Some years ago I published a series of articles about evolution in my local newspaper. It generated some interest and a spate of letters to the editor, and my publisher liked it. There was even a creationist who challenged me to a debate over it. I decided to reproduce it here. This is part two, which I called Tough Life. See also Part One and Part Three.
Life started on Earth just about as soon as it could. When the Solar System condensed out of a vast cloud of dust and gas about four and a half billion years ago, Earth was a molten globule. It was kept molten for a few hundred million years by a continuing bombardment of comets and planetoids among the thick debris. Eventually, as fewer collisions occurred, Earth cooled and its crust formed and hardened. It was still so hot that the huge quantity of water delivered by the comets was kept vaporized. Only after about 500 million years had it cooled enough for standing water to form on the surface. Not long after that we begin to see fossils of tiny organisms.
There is a question whether life formed through the evolution of chemistry on this planet, or arose from space-borne particles. Either way it’s been here for almost four billion years. The fact that it took root as soon as it could and has survived for so long both show how tough and persistent life is. It has had to be. There have been at least five major extinctions that we know of in the interim. There were probably more, but we haven’t unearthed the evidence yet. Some of the extinction events extinguished over ninety percent of the species on Earth. Once in a long while everything changes forever.
The first creatures were tiny, simple, single-celled organisms. They didn’t even have a nucleus, their DNA floating freely within the cell. From them evolved more complex forms of life, with DNA in a nucleus and other structures performing ever more complicated tasks. About two and a half billion years ago evolution produced a radical change. A microbe appeared which could use sunlight to synthesize food from water and carbon dioxide. Unfortunately for the existing life forms, the oxygen produced as a byproduct was a deadly poison. Things were changed forever.
About five hundred forty million years ago the abundance of free oxygen provided the energy for another radical change. Multicellular life arrived and proliferated in an explosion of diversity. It was a matter of time before some of it found a way to live on land, away from the competition and sharp teeth in the sea. It wasn’t long, in geological terms, before the land was covered in a riot of life. Evolution filled every niche with a profusion of species, including the majestic dinosaurs. Then the most famous mass extinction happened sixty-five million years ago. Dinosaurs were out and mammals were in.
Life took hold here early and has persisted by evolving and adapting to a state of permanent change.