Mammals, including humans, usually live on land. Experts say that this evolution of terrestrial animals has occurred at least twice in history. I.e., from Cetacea and Sirenia. Cetaceans, as we know them, include whales and dolphins, and cetaceans include dugongs and manatees. Whales are closest to the semi-aquatic-semi-terrestrial hippopotamus, while dugongs and manatees are closest to elephants.

How do whales adapt to the sea?
Remembering that all living things came from water, it can be said that some of these mammals returned to water. However, in order to adapt to the new marine environment, almost every organ of the animal must evolve. In the case of cetaceans, the forelegs became flippers and the hind legs disappeared to adapt to the movement of the sea. Visual and olfactory functions were also significantly reduced. In the case of the skin around their body, not only is it perfectly thick enough to protect the body from changes in body temperature and temperature, but it is also transformed to allow its cells to regenerate continuously.

A recent study in Current Biology analyzed the genomes of whales and hippos and revealed that there are genetic variations associated with the skin and that these are changes in their evolutionary process.

Similar and different whales and hippos
Cetacean skin is characterized by the absence of hair, sweat glands, or sebaceous glands, with the exception of some species that have scalp. Hippos have thick skin and, unlike cetaceans, have sparse hairs on the hair and sweat glands. The hippopotamus, which spends the day in the water and then comes to the ground at sunset, gives birth and lactation in the water, and hears sounds and understands its direction in the water, generally showing signs of adaptation to life in the water with cetaceans. Researchers believe that their common ancestor was a semiaquatic like the hippos.

If this is true, the researchers hypothesized that cetaceans and hippos have a normal skin evolution. To confirm this, researchers found skin cells of two species of hippopotamus, including the blue dolphin (Tursiops truncatus), the ghost whale (Escritius robustus), two species of cetaceans, the hippopotamus we know, and a relative of the pygmy hippo. (Choeropsis liberiensis) On the other hand, their genomes have been relatively analyzed.

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Whales have thick skin
First, they compared their skin cells to those of many terrestrial mammals, including humans, and found that the skins of cetaceans and hippopotamuses were very thick and devoid of sebaceous glands. In the structure of the skin layer, differences between cetaceans and hippopotamus species have also been observed.

At the same time, the researchers compared the genomes of cetaceans and hippos with the genomes of 63 other mammals and analyzed the inactivated genes in cetaceans and hippos. In addition to hippos and large dolphins, this includes the genes of the northern mink whale (Balenoptera actutorostrata), the sperm whale (Fister macrocephalus), and the killer whale (Orkinus orca).

As a result of the analysis, the researchers identified 38 genes that were inactive or completely extinct from cetaceans and hippos, nine of which were related to the skin or to skin appendages such as the sweat glands and hair roots. These include genes such as ALOX15, AWAT1, KPRP, and KRT2. In addition, the researchers identified a gene called ABCC11, which is inactivated only in cetaceans and is associated with the sweat glands in humans.

The skin features of whales and hippos do not come from a common ancestor
The researchers confirmed that the genes associated with normal dysfunctional skin in cetaceans and hippos have the same genetic mutation. To determine if skin-related evolution occurred in the common ancestor of these species, the analysis found that gene inactivation was caused by different genetic mutations.

To determine if each of these genes was inactivated, the researchers used modeling. As a result, they predicted that this genetic mutation would have occurred in the hippocampus about 37 million years ago and in the cetacean 49 million years ago. When dysfunctional genes were analyzed by dividing them into skin, hair root, and sweat glands, it was confirmed that the timing of each genetic mutation was slightly different. The research team interpreted this to mean that cetaceans and hippopotamuses each adapted to aquatic life through genetic mutations, and that the changes progressed in stages.

This is a study that examines changes in skin histology and genetics that occur when mammals that evolved from water and evolved on land went back into the water and presented evidence of subtle changes related to evolutionary features revealed macroscopically. The researchers said.

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