Fossils are the fragments from which, piece by laborious piece, the great mosaic of the history of life has been constructed. Here and there, we can supplement these meager scraps by the use of biochemical markers or geochemical signatures that add useful information, but, even with such additional help, our reconstructions and our models of descent are often tentative. For the fossil record is, as we have seen, as biased as it is incomplete. But...