temperate zones. As a result, people living in the U.S. and other developed nations will be exposed to these illnesses, and our immune systems will be forced to evolve new defenses. That, in turn, could cause other, noninfectious diseases.

Two blood disorders - sickle cell and thalassemia - arose and continue to exist because they have a beneficial side effect: resistance to malaria. Such disorders, or new ones, may soon appear if malaria moves into populated areas of North America, East Asia and Europe.

Similarly, our digestive systems will evolve in response to shifts in food availability - where crops and livestock can be cultivated. The ability to digest milk in adulthood evolved among groups in the Middle East and North Africa that began raising cattle. Future generations may evolve better abilities to tolerate sugar or fat.

Changing diets will also trigger changes in our microbiomes - the bacteria and other microorganisms that live in our guts and help to keep us healthy. Vegetarians tend to harbor a different mix of bacteria than meat eaters, and these changes could be exaggerated if prolonged droughts make it too costly to raise livestock for meat.

While these changes will be of enormous interest to biologists, they will be largely invisible. But as we change on the inside, we'll also be changing on the outside. Evidence suggests that a warming planet could melt away differences between human races - or population groups, as scientists more accurately call them.

The reason why climate change could reduce racial differences is that it will trigger massive migrations. In recent decades the world has become more urbanized, with people moving into large cities in coastal areas. But as polar ice melts and sea levels rise, large numbers of people will be forced to flee the coasts. And as droughts become more common and more severe, people living in more arid areas will have to move to places with more reliable sources of water.

These migrations will erode the geographic barriers that once separated human populations. In fact, this process is already underway. As of 2017, 258 million people were living in a country other than the one they were born in - an increase of 49 percent since 2000, according to a report from the United Nations. A World Bank report released in March predicts that climate change will cause 140 million people to migrate by 2050, with those now living in sub-Saharan Africa, South Asia and Latin America especially likely to migrate.

One consequence of large-scale migrations is what biologists call gene flow, a type of evolution caused by the blending of genes between populations. When people from different populations mate and reproduce, their genes intermingle in their children. That can lead to combinations of traits not seen in either parent or in the populations they come from - like the dark skin and blue eyes of Cape Verde islanders, the result of interbreeding between Portuguese and West Africans.

One of the most obvious effects of gene flow may be greater similarity in skin color.

Skin color differences came about a result of natural selection in different human populations. The pigment eumelanin makes skin darker, which helps protect against harsh sunlight. But too much eumelanin can make it hard for the body to produce vitamin D, which is needed to build healthy bones. So over many thousands of years, human populations evolved varying levels of skin pigmentation as they spread across the globe, with natural selection balancing the cost of having too much eumelanin (which can indirectly cause bone deformities) versus having too little (which can lead to cancer and birth defects).

As a result, skin color came to closely match the intensity of sunlight in different regions - darker near the equator and lighter near the poles.

But in today's world, with sunscreen and vitamin supplements, natural selection is less relevant to ongoing changes in human skin pigmentation than gene flow. Because skin color is controlled by many genes, parents whose skin color differs tend to have children with intermediate skin tones. And so in five to 10 generations (125 to 250 years), we may see fewer people with dark skin or pale skin and more with a brown or olive complexion. Having both dark skin and light eyes may become more common.

Blending of races is already well underway in ethnically diverse countries like Brazil, Singapore and the U.S. A Pew report from 2017 found that the number of multiracial births in the U.S. rose from 1 percent in 1970 to 10 percent in 2013. And the increase will continue - the multiracial population is projected to grow by 174 percent over the next four decades.

The bottom line? As people around the world become more physically similar to one another, it's possible that racism might slowly fade.
Scott Solomon teaches ecology, evolution and scientific communication at Rice University. He is the author of "Future Humans: Inside the Science of Our Continuing Evolution."
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Climate change could affect human evolution. Here's how.

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As climate change brings rising temperatures, droughts, shifting patterns of precipitation and longer growing seasons, plants and animals are evolving to keep pace. Biologists have observed squirrels and salmon developing at an accelerated pace, causing them to reproduce at a younger age. Earlier summers have caused some flowers to bloom earlier in the year.

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The environment is just one of many factors that influenced the progression of human evolution. Theories abound about what makes us human and how and when it happened.

