A second look at some 40-million-year-old fossils provides a "missing link" to suggest that the closest living relative of whales is the hippo, a group of scientists said on Monday.
Although the hippopotamus does not seem a likely relative of whales, genetic study has suggested they are close. Now, a team at the University of California, Berkeley, the University of Poitiers in France and the University of N'djamena in Chad say they have found more evidence in the fossil record.
"The problem with hippos is, if you look at the general shape of the animal it could be related to horses, as the ancient Greeks thought, or pigs, as modern scientists thought," researcher Jean-Renaud Boisserie said in a statement.
In Greek the name hippopotamus means "river horse".
"But cetaceans -- whales, porpoises and dolphins -- don't look anything like hippos. There is a 40-million-year gap between fossils of early cetaceans and early hippos," Boisserie added.
The earliest cetacean fossils date back 53 million years while the first hippopotamus fossils date to about 16 million years.
{{/usCountry}}The earliest cetacean fossils date back 53 million years while the first hippopotamus fossils date to about 16 million years.
{{/usCountry}}Writing in the Proceedings of the National Academy of Sciences, Boisserie and colleagues propose a new theory that whales and hippos had a common water-loving ancestor that lived 50 to 60 million years ago.
From it evolved two groups -- the early cetaceans, which gradually moved into the water full-time, and a large and diverse group of pig-like animals called anthracotheres.
These animals flourished, forming 37 distinct genera across the world before dying out and leaving just one descendant 2.5 million years ago -- the hippopotamus.
The theory would class whales, dolphins and porpoises with cloven-hoofed mammals such as cattle, pigs, and camels.
Boisserie argues that some of the older, time-tested ways of classifying animals by body shape and even teeth are not always the most accurate.