Evolutionary biologists believe that dolphins emerged 20 to 25 million years ago during the early Miocene period. Modern species may trace to kentriodontids, which are small-toothed whales believed to have the echolocating ability as modern dolphins do.
The earliest proto-whale, Pakicetus, was a hoofed mammal related to the ancestors of modern deer, goats and cattle that are referred to as artiodactyls or even-toed ungulates. Pakicetus fossils were discovered in northern Pakistan in 1979 and were dated to about 50 million years ago. These animals were probably semi-aquatic waders. The first fully aquatic whales, basilosaurids and dorontids, date to the late Eocene period 41 to 35 million years ago, and the first toothed whales appeared about 33 million years ago.