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Philosophical Transactions of the Royal Society B: Biological Sciences

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Abstract

Current evidence suggests that many of the major events in hominin evolution occurred in East Africa. Hence, over the past two decades, there has been intensive work undertaken to understand African palaeoclimate and tectonics in order to put together a coherent picture of how the environment of Africa has varied over the past 10 Myr. A new consensus is emerging that suggests the unusual geology and climate of East Africa created a complex, environmentally very variable setting. This new understanding of East African climate has led to the pulsed climate variability hypothesis that suggests the long-term drying trend in East Africa was punctuated by episodes of short alternating periods of extreme humidity and aridity which may have driven hominin speciation, encephalization and dispersals out of Africa. This hypothesis is unique as it provides a conceptual framework within which other evolutionary theories can be examined: first, at macro-scale comparing phylogenetic gradualism and punctuated equilibrium; second, at a more focused level of human evolution comparing allopatric speciation, aridity hypothesis, turnover pulse hypothesis, variability selection hypothesis, Red Queen hypothesis and sympatric speciation based on sexual selection. It is proposed that each one of these mechanisms may have been acting on hominins during these short periods of climate variability, which then produce a range of different traits that led to the emergence of new species. In the case of Homo erectus ( sensu lato), it is not just brain size that changes but life history (shortened inter-birth intervals, delayed development), body size and dimorphism, shoulder morphology to allow thrown projectiles, adaptation to long-distance running, ecological flexibility and social behaviour. The future of evolutionary research should be to create evidence-based meta-narratives, which encompass multiple mechanisms that select for different traits leading ultimately to speciation.

1. Introduction

Human evolution is characterized by speciation, extinction and dispersal events that have been linked to both global and/or regional palaeoclimate records [ 1- 7]. Many theories have been proposed to link environmental changes to these human evolution events [ 8- 11]. This synthesis paper presents each of these theories in the context of the pulsed climate variability conceptual framework [ 11, 12], which has been developed from the latest tectonic and palaeoclimate data from East Africa. This greater understanding of the past climate of East Africa suggests that different hominin species or, at the very least, different emerging traits within a species could have evolved through various different mechanisms that are described by the turnover pulse hypothesis, aridity hypothesis, variability selection hypothesis, Red Queen hypothesis, allopatric or sympatric speciation.

2. Overview of human evolution

The recent expansion of the hominin fossil record has been dramatic, with 11 new species and four new genera named since 1987. This richer fossil record has provided two major improvements. First, this has led to a much greater understanding of the range of variation in the hominin phenotype, including in 'real' biological populations with evidence from Atapuerca, Dmanisi and Hadar. Second, extensive use of new dating techniques has provided chronological precision to link those phenotypes to the environments in which they evolved. However, the fossil record is still very limited with many gaps ( figure 1); the most significant for this study is the lack of cranial capacity data between 2 and 2.5 Ma [ 7]. There is also considerable discussion about defining the new species and genera [ 13, 14], which has an influence on understanding changes in overall hominin diversity. However, conflating or expanding the defined species has little overall influence on the diversity pattern, the pattern of species first appearance dates suggests contemporary speciation events [ 11]. First appearance dates are dependent on taphonomy and sampling biases; however, the consistency of hominin first appearance dates (FAD) in East Africa supports this region as the primary location of speciation events. The other key debate is where all the new hominin species evolved. The fossil record at the moment suggests that the majority of the new species evolved in East Africa and then dispersed outwards. This is supported by the current brain capacity evidence, which suggests brain expansion occurs first in East Africa and only appears elsewhere once there has been a dispersal event [ 15]. However, it should be noted that other authors suggest the possibility of South Africa, European and Asian origins for hominin speciation (e.g. [ 16, 17].

The fossil record suggests four main stages in hominin evolution: (i) the appearance of the earliest (proto) hominins attributed to the genera Sahelanthropus, Orrorin and Ardipithecus between 4 and 7 Ma, (ii) the appearance of the Australopithecus genus around 4 Ma and the appearance of the robust Paranthropus genus around 2.7 Ma, (iii) the appearance of the genus Homo around the Plio-Pleistocene boundary between 1.8 and 2.5 Ma, and (iv) the appearance of H. heidelbergensis at 800 ka and anatomically modern humans around 200 ka. The taxonomic classification of many specimens, as well as their role in the evolution of modern humans, is continually discussed (e.g. [ 13, 14]). What is not disputed is that, apart from Sahelanthropus remains from Chad, all the earliest specimens for each of the main genera were found in the East African Rift System [ 18].

