A study published in the latest issue of the Proceedings of the National Academy of Sciences shows that the whale and the hippo are each other's closest living relatives. The genetic analysis was conducted by Masato Nikaido and Norihiro Okada of the Tokyo Institute of Technology, and by Alejandro P. Rooney in the Institute of Molecular Evolutionary Genetics at Penn State.
Rooney says, "We knew from previous work that whales were closely related to even-toed hoofed mammals, but the studies had been inconclusive or unreliable regarding exactly where they fit in the family tree of this group of mammals." This new study, like previous ones, found that animals such as the hippo, camel, pig, giraffe, sheep, and cow do share many segments of DNA with whales, porpoises, and dolphins, indicating that at some point they all had a common ancestor. However, DNA segments found only in whales and hippos indicate that they have a common ancestor that is not part of the evolutionary history of the other animals. "Ours is the first study to provide reliable confirmation that hippos are the sister-group to whales," says Rooney. Whales and hippos share several adaptations to life in an aquatic environment, including oil-producing skin glands, the lack of hair, and the use of underwater vocalizations for communication.
The study by Nikaido, Rooney, and Okada compared gene sequences from several species of whales and dolphins to the hippopotamus and other hoofed mammals by using a totally new approach: analysis of short interspersed element (SINE) and long interspersed element (LINE) insertion events. These genetic markers are mobile DNA elements that "jump" around the chromosomal DNA of a species. Rooney expects that this method will provide researchers with a new, highly robust way to look at the relationships among living things. "We know that carnivores form a unique group that includes all animals with modified teeth for ripping and tearing flesh, such as cats, dogs, hyenas, mongooses, otters, weasels, and bears, but we have no idea what the closest relative of this group is. I expect that this new method will answer that question," says Rooney. "This method might also be applied to solve the question of relationships in the family tree of birds, which is very problematic."