The puzzle over how the air-breathing ancestors of whales and dolphins took to the sea after millions of years living on land may have been solved with the discovery of a distant relative of marine mammals.
A fossilised, deer-like animal that grew no bigger than a fox and lived 48 million years ago has been identified as the closest ancestral relative of the cetacean group of marine mammals, which includes anything from the smallest porpoise to the largest whale. Since Charles Darwin, biologists have known whales are mammals, suckling their young on milk and breathing through lungs, so they must have originated from terrestrial ancestors. But until now there has been no definite "missing link" between the cetaceans and land mammals.
Scientists have analysed the anatomy of the fossil and have concluded that Indohyus an "even-toed ungulate" like modern deer possesses a bony structure in its auditory system that is shared with only one other group of mammals, the cetaceans. In the journal Nature, the scientists, led by Professor Hans Thewissen of Northeastern Ohio Universities Colleges of Medicine and Pharmacy, said the discovery, "has profound implications".
It had been thought that the ancestor of whales was a carnivore that took to an aquatic life in pursuit of fish and other prey, as do modern-day seals. But Indohyus, which belonged to a group of extinct mammals called the raoellids, was almost certainly a herbivore and probably became semi-aquatic to escape from its predators.