Why Evolution Is True

by Jerry A. Coyne

The first organisms, simple photosynthetic bacteria, appear in sediments about 3.5 billion years old, only about a billion years after the planet was formed. These single cells were all that occupied the earth for the next 2 billion years, after which we see the first simple “eukaryotes”: organisms having true cells with nuclei and chromosomes. Then, around 600 million years ago, a whole gamut of relatively simple but multicelled organisms arise, including worms, jellyfish, and sponges. These groups diversify over the next several million years, with terrestrial plants and tetrapods (four-legged animals, the earliest of which were lobe-finned fish) appearing about 400 million years ago. Earlier groups, of course, often persisted: photosynthetic bacteria, sponges, and worms appear in the early fossil record, and are still with us. Fifty million years later we find the first true amphibians, and after another 50 million years reptiles come along. The first mammals show up around 250 million years ago (arising, as predicted, from reptilian ancestors), and the first birds, also descended from reptiles, show up 50 million years later. After the earliest mammals appear, they, along with insects and land plants, become ever more diverse, and as we approach the shallowest rocks, the fossils increasingly come to resemble living species. Humans are newcomers on the scene—our lineage branches off from that of other primates only about 7 million years ago, the merest sliver of evoluti...