This paper documents the early cetacean transition from land to water and describes early forms such as Pakicetus and Ambulocetus as key evidence. It states that early cetaceans were terrestrial artiodactyls and that the fossil record from India and Pakistan captures this land-to-sea transition in detail.

Cetaceans (whales, dolphins, and porpoises) are an order of mammals that originated about 50 million years ago in the Eocene epoch. Even though all modern cetaceans are obligate aquatic mammals, early cetaceans were amphibious, and their ancestors were terrestrial artiodactyls, similar to small deer. The transition from land to water is documented by a series of intermediate fossils, many of which are known from India and Pakistan. Cetaceans originated from land mammals (Thewissen and Williams 2002; Fordyce and Muizon 2001).

The ancestors of modern cetaceans (whales, dolphins, and porpoises) transitioned from a terrestrial to a fully aquatic lifestyle during the Eocene about 50 million years ago.[1]

The article says that the first whales, such as Pakicetus, were typical land animals. It also says that Ambulocetus looked more aquatic and that its anatomy supports a life in and out of water, with evidence from stratigraphy and oxygen isotopes.

Cetaceans (whales, dolphins, and porpoises) have undergone the most dramatic morphological transformation of all mammals, having originated from a clade of terrestrial even-toed ungulates >50 Ma.[16]

Approximately 56–53 million years ago, cetacean ancestors transformed their habitat from a completely terrestrial to a fully aquatic environment and subsequently radiated around the world with a series of outstanding morphological changes (e.g., degeneration of limbs, lengthening of the skull, thickening of the blubber, loss of hair, etc.).[18]

The resource explains that Pakicetus had key features transitional between terrestrial mammals and the earliest true whales. It also describes Ambulocetus as amphibious, able to move on land and swim by pushing with its hind feet and undulating its tail.

All cetaceans, including whales, dolphins, and porpoises, are descendants of land-living mammals. The earliest whales are known from India and Pakistan from around 52.5 million years ago, during the early Eocene. The earliest whales that we think were fully aquatic, that is, they never left the water, are found around 40 million years ago, during the middle Eocene. That means that the transition from terrestrial animals to fully aquatic animals took about 12 million years.

Fossils collected in the last decade document the ways in which Cetacea (whales, dolphins, and porpoises) became aquatic, a transition that is one of the best documented examples of macroevolution in mammals.[5] Whales indisputably are mammals… This implies that whales evolved from other mammals and, because ancestral mammals were land animals, that whales had land ancestors.[5]

DNA-DNA hybridizations support ungulate ancestry of Cetacea.[2] Recent morphological data on Pakicetus spp. and Basilosaurus spp. fossils suggest that cetaceans (whales, dolphins and porpoises) originate from carnivorous Mesonychid land mammals…and made a gradual transition from land to sea in early Eocene.[2] The Tm and Tmode…show a greater sequence similarity—and thus a lower genetic distance—between cetaceans and ungulates than between cetaceans and other mammals.[2]

"Fossils reveal that Cetacea originated 56–53 Ma from terrestrial artiodactyl ancestors (Thewissen et al. 2007)." The paper continues: "Early archaeocetes or ‘ancient whales’ (stem Cetacea) diversified through amphibious stages to become fully pelagic by 40–38 Ma." This documents a transition from land-dwelling artiodactyl mammals through amphibious forms to fully aquatic whales, dolphins and porpoises.

"Our present phylogenetic analyses of protein and mitochondrial DNA sequence data indicate that cetaceans are not only intimately related to the artiodactyls; they are in fact deeply nested within the artiodactyl phylogenetic tree…" The authors estimate "that the whale lineage has branched off a protoruminant lineage < 50 Mya. By implication, the cetacean transition to aquatic life is inferred to be a relatively recent evolutionary event." This molecular evidence supports that modern cetaceans (including dolphins) evolved from terrestrial even-toed ungulates that later became aquatic.

The study reports that its results "indicated (i) that cetaceans are deeply nested within Artiodactyla…" and concludes: "These findings lead us to conclude that cetaceans evolved from an immediate artiodactyl, not mesonychian, ancestor." Artiodactyls are terrestrial even-toed ungulates, so this places whale and dolphin ancestors among land-dwelling mammals before their return to aquatic life.

One of Pakicetus' relatives was Ambulocetus. It lived about 50 to 48 million years ago. Like Pakicetus, it spent time both in and out of water, and within 10 million years cetaceans were completely adapted to life in the water.

This museum page states that cetaceans evolved from land-dwelling mammals and that Pakicetus and Ambulocetus are part of the documented transition from land to sea. It presents the museum's educational summary of the fossil sequence.

Though they have a fishlike body, cetaceans (a group comprised of whales, dolphins, and porpoises) are mammals that descended from land-dwelling ancestors – same as cats, dogs, mice, elephants, cows, and humans. Unlike their land-dwelling relatives, however, cetaceans have undergone profound changes in their bodies and skeletal structures to thrive in aquatic environments; including the reduction of hindlimbs and the evolution of flippers and tail flukes, resulting in a streamlined body.

The page states that cetaceans did not always live in the sea and that up until about 55 million years ago they were small terrestrial omnivores. It says Pakicetus was mostly land-dwelling and that Ambulocetus was much better suited to paddling and swimming than walking on land.

When marine mammals such as whales, dolphins, manatees and walruses moved from land to water, a series of physical abilities – limbs adapted for swimming, less dense bones that make them more buoyant and a large store of oxygen…made it possible.[9] An international consortium…looked at the genomes of these four marine mammals and compared them to their closest land kin…They found 191 genes likely to be associated with the move from land to sea that were selected during evolution across the four species.[9]

The page states that cetaceans "are marine mammals belonging to the infraorder Cetacea, a secondarily aquatic clade under the order Artiodactyla that include whales, dolphins, porpoises…" It explains: "The fossil record traces the gradual transition from terrestrial to aquatic life. The regression of the hind limbs allowed greater flexibility of the spine… The front legs transformed into flippers, costing them their mobility on land." It also notes: "The transition from land to water was completed in about 10 million years." This summarizes that dolphins and other cetaceans are secondarily aquatic mammals derived from land-dwelling artiodactyls.

