Hippos and cetaceans are direct relatives, but their mutual "aquatic" skin traits weren't gotten from a common ancestor. A recent study shows that the closely smooth, almost hairless skin of whales and hippopotamuses developed independently.

(Photo : Getty Images)

The Flight Evolution 

The work proposes that their final common ancestor was probably a terrestrial mammal, uprooting recent thinking that the skin came adjusted for life in the water from an amphibious ancestor that is shared. 

The study was published on the 1st of April in the journal Current Biology and was conducted by scientists at the American Museum of Natural History; University of California, Riverside; University of California, Irvine; the LOEWE-Centre for Translational Biodiversity Genomics, Germany; and Max Planck Institute of Molecular Cell Biology and Genetics.

A senior research scientist, John Gatesy, in the American Museum of Natural History's Vertebrate Zoology Division and a corresponding author on the study said that the way mammals migrated from terra firma and became completely aquatic is one of the most amazing stories of evolution, perhaps opposed only by how animals swapped water for land in the first place or by the flight evolution. 

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Ancestor of Hippos and Cetaceans 

Recent findings dispute the current field's dogma --that families of the amphibious hippo might have been among the alteration as mammals re-entered life in the water.
In spite of their different appearances, completely aquatic cetaceans - the group that comprises dolphins, whales, and porpoises - and semi-aquatic hippopotamuses are one another's nearest living relatives and share the same ancestor that lived about 55 million years ago.

Features that are unusual for most mammals are also shared among them: they deliver their babies and nurse them underwater and doesn't have sebaceous glands ( gland that secrete oily sebum) and scrotal testes as well as most of their hair.

Since these traits are hardly seen in other mammals, one would take it that they were present in the common ancestor of hippos and cetaceans already. But how and when cetacean ancestors became entirely aquatic still remains a topic of intense debate.

(Photo : Getty Images)

Genomic Screens 

The scientists made a comparison of the anatomy of hippo and cetacean skin centered on histology and used genomic screens to put together a complete list of "skin genes" that have been deactivated in both cetaceans and hippos. This was assisted by studying for the initial time the genome of the pygmy hippo, (Choeropsis liberiensis) one of the only two living hippo species.

"When you glance at the molecular signatures, there is a noticeable and clear answer," study co-corresponding author and evolutionary genomicist Michael Hiller, from the Max Planck Institute of Molecular Cell Biology and Genetics and the LOEWE-Centre for Translational Biodiversity Genomics in Germany said.

The outcomes firmly support the notion that skin traits of 'aquatic' found in both hippos and cetaceans developed independently. And not that only, the losses of gene can be seen in the hippo's lineage that occurred much in the future than in the cetacean lineage. These gene outcomes are in line with studies of the skin itself. Hippos actually have a very specialized type of sweat gland that manufactures "blood sweat," a substance that is orange-colored speculated to have natural anti-microbial and properties of sunscreen unlike whales.

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