Scientist know, so we know, that there was liquid water early in earth's history, this is known by the presence of zircons formed in the presence of water from 4.3 billion years ago. They speculate that life could have first evolved during this time, but was repeatedly scoured away by large impacts. After the Hadean, large impacts became rare and it is theorized that our deep ancestors evolved then.
Life was very different then. The world was mostly ocean and the atmosphere was mostly nitrogen and carbon dioxide. Maybe methane too. Undoubtedly life was very slow at this point, DNA had not yet come about, RNA may have existed. Life would have been in the coolest phase of experimentation. And by that I mean something would happen and something else and in a unlikely context and boom it would sustain itself. The start of life. And then something else would happen and it would die. Or it would get a better form and it would survive. Life is funny like that.
The important thing to realize is that the important thing about early life is that there was not a lot going on in the world. There were volcanoes, there was weather, there was plate-tectonics and the was time. Let's look at plate-tectonics. A plate may move a centimeter a year, many move faster but let us assume only a centimeter. It would take one hundred years to move a meter, it would take 100,000 years to move a km and it would take 100 million years to move a thousand km and four billion years to move around the planet, but it would move. Little changes add up to big changes when one considers the vastness of time. So is life, and so is the start of life. The first bacteria probably took millions of years just to evolve from something that did not have DNA into something that had DNA and became a single felled organism. The big reason was because nothing to start with and the second reason was that without oxygen in the world, everything is really slow.
Life continued to evolve for hundreds of millions of years, all with baby steps. The first big change was likely a container to keep everything together next to create DNA. Not that DNA is really important for life at first, but it is a way to store all the changes that have been made, and by store I mean save the changes to the blue prints of the life that did not die.
Somewhere around two billion years, or more, ago, one of the bacteria evolved that did something horrible. It used light from the sun to create food for itself, with a by product of oxygen. Before this time all life relied upon energy from the Earth and from other chemical reactions. The oxygen was free floating for about a second and then it oxidized an iron atom or something similar. But the beginning of the end of life had started right then and there. Slowly the oxygen would get created and slowly the metals and elements that oxidize were oxidized. All the while life carried on evolving in the oceans without oxygen gaining strong footholds all through the oceans. Other bacteria developed, one that efficiently used sugars to make useable energies and others that did other things. One group of organisms incorporated the energy producing critters now known as mitochondria, within its walls, another took the oxygen producing critters, now known as chloroplasts.
Then one horrible day all the oxidation stopped. Everything was full of oxygen. The rest of the oxygen floated up into the atmosphere and into the water. The single celled organisms with mitochondria and chloroplasts used the oxygen to make energy. Oxygen based life is fast; it has more energy. Soon the first great extinction occurred and most of the non-oxygen based life died; 99% of all there was died. This happened about 542 million years ago.
The new life lived near the surface of the oceans and they were exposed to new things like radiation from the sun, and it had mutating properties. The efficient life began to evolve rapidly. Plant evolved, multi-celluar life, cells that specialized their functions to be more efficient. Things that evolved to eat the plants, things that ate the things that ate the plants and things that ate them too. Everything seemed to explode out of nothing to fill the world. Over the next 40 million years, known as the Cambrian Explosion, all the simple body plans developed; worms, plants, sponges, jellyfish, starfish, trilobites, and the first critter with a backbone as part of its life cycle: the turnicates. Sexual reproduction became the standard form of reproduction, two parents produce young that are not duplicates of the parents, new combinations, new mutations, new life. Everything exploded, and everything continued to explode and diversify. Life did what life does best, exploit every possible niche in the world.
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