The original forerunner is named Pakicetus and lived about 50 million years ago. In those days, the region of Earth known now as the Middle East was then a vast marshland bordering a shallow sea. Along the rivers and estuaries, Pakicetids had learned to feed on small fish. This species was a member of a group of animals referred to as artiodactyls.
Modern artiodactyls, (also called even-toed ungulates), include deer, sheep, cattle, and pigs. Chromosomes, blood composition, fetal blood sugar, shape of the uterus, and structure of the tooth enamel are common traits shared by both whales and these ungulates. DNA evidence links the hippopotamus as the ungulate most closely related to whales.
As Pakicetids perfected their fishing skills over millennia, natural selection favored the subtle adaptations that improved these animals’ foraging and feeding success. Eventually succeeding generations would inherit specific traits that allowed for better swimming ability, as well as eyes and ears adapting to function better underwater.
Evolution can cause whole populations of animals to change genetically over long periods time. This results in speciation, where one species can gradually split into two or more species and slowly give rise to new groups of animals.