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Hippos and whales are first cousins


Hippos and whales had a common water-loving ancestor 50 to 60 million years ago, scientists say (Image: Reuters/Enrique Marcarian)

Hippos are the closest living relative of whales, scientists say after taking a second look at some 40-million-year-old fossils that may be a "missing link".

Although hippos and whales don't look alike, a genetic study has suggested they are close.

Now, an international team of scientists says it has found more evidence in the fossil record, publishing its results online today ahead of print in the journal Proceedings of the National Academy of Sciences.

"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," says researcher Dr Jean-Renaud Boisserie, from the University of California, Berkeley.

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 adds.

The earliest cetacean fossils date back 53 million years while the first hippopotamus fossils date to about 16 million years.

Who is the common ancestor?

Boisserie and his French 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, except in Oceania and South America, 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.

According to the researchers, most biologists agree that whales and hippos are first cousins. But they are still debating how they are related and where in the animal family tree they belong.

Tags: science-and-technology, palaeontology, zoology