The earliest disputed hominin is Sahelanthropus tchadensis, dated to approximately 7 Ma [ 19]. The remains are limited to cranial fragments that suggest a mosaic of hominin and non-hominin features and a brain size equivalent to modern chimpanzees [ 20]. The lack of post-cranial remains makes it extremely difficult to reconstruct its lifestyle and whether it was bipedal or whether it was truly a hominin. The next putative hominin is Orrorin tungenesis from Western Kenyan deposits aged around 6 Ma [ 21], but its taxonomic position, lifestyle and locomotion are all disputed owing to the fragmentary nature of the specimens. Both Sahelanthropus and Orrorin have been suggested to be members of a clade that includes Ardipithecus [ 20]. The oldest member of the Ardipithecus genus is A. kadabba, whose fossil evidence consists only of fragmentary teeth and skeletal remains dated to approximately 5.5 Ma [ 22]. A much more extensive fossil record exists for the second member of the genus, A. ramidus. Ardipithecus had brain and body sizes roughly equivalent to modern chimpanzees, their teeth indicate a highly omnivorous diet and their post-crania suggest a lifestyle of arboreality coupled with primitive bipedality [ 23]. The fauna and vegetation associated with the A. ramidus specimens in the Awash Valley, Ethiopia, dating to around 4.4 Ma suggest woodland-forest matrix habitats, associated with significant rainfall and water availability [ 23- 25]. This appearance of bipedality in closed woodland environments undermines theories of bipedality evolving exclusively as an adaptation to open habitats.

The first members of the Australopithecus genus, attributed to A. anamensis, appeared around 4 Ma [ 26]. These individuals show strong evidence of bipedality combined with primitive cranial features. They are followed by A. afarensis, which is very well known from the fossil record and includes the remarkably complete 'Lucy' specimen. Afarensis still retains a small brain size, yet the post-cranial morphology is more similar to modern humans than to apes and suggests a lifestyle strongly adapted to long-distance walking [ 27]. Australopithecus africanus, the first hominin found in South Africa, is similar to A. afarensis but with more ape-like limb proportions yet less primitive teeth [ 28]. The longer femur in A. afarensis as compared with A. africanus suggests longer strides and a more efficient walking style [ 28]. The final gracile australopith is A. garhi, associated with 2.5 Ma old deposits in the Awash Valley [ 29]. In a separate development, a group of hominins with robust dentition and jaw muscles appeared around 2.5 Ma. These hominins, generally attributed to the Paranthropus genus, include the East African P. aethiopicus (2.5 Ma) and P. boisei (2.3-1.2 Ma), and the Southern African P. robustus (1.8-1.2 Ma). These species have been attributed to more open habitats [ 25], though the evidence to support this inference has been questioned [ 30].

The earliest fossil evidence of Homo comes from 1.8 to 1.9 Myr old deposits in the East African Rift Valley. H. habilis had a gracile morphology similar to the australopithecines [ 18], and a brain size only slightly larger, leading to some arguing it should not be classified as Homo [ 31]. H. habilis was then followed by the appearance of H. erectus sensu lato, which is associated with sweeping changes in brain size, life history, and body size and shape. Post-cranially, H. erectus is very similar to anatomically modern humans. Inferences from fossil demography are that development slowed down, coupled with decreased inter-birth intervals. The final stages in the evolution of modern humans were the appearance of H. heidelbergensis around 800 ka and anatomically modern humans around 200 ka.

Arguably, the most important episode in hominin evolution occurred in East Africa around 1.8-1.9 Ma when hominin diversity reached its highest level with the appearance of the robust Paranthropus species, as well as the first specimens attributed to genus Homo ( sensu stricto). In addition to speciation, a second major process that began during this period was the episodic migration of hominins out of the Rift Valley and into Eurasia. This period also witnessed the most dramatic increases in hominin brain size; early representatives of the H. erectus sensu lato ( H. erectus and H. ergaster) in Africa had a brain that was more than 80% larger than the gracile australopithecine A. afarensis and approximately 40% larger than Homo (Australopithecus) habilis ( figure 1). By contrast, from the appearance of the early australopithecines until the appearance of the first member of the genus Homo, there was remarkably little change in hominin brain size.