The Smithsonian educational animation on whale evolution presents the standard fossil sequence showing whales evolving from land mammals into fully aquatic cetaceans. It is explicitly framed as an educational explanation of that transition.

Cetaceans are thought to have transitioned from land to water at the tail end of the Paleocene–Eocene transition about 56 Mya, which was marked by increased global temperatures of 5.6 °C, warming the earth’s oceans. Cetaceans are fully aquatic marine mammals, and scientists think they evolved during the Eocene or earlier, sharing a common ancestor with hippopotamuses, making them their closest living relative. Fossil evidence from early cetaceans such as Pakicetus, Ambulocetus, and Remingtonocetus documents stages in the transition from land-dwelling to fully aquatic life.

The cetaceans, including whales, dolphins and porpoises, are thought to be among the most successful animals to have ever transitioned from land back to the sea, beginning about 50–55 Ma.[4]

The article says whale evolution involved land-dwelling ancestors, with Pakicetus as a terrestrial early form and Ambulocetus as an intermediate amphibious form. It describes the broader transition of cetaceans from land to sea over millions of years.

Dolphins evolved from land mammals that reentered the water over 50 million years ago. Early ancestors like Pakicetidae and Ambulocetidae exhibited whale-like features as they adapted to freshwater and saltwater environments, beginning to hunt in the water. Later groups like Remingtonocetidae and Basilosauridae lost the ability to live on land and became fully aquatic, eating fish and developing sinuses and echolocation abilities.

"Recent molecular evidence suggests that Cetacea evolved from artiodactyl ancestors, making Artiodactyla non-monophyletic unless Cetacea is included." The account explains that experts recognize a combined clade "Cetartiodactyla" for artiodactyls and cetaceans. Since artiodactyls are land-dwelling even-toed ungulates, this supports that whales and dolphins evolved from terrestrial mammals within this group before becoming aquatic.

Around 50 million years ago, certain four-legged land creatures started spending more and more of their time in water. (An animal that there are fossil records of from around this time named Pakicetus may represent this initial stage). For some reason, this change must have suited them, as they gradually evolved, and their bodies changed form, so that they eventually lost the ability to move onto the land at all. An interim stage in this development is represented by the Protocetids and examples of the first exclusively aquatic dwelling form are the Dorudon and the 60 foot/18 metre long Basilosaurus.

Discussing the stem lineage of cetaceans, the article notes that the phylogenetic position of †Indohyus "suggests that the cetacean stem lineage included herbivorous and carnivorous aquatic species." Indohyus and related forms are described as semi-aquatic artiodactyls closely related to whales, implying that fully aquatic cetaceans evolved from terrestrial or semi-aquatic ungulates that progressively adapted to life in water.

"The Cetaceans (whales, dolphins and porpoises) are mammalian marine descendants of land mammals. Their terrestrial origins are indicated by: their need to breathe air from the surface; the bones of their fins, which resemble the limbs of land mammals; [and] the vertical movement of their spines…" It further states that DNA analysis showed "the closest relationship was with artiodactyls, the even-toed ungulates" and that early cetaceans "gradually became adapted to life in the sea" and "became the completely aquatic cetaceans."

Land animals, including mammals, originally evolved from creatures that lived in the sea. Dolphins, rather surprisingly, are mammals that evolved from animals that used to live on the land; in other words they went back to live in the sea. Scientists believe that dolphins evolved from a hoofed, land-living mammal called 'Mesonyx', and returned to live in the seas some fifty million years ago.

New skeletal discoveries seem to back this claim, giving possible morphological evidence as testimony. This is one of the many internal physiological structures leading scientists to believe that dolphins and whales evolved from a terrestrial ancestor. Modern dolphins retain vestigial pelvic bones and limb girdles that no longer support functional hind legs, consistent with descent from land mammals.

The ancestors of dolphins that we know and love today entered the water about 50 million years ago.[10] Researchers have carefully examined the bones of the fins for dolphins. What they find is that they strongly resemble the limbs of many animals that live on land…This is one of the many telling signs that the dolphin is related to terrestrial origins.[10]

The museum page describes the ancestor of today’s whales as "Pakicetus, a quadruped measuring 1 to 2 metres long" and notes that "the descendants of the first land-roaming cetaceans increasingly gravitated to an aquatic environment… to seek refuge from danger… or to feed." It explains that today’s artiodactyls are herbivores but "biologists believe that cetaceans evolved because some of the herbivores in this group appear to have changed their diets to become carnivores," indicating a shift from terrestrial ungulate ancestors into aquatic predators such as whales and dolphins.

Just a few million years later, the descendants of the Pakicetus had begun wading back into the water, transitioning to an amphibious lifestyle. As marine food sources became more abundant, these pre-whales — known collectively as archaeocetes or ‘ancient whales’ — gradually gave up their life on land. Over tens of millions of years, their bodies transformed: back legs shrank, tails broadened into flukes, and forelimbs became flippers, giving rise to modern whales and dolphins.

This summary says early cetaceans were terrestrial artiodactyls, similar to small deer, and that the fossil evidence documents the transition from land to water during the Eocene. It is a secondary educational explanation rather than a primary study.

The page identifies Ambulocetus as a semiaquatic early cetacean from Pakistan and says it is among the best-studied Eocene cetaceans, important for understanding the transition from land to sea. It also notes that Archaeoceti span that transition.