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Witnesses were absent for the comings and goings of the first life some four billion years ago, but scientists are pretty sure the typical Earth creature in those days consisted of no more than a single cell. That doesn't mean the planet was a dull sea of sameness. Single-celled creatures may have acquired genetic diversity early on.
Here's why. When cells divide, mistakes have a way of creeping into genetic material. Variants that enhance a cell's ability to survive and reproduce become more common over successive generations. This basic fact of evolution applied to the early Earth. “Variation is necessary for there to be evolution by natural selection in the first place,” explains Andrew Hamilton, a philosopher of science at Arizona State University. “Biodiversity originated at the point that there was variation on which selection could operate.”
Today we think of biodiversity in terms of multicellular life, but flowering plants and animals didn't arrive until relatively recently (540 million years ago). Although some evidence suggests that having a wide variety of species makes an ecosystem more stable, the jury is still out. It is of no comfort to know that the worst catastrophe would still preserve some biodiversity—even if only for the lowly cell.