The emergence of the H. erectus sensu lato in East Africa represents a fundamental turning point in hominin evolution. The dramatic increase in brain size was also accompanied by changes in life history (shortened inter-birth intervals, delayed development), pelvic morphology (see [ 32, 33] in this issue), body size and dimorphism, shoulder morphology allowing throwing of projectiles [ 34], adaptation to long-distance running [ 35], ecological flexibility [ 36] and social behaviour [ 37]. Some of these changes are consistent with a change in strategy towards flexibility and the ability to colonize novel environments. By contrast, the robust Australopithecus sp. adopted specialized habitat and dietary strategies [ 38, 39]. Thus, two strategies arose during this period, one of increased flexibility and one of increased specialization. With the appearance of H. erectus, brain size increased significantly and continued to increase over the following 500 kyr, followed by additional step increases between 0.8 and 1 Ma, at 200 ka, and finally again at 100 ka ([ 7], figure 5). These final stages of increased brain capacity were due to the appearance of H. heidelbergensis around 800 ka, H. denisovan around 600 ka, H. neanderthal around 300 ka and anatomically modern humans around 200 ka.

3. Theories of early human evolution

Environmental pressures have long been assumed to play a key role in hominin speciation and adaptation [ 40] and a number of iconic theories have been developed to frame and develop the discussion of hominin evolution. Table 1 tries to put these key theories into the context of overarching evolutionary theory. Although the split between phylogenetic gradualism and punctuated equilibrium is artificial, it does provide a starting point with which to discuss theories of early human evolution. In table 1, gradualism has been split into constant and variable evolution rates to reflect the full range of current opinions.

The first key environmental theory to explain bipedalism was the savannah hypothesis, which suggested that hominins were forced to descend from the trees and adapted to life on the savannah facilitated by walking erect on two feet. This theory was refined as the aridity hypothesis, which suggested that the long-term trend towards increased aridity and the expansion of the savannah was a major driver of hominin evolution [ 1, 38, 45]. A key addition to this theory was the suggestion that during periods when aridification accelerated, owing to thresholds in the global climate system, then thresholds in evolution were reached and major hominin speciation events occurred [ 1].

The turnover pulse hypothesis [ 42, 46- 48] was originally developed to explain discrete patterns in ungulate speciation, and suggests that acute climate shifts drove adaptation and speciation. Vrba [ 46] recognized that environmentally induced extinctions hurt specialist species more than generalist species. Hence, when there is an environmental disruption, the generalists will tend to thrive by using new environmental opportunities and by moving elsewhere to take advantage of other areas that have lost specialist species. The specialists will experience more extinctions, and therefore an increased speciation rate within their group. This would lead to more rapid evolution in isolated areas, i.e. allopatric speciation, whereas the generalists will become more spread out.

The variability selection hypothesis advocates the role of environmental unpredictability in selecting for behavioural or ecological flexibility [ 10, 43, 49- 51]. This theory develops the original turnover pulse hypothesis but instead splits species into their varying ability to adapt and evolve to a more variable and unpredictable environment. The variability selection hypothesis emphasizes the long-term trends toward a drier and more variable climate. It, however, struggles to explain the current palaeoanthropological evidence that suggests a pulsed/threshold nature of hominin speciation and migration events.

More recently, it has been suggested that periods of climate stability may be equally important in driving human evolution, dispersal and technological innovation (e.g. [ 15, 52, 53]). Relatively long periods of climate stability could invoke the Red Queen hypothesis or sympatric evolution owing to sexual selection. The Red Queen hypothesis suggests that continued adaptation is needed in order for a species to maintain its relative fitness among co-evolving systems [ 54] and that biotic interactions, rather than climate, are driving evolutionary forces. It is based on the Red Queen's race in Lewis Carroll's Through the Looking-Glass, when the Queen says; 'It takes all the running you can do, to keep in the same place' [ 44]. However, for this to occur, it is reasonable to assume that a relatively highly productive environment has to exist so that competition rather than resources is the dominant control. At Koobi Fora (Northern Kenya) there is evidence for multiple hominin species, including P. boisei, H. erectus spp., H. habilis and H. rudolfensis attributed to the period of maximal lake coverage (approx. 1.8-1.9 Ma), and hence the highest availability of resources; it might be postulated that these hominins evolved as a result of competition with each other and other animals.

