Back past the Cenozoic, the time when our ancestors descended from the trees; back past the Mesozoic, when dinosaurs ruled the land; back past the Permian and the rise of the ancestors of the mammals; back, past the Carboniferous’s coal-forming swamp forests; back past the Devonian, when amphibians emerged from the water; back past the Silurian, when plants first took root on land—all the way to the Ordovician, to a time before the advent of wings or knees or lungs, before the fishes had bony jaws, before blood pumped from a multichambered heart.
