Video: scientists explain their discovery

A small deer-like mammal about the size of a modern fox or racoon was the ancestor of whales and dolphins, according to research that fills a missing link in their evolutionary history.

The creature, indohyus, which lived in what is now India around 48 million years ago, may have been the land animal that first took to the water to escape predators, leading ultimately to the evolution of the cetaceans - the order that includes whales, dolphins and porpoises.

While it had long been understood that cetaceans are mammals that had forerunners that lived on land, and a series of intermediate fossils have been found, scientists had not previously identified a species from the era in which cetacean ancestors took up an aquatic lifestyle.

A new analysis of fossils belonging to indohyus has now filled this important evolutionary gap. Details of its anatomy have shown that while it belongs to a terrestrial group called the raoellids, it spent much of its time in water.

Further similarities with other intermediate cetacean fossils suggest that either indohyus or something like it was probably the ancestor of modern whales and dolphins, scientists said.

As indohyus was a herbivore, the new research also challenges a standard view that as whales are carnivorous, they are most likely to be descended from predatory land ancestors which moved to the water to feed on fish.

“Clearly, this is not the case,” said Hans Thewissen, of Northeastern Ohio Universities, who led the research. “Indohyus is a plant-eater, and already is aquatic. Apparently the dietary shift to hunting animals, as modern whales do, came later than the habitat shift to the water.”

The small mammal's aquatic lifestyle has been inferred from an examination of its bones, which have a much thicker outer layer than is usual for land animals. It is, however, found in modern mammals that are largely aquatic, such as the hippopotamus.

The hippo is also known from DNA evidence to be the closest relative of cetaceans still living today, though as it is found in the fossil record only from 35 million years ago it is not a candidate for being a direct ancestor.

Further evidence for aquatic adaptation comes from indohyus's teeth, which have a similar ratio of oxygen isotopes to modern animals that live principally in water. Details of the discovery are published in the journal Nature.

Dr Thewissen said that while it seems odd that a mammal similar to a deer might have lived mainly in water, there is a modern herbivore that follows just such a lifestyle. The African mouse-deer, also known as the chevotain, is known to jump into water when in danger from predators.

While the chevotain is not closely related to whales, its behaviour suggests that cetacean ancestors might first have taken to the water to escape predators, before adapting to a mainly aquatic lifestyle.

Dr Thewissen's previous research has established that whales and dolphins are evolved from a group of land mammals known as the artiodactyls - the even-toed ungulates that include cattle, pigs and camels.

In 1994, his team discovered Ambulocetus natans, an amphibious whale that was a later ancestor of cetaceans, and in 2001 it identified Pakicetus attocki, the oldest known true whale. Nothing, however, was previously known about the first artiodactyls to take to the water.

Walt Horton, vice-president for research at Northeastern Ohio Universities, said: “This remarkable research demonstrates that the study of the structure and composition of fossil bones can tell us about how the skeleton of whales and, by extension, other mammals like humans, interacts with the environment and changes over time."

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