Aquatic adaptation
Several mammal groups have undergone aquatic adaptation, going from being purely terrestrial animals to living at least part of the time in water. The adaptations in early speciation tend to develop as the animal ventures into water in order to find available food. As successive genereations spend more time in the water, natural selection causes the acquisition of more adaptations. Animals of later generations may spend the majority of their life in the water, coming ashore for mating. Finally, fully adapted animals may take to mating and birthing in water.
During the Paleocene Epoch (about 55-65 million years ago), a wolf-like pack animal, possibly either Ambulocetes natans whose fossils were found in Pakistan or Andrewsarchus, started wading into the Tethys Ocean. Rodhocetus balochistanensis fossils, found in Balochistan Province, Pakistan in 2001 by Philip Gingerich, may be a prehistoric link between whales and artiodactyls (pigs, cattle, deer, hippos, etc.). It's front flipers were more hand-shaped, and it still had external legs.
Archaeoceti is the group of whale-like animals that flourished during the Eocene Epoch. They had the basic whale streamlined body pattern, but heterodont teeth.
The zeuglodon, Basilosaurus cetoides (Owen), was discovered in 35-40 million year old marine sediments in central Louisiana and in similar structures in Alabama. It had a long, lizard-like body and only front flippers.
See also: Evolution of cetaceans
The ancestors of the dugong and manatees first appeared in the fossil record about 45 to 50 million years ago in the Eocene.
The fossil records show that phocids existed 12 to 15 million years ago, and odobenids about 14 million years ago. Their common ancestor must have existed even earlier than that.
Lutrins
Otters have existed along the coast of California for about 5 million years.
Although still primarily a terrestrial animal, the polar bear shows the beginnings of aquatic adaptation to swimming (body fat, closable nostrils), diving, and thermoregulation. Distinctly polar bear fossils can be dated to about 100,000 years ago.
Some people believe that part of human evolution includes some aquatic adaptation and have put forward an Aquatic ape hypothesis.
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