The suggestion comes from scientists at the Tokyo Institute of Technology, who have developed a new way of tracking an animal's evolutionary history through DNA sequences millions of years old.

Every animal inherits chunks of DNA which are specific to its species. These bits appear at precisely the same place in the animal's genetic code - or genome - as in that of its ancestors.

By comparing the DNA of different animals, the researchers have been able to redefine their family trees.

New beginnings

Their results show that whales and dolphins are more closely related to cows, camels and pigs than horses, elephants and sea cows.

The camel - went its own way first

The scientists have also been able to show that camels formed their own evolutionary branch long before other species of four-legged grazers.

A close relationship between whales and ungulates - hoofed animals - was first suggested 100 years ago, but until recently there has been little evidence to link whales with any particular group of ungulates.

Part of the problem has been the profound changes the whale has undergone in adapting to its watery environment.

The hind limbs have been lost and the front limbs have become flippers, so the only way whales could be compared to ungulates was through other links.

Underwater talking

"Hippopotami and whales share several specialised aquatic adaptations, including lack of hair and sebaceous glands, and underwater vocalisations that are apparently communicative," writes Masato Nikaido in the Proceedings of the National Academy of Sciences.

These shared characteristics have been interpreted as examples of convergence - both species developed the same features independently whilst adaptating to life in the water.

Hippo and whale both "talk" underwater

"In fact, our study shows that the two animals have the similarities because they share a common ancestor," said Professor Nikaido.

Missing link

But just what that ancestor could be is more of a mystery now than before.

Modern whales are thought to be descended from an extinct primitive whale that first appeared 50 million years ago.

It, in turn, is thought to have arisen from an extinct group of land mammals called the mesonychians.

Professor Nikaido's research, together with other evidence, now suggests that there may have been a 'missing link' between the mesonychians and the first whales - if they were related at all.

The new findings have been cautiously welcomed by other biologists

"These studies provide a useful and important new source of data but they are not magic bullets.