The most convincing evidence for the occurrence of evolution is the discovery of extinct organisms in older geological strata. Some of the remnants of the biota that lived at a given geological period in the past are embedded as fossils in the strata laid down at that period. Each earlier stratum contains the ancestors of biota fossilized in the succeeding stratum. The fossils found in the most recent strata are often very similar to still living species or, in some cases, even indistinguishable. The older the strata are in which a fossil is found—that is, the further back in time—the more different the fossil will be from living representatives. Darwin reasoned that this is to be expected if the fauna and flora of the earlier strata had gradually evolved into their descendants in the later, more recent strata.
Given the fact of evolution, one would expect the fossils to document a gradual steady change from ancestral forms to the descendants. But this is not what the paleontologist finds. Instead, he or she finds gaps in just about every phyletic series. New types often appear quite suddenly, and their immediate ancestors are absent in the earlier geological strata. The discovery of unbroken series of species changing gradually into descending species is very rare. Indeed the fossil record is one of discontinuities, seemingly documenting jumps (saltations) from one type of organism to a different type.
Transitional forms in evolution of the whale are illustrated by Hans Thewissen’s discovery of fossils of the Ambulocetus from 47 million years ago.
