The study, by researchers at the Tokyo Institute of Technology in Japan and Pennsylvania State University at University Park, Pa., compared gene sequences from the minke whale, the sperm whale and the hippopotamus. It found the diverse animals share a sequence of genes inherited from an ancient common ancestor.
A report of the study appears Tuesday in the Proceedings of the National Academy of Sciences.
The study found that the camel, pig, giraffe, sheep and cow share some gene sequences with the whale and hippo, indicating that far back in time all had a common ancestor, according to evolution theory. The animals diverged genetically. A gene sequence found only in whales and the hippo show that they share a common ancestor that was not part of the evolutionary history of the other animals.
Until 1985, it was generally thought that pigs were more closely related to whales. Other genetic studies since have suggested the whale-hippo link. The new study, researchers say, confirms that the hippo is the closest living relative to the whale.
The genetic analysis was conducted by Masato Nikaido and Norihiro Okada of the Tokyo Institute and by Alejandro Rooney of Penn State.
It has long been believed that the whale's ancient ancestor was a land animal. It is thought that a lineage leading to the modern whale returned to the sea and evolved into a group of marine mammals called cetacean. Along the way, hind legs were lost, and forelegs were replaced by flippers.
David Hillis of the University of Texas, in a PNAS commentary, said whales and hippos share several adaptations, including the lack of hair and oil-producing skin glands, and the ability to communicate and to nurse offspring under water.