It may also be possible that certain traits, such as a large brain, became a key characteristic in sexual selection that hence drove sympatric evolution. The social brain hypothesis [ 55, 56] suggests that enhanced cognitive ability would provide the ability to strongly influence groups or tribes of hominins and hence control the distribution of resources. It would also help social cohesion and thus the ability of individuals to ensure allomaternal care, reducing the effects of the obstetric dilemma (see [ 57] in this issue). The social brain hypothesis could apply equal well to periods of high or low resources as it can be seen as an internal arms race to develop great cognitive skills to enable greater social control. In additional to this is the expensive brain framework [ 56, 58] which tries to understand the advantages of having enhanced cognitive ability in terms of food production and sharing, predation reduction and allomaternal care compared with the negative impacts such as increase food requirement and increased infant and mother mortality ( figure 2). Both of these theories provide an essential link between the mechanisms driving evolution and the biological response.

Finally, a direct development of the variability selection hypothesis, which incorporates the latest palaeoenvironmental reconstructions and the role of both stability and instability is the pulsed climate variability framework, which highlights the role of short periods of extreme climate variability specific to East Africa in driving hominin evolution [ 12]. This framework is discussed in §4 along with how the other evolutionary theories may be applied given the new environmental context.

4. Pulsed climate variability conceptual framework

Over the past two decades, intense work on African palaeoclimate and tectonics has allowed us to begin to put together a coherent picture of how the environments of eastern and southern Africa have changed over the past 10 Myr. Tectonics has altered the landscape of East Africa dramatically over this period of time. It changed from a relatively flat, homogenous region covered with tropical mixed forest, to a heterogeneous region, with mountains over 4 km high and vegetation ranging from desert to cloud forest. Tectonic events such as these are associated with a variety of biotic changes. Over the Oligocene and Miocene progressive uplift of East Africa split the pan-Africa rainforest which joined the Congo with East Africa resulting in endemic species in East Africa emerging at 33, 16 and 8 Ma [ 59]. During the Plio-Pleistocene, there is evidence from soil carbonates [ 60- 63], marine sediment n-alkane carbon isotopes [ 64- 66] and fossilized mammal teeth [ 67, 68] that there was a progressive vegetation shift from C ₃ plants to C ₄ plants during the Pliocene and Pleistocene. This vegetation shift has been ascribed to increased aridity owing to the progressive rifting and tectonic uplift of East Africa [ 45]. The aridity trend is also supported by a number of climate model simulations [ 69- 71]. These studies demonstrate that as uplift increases, wind patterns become less zonal resulting in a decrease in regional rainfall. Hence as elevation increases, a rain shadow effect occurs that reduces moisture availability on the Rift Valley mountain side, producing the strong aridification trend evident in palaeoenvironmental records [ 69, 70].

In addition to contributing towards the aridification of East Africa, the tectonic activity also produced numerous basins suitable for lake formation [ 72]. The southward propagation of rifting, including the formation of faults and magmatic activity, is also reflected in the earliest formation of lake basins in the northern parts of the rift. For example, the Middle and Upper Miocene saw the beginning of lakes in the Afar, Omo-Turkana and Baringo-Bogoria Basins, but the oldest lacustrine sequences in the central and southern segments of the rift in Kenya and Tanzania are of Early Pliocene age [ 73, 74]. Palaeo-lakes in the northern part of the East African Rift Valley thus formed earlier than in the south. However, if tectonics were the sole control over the appearance and disappearance of lakes, then either a north-south or north-west-south-east temporal pattern would be expected. By contrast, what is observed is the synchronous appearance of large deep lakes across a large geographical area at specific times [ 2], suggesting a regional climatic control. Moreover, there is growing evidence for significant late Cenozoic lake periods between 4.6-4.4 Ma, 4.0-3.9 Ma, 3.6-3.3 Ma, 3.1-2.9 Ma, 2.7-2.5 Ma, 2.0-1.7 Ma, 1.1-0.9 Ma and 0.2-0 Ma before present in East Africa [ 2, 11, 74]. These occurrences correlate with the 400- and 800-kyr components of the eccentricity cycle, suggesting a major role in lake formation for extreme amplitude fluctuations in precession ( figure 4). During each of the lake phases there is evidence that the lakes appear and disappear rapidly in time with precessional forcing [ 1, 45, 75- 86]. Deino et al. [ 77] and Kingston et al. [ 78] have found that the major lacustrine episode of the Baringo Basin in the Central Kenyan Rift between 2.7 and 2.55 Ma actually consisted of five palaeo-lake phases separated by a precessional cyclicity of approximately 23 kyr, while Magill et al. [ 84] have found biomarker stable carbon isotope evidence in Olduvai lake sediment of precessional forced variations between open C ₄ grasslands and C ₃ forest between 1.8 and 1.9 Ma. There is also evidence for precessional forcing of the 1.9-1.7 Ma lake phase indentified in the KBS Member of the Koobi Fora Formation in the northeast Turkana Basin in Kenya [ 80, 87]. During the same period, an oxygen isotope record from the Buffalo Cave flowstone (Makapansgat Valley, Limpopo Province, South Africa) shows clear evidence of precessionally forced changes in rainfall [ 79]. The occurrences of these environmental changes are in phase with increased freshwater discharge and thus sapropel formation in the Mediterranean Sea [ 88- 90] and coincide with dust transport minima recorded in sediments from the Arabian Sea [ 1, 45, 91]. Hence, the lake records from East Africa and the Arabian Sea dust records document extreme climate variability with precessionally forced wet and dry phases.

