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Earth is the only planet we know of that can support life. This is an amazing fact, considering that it is made out of the same matter as other planets in our solar system, was formed at the same time and through the same processes as every other planet, and gets its energy from the sun.

To a universal traveler, Earth may seem to be a harmless little planet in the far reaches of one of billions of spiral galaxies in the universe. It has an average size star of average brightness and is joined by seven other planets - which support no known life forms - in its solar system. While this may be fitting for a passage from The Hitchhikers Guide to the Galaxy, by Douglas Adams, in the grand scheme of the universe, it would be a fairly accurate description.

However, Earth is a planet teeming with vitality and is home to billions of plants and animals that share a common evolutionary track. How and why did we get here? What processes had to take place for this to happen? And where do we go from here? The fact is, no one has been able to come close to knowing exactly what led to the origins of life, and we may never know. After 5 billion years of Earth's formation and evolution, the evidence may have been lost. But scientists have made significant progress in understanding what chemical processes that may have led to the origins of life.

There are many theories, but most have the same general perspective of how things came to be the way they are. Following is an account of life's beginnings based on some of the leading research and theories related to the subject, and of course, fossil records dating back as far as 3.5 billion years ago.

Earth's Beginnings

Earth began to form over 4.6 billion years ago from the same cloud of gas (mostly hydrogen and helium) and interstellar dust that formed our sun, the rest of the solar system and even our galaxy. In fact, Earth is still forming and cooling from the galactic implosion that created the other stars and planetary systems in our galaxy. This process began about 13.6 billion years ago when the Milky Way Galaxy began to form.

As our solar system began to come together, the sun formed within a cloud of dust and gas that continued to shrink in upon itself by its own gravitational forces. This caused it to undergo the fusion process and give off light, heat and other radiation. During this process, the remaining clouds of gas and dust that surrounded the sun began to form into smaller lumps called planetesimals, which eventually formed into the planets we know today.

The Earth went through a period of catastrophic and intense formation during its earliest beginnings 4.6-4.4 billion years ago. By 3.8 to 4.1 billion years ago, Earth had become a planet with an atmosphere (not like our atmosphere today) and an ocean. This period of Earth's formation is referred to as the pre-Cambrian Period. The pre-Cambrian is divided into three parts: the Hadean, Archean and Proterozoic Periods.

Pre-Cambrian Period

The Earth formed under so much heat and pressure that it formed as a molten planet. For nearly the first billion years of formation (4.5 to 3.8 billion years ago) - called the Hadean Period (or hellish period) - Earth was bombarded continuously by the remnants of the dust and debris - like asteroids, meteors and comets - until it formed into a solid sphere, pulled into orbit around the sun and began to cool down.

As Earth began to take solid form, it had no free oxygen in its atmosphere. It was so hot that the water droplets in its atmosphere could not settle to form surface water or ice. Its first atmosphere was also so poisonous, comprised of helium and hydrogen, that nothing would have been able to survive.

Earth's second atmosphere was formed mostly from the out gassing of such volatile compounds as water vapor, carbon monoxide, methane, ammonia, nitrogen, carbon dioxide, nitrogen, hydrochloric acid and sulfur produced by the constant volcanic eruptions that besieged the Earth. It had no free oxygen.

About 4.1 billion years ago, the Earth's surface - or crust - began to cool and stabilize, creating the solid surface with its rocky terrain. Clouds formed as the Earth began to cool, producing enormous volumes of rainwater that formed the oceans. For the next 1.3 billion years (3.8 to 2.5 billion years ago), the Archean Period, first life began to appear and the world's landmasses began to form. Earth's initial life forms were bacteria, which could survive in the highly toxic atmosphere that existed during this time.

Toward the end of the Archean Period and at the beginning of the Proterozoic Period, about 2.5 billion years ago, oxygen-forming photosynthesis began to occur. The first fossils were a type of blue-green algae that could photosynthesize.

Some of the most exciting events in Earth's history and life occurred during this time, which spanned about two billion years until about 550 million years ago. The continents began to form and stabilize, creating the supercontinent Rodinia about 1.2 billion years ago. Although Rodinia is composed of some of the same land fragments as the more popular supercontinent, Pangea, they are two different supercontinents. Pangea formed some 225 million years ago and would evolve into the seven continents we know today.

Free oxygen began to build up around the middle of the Proterozoic Period - around 1.8 billion years ago - and made way for the emergence of life as we know it today. This increased oxygen created conditions that would not allow most of the existing life to survive and thus made way for the more oxygen-dependent life forms.

By the end of the Proterozoic Period, Earth was well along in its evolutionary processes leading to our current period, the Holocene Period, or Anthropocene Period, also known as the Age of Man. Thus, about 525 million years ago, the Cambrian Period began. During this period, life "exploded," developing almost all of the major groups of plants and animals in a relatively short time. It ended with the massive extinction of most of the existing species about 500 million years ago, making room for the future appearance and evolution of new plant and animal species.

About 498 million years later - 2.2 million years ago - the first modern human species emerged.

(Compiled with the assistance of a broad range of science and research resources and review by Dr. Jack C. Hall, Director of Environmental Studies, UNC Wilmington)

Did You Know?

The first modern human being was called homo habilis, the first of the homo genus. This species developed stone tools for use in daily life. Homo habilis means "Handy Man." He existed from about 2.2 to 1.5 million years ago. There are earlier species related to modern man, called hominids. The images show the skull shape and probable appearance of homo habilis.

The Pre-Cambrian Period - accounts for about 90 percent of Earth's history. It lasted for about four billion years until about 550 million years ago.

About 70 percent of the world's land masses were created in the Archean Era, between 3.8 and 2.5 million years ago. Rodinia, widely recognized as the first supercontinent, formed during the Proterozoic Era, about 2.5 billion years ago.

It is believed that the oldest human family member was discovered in Ethiopia and lived 4.4 million years ago. It was named "Ardi," short for Ardipithecus ramidus
Other Sources of Information The Planets Evolution of Life Earth at a Glance... You Are Here!
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