Secondarily aquatic tetrapods

Several clades of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least partly aquatic. These animals are called "secondarily aquatic" because although all tetrapods descended from freshwater lobe finned fish, their more recent ancestors are terrestrial vertebrates that evolved on land for hundreds of millions of years, and their clades only re-adapted to aquatic environment much later.

Unlike primarily aquatic vertebrates (i.e. fish), secondarily aquatic tetrapods (especially aquatic amniotes), while having appendages such as flippers, dorsal fin and tail fins (flukes) that resemble fish fins due to convergent evolution, still have physiology based on their terrestrial ancestry, most notably their air-breathing respiration via lungs instead of aquatic respiration via gills.

Nearly all extant aquatic tetrapods are secondarily aquatic, with only larval amphibians (tadpoles) being primarily aquatic with gills, and only some species of paedomorphic mole salamanders (most notably the fully aquatic axolotl) retain the gill-based larval physiology into adulthood.

Secondary aquatic adaptations of tetrapods tend to develop in early speciation of semi-aquatic animals that venture more and more frequently into water bodies in search of suitable habitats and foraging/hunting for food. As successive generations spend more time in water, natural selection favors those with traits that fair better in water, hence leading to more specialized aquatic adaptations via convergent evolution that make them become more fish-like in morphology.

Marine reptiles

Main article: Marine reptile

Mesosaurs were small aquatic reptiles that lived during the early Permian period, about 299 to 270 million years ago. They were among the first reptiles to live in water again after their ancestors lived on land.

Archelon was a giant sea turtle from the Cretaceous Period that is now extinct. Today, we still have sea turtles, and there are also softshell turtles that can stay underwater for long periods by "breathing" through rhythmic movements of their mouth cavity.

Squamata is a large group of reptiles that includes lizards, snakes, and worm lizards. Many of them have adapted to live in water. Mosasaurs, which lived at the same time as dinosaurs, looked like crocodiles but were better suited for life in the ocean. Modern aquatic squamates include marine iguanas and sea snakes, which are well adapted to life in the ocean. Sea snakes give birth to live babies and can’t live on land very well.

Ichthyosaurs were marine reptiles that lived in the oceans and were well adapted, even giving birth to live offspring. Crocodilomorphs include crocodiles and their extinct relatives. Some of them, like the Metriorhynchidae, had extreme adaptations for life in the open ocean, such as flippers and a tail fluke.

Sauropterygians evolved from land reptiles shortly after a major extinction event and thrived during the Triassic period. Most went extinct, except for the plesiosaurs, which disappeared at the end of the Cretaceous period along with the dinosaurs.

Main article: Ichthyosauria

Main article: Sauropterygia

Main article: Mesosaur

Marine mammals

Main articles: Marine mammal and Aquatic mammal

Some animals that started life on land later returned to live in the water. These are called secondarily aquatic animals. They are different from animals that have always lived in water.

Cetacea

Main articles: Cetacea and Evolution of cetaceans

During the Paleocene Epoch (about 66 - 55 million years ago), an ancient whale called Pakicetus began living partly in rivers and shallow seas. It was the ancestor of today’s whales, dolphins, and porpoises. These animals are now fully adapted to life in the ocean and cannot live on land. They have special features like a dorsal blowhole, special teeth called baleen, and a special organ called the 'melon' that helps them find things underwater using echolocation. The closest living land relative to whales is the hippopotamus, which often stays in water and whose name means "horse of the river".

Sirenians

Main articles: Sirenian and Evolution of sirenians

The ancestors of today’s dugong and manatees first appeared in the ocean about 45 to 50 million years ago.

Pinnipeds

Main article: Pinniped

Fossils show that phocids lived 12 to 15 million years ago, and odobenids lived about 14 million years ago. Their common ancestor lived even earlier.

Polar bears

Main article: Polar bear

Polar bears mostly live on ice but have started to adapt to water. They have lots of body fat and can close their nostrils, which helps them swim and stay warm. Fossils show polar bears lived about 100,000 years ago. They have thick fur and fat to stay warm in cold places.

Speculative theories

Humans

Main article: Aquatic ape hypothesis

Some people think that part of how humans evolved included living near water. They say this might explain why humans don’t have much hair, walk on two legs, have a layer of fat under their skin, and other body features. However, most scientists who study human evolution do not agree with this idea.