In summary, the pulsed climate variability framework suggests there are periods of extreme climate variability every 400 or 800 kyr driven by the eccentricity maxima when lakes rapidly grow and fill much of the Rift Valley and then rapidly disappear. Wilson et al. [ 86], using evidence from Pliocene diatomite deposits in the Baringo Basin, suggest that the lakes appear rapidly, remain part of the landscape for thousands of years, then disappear in a highly variable and erratic way [ 77, 82]. In fact, the absence of shallow-water (littoral) diatom species at key Plio-Pleistocene lake deposits [ 78, 82, 86] suggests that the lakes appeared in only a few hundred years. Figure 3 shows a compilation of what a generic extreme wet-dry cycle may have looked like with a threshold at the beginning of the wet phase and a prolong highly variable period at the end of the wet phase. There would be four or five of these cycles during each of the periods of extreme climate variability. The different appearance and disappearance of the lakes is also consistent with the idea of a bifurcated relationship between climate and lake presence [ 92]. Figure 4 shows that precipitation needs to increase significantly before lake growth can initiate, but once it has there are some key feedbacks which accelerate the expansion of the lakes. The most important is the change in the local lapse rate owing to increased moisture in the atmosphere. Hence the increased local relative humidity reduces the evaporation-precipitation balance, increasing the moisture in the atmosphere. When lake become more established the increased moisture changes the vegetation and more bushes and trees appear which subsequently increase the evapo-transpiration, further increasing the moisture in the atmosphere. These same feedbacks also resist the drying out of the lake when precessionally driven rainfall starts to reduce. This leads to a period of up to 2 kyr when the lake expands and contracts finally before there is not enough moisture in the region to sustain any sort of lake. Recent evidence of this lake behaviour has been found using radiocarbon dating of the palaeo-Lake Suguta in Northern Kenya [ 93, 94].

It should be stressed, however, that the pulsed climate variability framework only applies up to about 800 ka. After this time, the Early-Middle Pleistocene transition (which was previously known as the Mid-Pleistocene Transition or Revolution [ 95]) occurs which marks a prolongation and intensification of glacial-interglacial climate cycles, which have an increasing influence on tropical climates. Hence post 800 ka, the climate of the tropics becomes more complicated and fragmented as it is influenced both by localized influences of orbital forcing as well as the evermore global influence of the glacial-interglacial cycles [ 11].

