General Astronomy/The Biosphere
The story of life on Earth is a fascinating one. Life is more interesting than almost anything else—certainly more interesting than simple rocks, water, and gases. Life is above all else a very complex set of chemistry known as biochemistry—the chemistry of life itself.
So far as we know the Earth is the only place in the universe with life. We may find other places in time, places with very different life. We may even create life. But life has to come from somewhere.
When it formed, the Earth had no life. It couldn't hold life; its surface was hot from either a glob of congealing, molten rock or from giant pieces of rock slamming together. It may have had no gases, and certainly no liquid water. Deadly ultraviolet radiation would have killed anything. But eventually as the rocks cooled, some of them released some of the chemicals of life. Most importantly, liquid water emerged from the rocks or fell to the early Earth and formed early lakes, seas, and oceans. Some of the water evaporated and formed clouds from which rain fell. Other substances emerged from the rocks, like rock phosphates and salts of magnesium, calcium, sodium, potassium, and iron. Some others may have been captured from space when the planets were young, such gases as carbon dioxide or carbon monxide, methane, ammonia, and hydrogen sulfide. Some of those chemicals are dangerous to us—like hydrogen sulfide, ammonia, methane, and carbon monoxide. Don't worry; we weren't around for them to hurt us. There was no oxygen; that would come into existence later.
A remarkable experiment in 1954 by the physicist Harold Urey and the chemist Stanley Miller showed how the early chemicals of life came into existence. They put small amounts of water, ammonia, and methane into chemical flasks and simulated lightning upon the chemicals. (Don't try this at home!) They got some chemicals known as amino acids necessary for proteins. Other people have done similar experiments with slight changes and gotten slightly different effects—but also the chemicals essential to life.
Sulfur was necessary for some vital proteins for all later life, and where there a little hydrogen sulfide, some of those proteins were possible. But proteins are not life themselves. Life cannot form until it has some unique substances called RNA and DNA that control the chemistry of life and define life itself. DNA and RNA require phosphates, compounds that contain phosphorus and oxygen. Life came into existence when it controlled its own chemistry, defined itself in cells, and reproduced.
We don't know the whole story of life. Much of it is genetics whose chemical basis is DNA or RNA—and every living thing has its own DNA or RNA (if not both). They are very complicated compounds with subtle effects. We don't know how cells came into existence—yet. We can't create life—but we know that it came into existence.
Life became increasingly complex. Some of it simply devoured the early material of life. Some devoured creatures that came into existence before it. Some (most of which would become plants) used sunlight to get energy to store in sugars. Plants would do a process called photosynthesis—taking carbon dioxide, water, and sunlight and turning them into sugar (their own food) and oxygen.
Plants released oxygen into the atmosphere, but only from water. The lands were barren of life of any kind for millions of years. Some animals began to eat plants, and some animals ate the animals that ate the plants. All animals would use oxygen. But oxygen would enter the atmosphere. Some of the oxygen would go to the top of the atmosphere and become ozone that would absorb deadly ultraviolet radiation. Then, and only then could animals and plants enter the land. By then the Earth was no longer young. But the chemistry of life was itself very old by then, and that chemistry has changed little to our time—but the great variation possible due to subtle differences DNA and RNA now makes possible the great variety of life—and even the uniqueness of you and me.