Courtesy American Museum of Natural History

Paleontologists have long asked if hippopotamuses and cetaceans – whales, dolphins and porpoises – share a common, amphibious ancestor. The answer, which is no, according to a study published today in Current Biology, was derived by a molecular and genetic analysis of the animals’ hides which are highly adapted to aquatic environments. Co-senior author Maksim Plikus, a UCI skin science expert, said that when a group of animals becomes aquatic their outer layer becomes much more streamlined and uniform.

“Complex derivatives like hairs, nails, or sweat glands are no longer needed, and in fact, can become a hinderance to life under water, so those go away,” said Plikus, UCI professor of developmental & cell biology. “And it loses the barrier function performed by the outer layer of skin, which in terrestrial mammals, is vital to keeping water from evaporating out of the body and preventing pathogens from getting in.”

The researchers compared hippo and cetacean skin and went so far as to examine newly available genome of the pygmy hippo to compile a list of the so-called “skin genes” inactivated in the species. The molecular signatures they uncovered convinced them that traits in hippos and cetaceans evolved independently, and that the gene losses in hippos happened much later than in cetaceans.

“The project provides convincing evidence that the last common ancestor for hippos and cetaceans was likely a land-dwelling mammal, uprooting current thinking that the skin came fine-tuned for life in the water from a shared amphibious relative,” said Plikus, whose collaborators included researchers at UC Riverside, the American Museum of Natural History and the Max Planck Institute of Molecular Cell Biology.