5. Human evolution theories within the new framework

Figures 5- 9 illustrate how the pulsed climate variability framework helps to conceptualize the different theories of early human evolution. Figure 5 shows how the turnover pulse hypothesis would operate through one of these extreme climate cycles. Vrba [ 46] suggested that environmental changes would affect specialist and generalist species differently. During dry periods, the extinction rates of generalist species would reduce as they would be better able to find resources, while specialist species would struggle having lost their environmental niche and their competitive advantage ( figure 5). Speciation would be much higher in the specialist species during dry periods as they try to adapt to the new habitats. By contrast, during the wet periods and to a lesser extent the high variable periods generalist species would suffer as specialists would have a lot more niches to fill and thus would outcompete the generalists. Figure 6 illustrates possible changes that could have occurred owing to the aridity hypothesis [ 1, 45], which suggest that speciation mainly occurs during periods of dryness with low resources. Figure 7 illustrates the variability selection hypothesis [ 10, 43, 50, 51] that develops the original turnover pulse hypothesis but instead splits species into their varying ability to adapt and evolve to a more variable and unpredictable environment. Hence, generalists undergo more extinction and specialists more speciation during the highly variable climate period in between the long wet and arid phases. Figure 8 illustrates the Red Queen hypothesis that suggests continued adaptation is needed in order to keep up with other species which are also evolving. Figure 8 assumes that a relatively high-energy environment would provide more resources and therefore more energy in the system to enable inter-species competition. The structure of figure 8 could also apply to sympatric evolution owing to sexual selection. Finally, figure 9 illustrates allopatric evolution that suggest geographically isolated populations can evolve independently. In the Rift Valley during the extreme dry periods north-south and east-west migration was very difficult so it would have created isolated populations. The same is true of extreme wet periods because when the lakes completely fill the rift basins north-south and east-west migration would be again difficult, creating isolated populations. Only during the highly variable period and the threshold change would it be possible easily to move up and down and across the Rift Valley. Recent evidence from Wilson et al. [ 86] suggests there were millennial-scale fluctuations in lake level during the extreme wet periods; hence movement between populations may have been possible during the wet phases, limiting the isolation.

Figures 5- 9 are just our interpretations of how these major theories of human evolution could be placed within the pulsed climate variability framework. We would encourage colleagues to use this more visual approach to provide their own interpretation of how changing environments would interact with different theories of early human evolution.

6. Conclusion

The pulsed climate variability framework therefore takes the latest palaeoclimate understanding of East Africa and provides a framework within which to understand the causes of early human evolution. Different species or, at the very least, different emerging traits within a species could have evolved through various mechanisms including the turnover pulse hypothesis, aridity hypothesis, variability selection hypothesis or allopatric speciation. This is exemplified by the case of H. erectus ( sensu lato) which exhibits changes in life history (shortened inter-birth intervals, delayed development), pelvic morphology, body size and dimorphism, a shoulder morphology that enables projectile use, adaptation to long-distance running, ecological flexibility, social behaviour which may have included cooking. Each one of these traits could have been forced by a different evolutionary mechanism operating at a different part of the environmental cycle.

Acknowledgements

We would like to thank Beth Christensen, Annett Junginger, Kit Opie, Robin Dunbar, Richard Leakey, Meave Leakey, Rob Foley, Marta Lahr, Mark Thomas and Mark Collard for their comments and support. We would also like to thank the reviewers whose detailed comments greatly improved this paper. We would like to thank the UCL Drawing Office (Department of Geography) for compiling the figures.

© 2015 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

View Abstract

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A synthesis of the theories and concepts of early human evolution

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Current evidence suggests that many of the major events in hominin evolution occurred in East Africa. Hence, over the past two decades, there has been intensive work undertaken to understand African palaeoclimate and tectonics in order to put together a coherent picture of how the environment of Africa has varied over the past 10 Myr.

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Discuss the differences between human-infectious diseases of temperate versus tropical origin and explain how understanding the origins of infectious diseases may aid in identification and treatment and prevention. (Wolfe, et al, 2007) Please cite outside references to substantiate your claim.

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Discussion from this prompt showed that with the same origin from the Old World, tropical diseases are more vector-born (flies, mosquitos, kissing beetle) to transmit from the host to the human. For example: dengue, malaria, Zika, sleeping sickness, river blindness. Temperate diseases are more acute and can be easily transmitted from human to human. For example: flu, SARS, HIV. More on differences between temperate and tropical diseases can be further found at: https://www.ncbi.nlm.nih.gov/books/NBK114494/ .

Understanding the origins of infectious diseases also means understanding about the vector, the host, transmission route, incubation and more about the diseases. With the impact of climate change and global warming, convenient of international travel and connected world, human infectious diseases are spreading easily through regions and continental. These require the development of nut only public health, veterinary medicine but environmental health (One Health approach) to detect, monitor, prevent, control and eliminate and protect human population from outbreaks.

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Throughout the discussion, participants brought up examples of how evolution and cultural shifts has happened in hominin history. The use of certain tools that have been uncovered globally during early Homo years, such as the Acheulean handaxe (pictured below). Another mentioned example is the advent of hunting and gathering directly following the development of grasslands in Africa.
Image below: Bostrom, 2008 Paleolithic era Achuelan handaxes