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
The
solar system was created from gas clouds and dust that remained from
the Sun's formation some 6-7 billion years ago. This material contained
only about .2% of the solar system's mass with the Sun holding the rest.
(NASA graphic, by N. A. Cabrol)
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.
A
large number of small objects, called planetesimals, began to form
around the Sun early in the formation of the solar system. These objects
were the building blocks for the planets that exist today. (NASA
graphic, by N. A. Cabrol.)
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.Jupiter Atmosphere
Earth's
early atmosphere most likely resembled that of Jupiter's atmosphere,
which contains hydrogen, helium, methane and ammonia, and is poisonous
to humans. (Photo: NASA, from Voyager 1)
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.
Earth's
atmosphere was first supplied by the gasses expelled from the massive
volcanic eruptions of the Hadean Era. These gases were so poisonous, and
the world was so hot, that nothing could survive. As the planet began
to cool, its surface solidified as a rocky terrain, much like Mars'
surface (center photo) and the oceans began to form as the water vapor
condensed into rain. First life came from the oceans. Source: NASA, NWS
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
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