Mammals, including humans, usually live on land. This evolution of land-dwelling animals to sea life has occurred at least twice in history, experts say. That is, from Cetacea and Sirenia. Cetaceans include things like whales and dolphins as we know them, and dugongs and manatees belong to cetaceans. Whales are phylogenetically closest to the half-aquatic half-terrestrial hippopotamus, while dugongs and manatees are closest to elephants.
How did whales adapt to the sea?
Recalling that all living things came from water, it could be said that some of these mammals returned to water. However, in order to adapt to the new environment of the sea again, almost all organs of animals must evolve. In the case of cetaceans, to adapt to movement in the sea, the front legs became flippers and the hind legs disappeared. Visual and olfactory functions also declined significantly. In the case of the skin surrounding their body, it is not only quite thick to protect the body from physical and temperature changes, but its cells have also been changed to continuously regenerate.
A study recently published in ‘Current Biology’ analyzed the genomes of whales and hippos, revealed that they had skin-related genetic changes, and revealed that these appear to be changes in their evolutionary processes.
Similar but different whales and hippos
Cetacean skin is characterized by the absence of hair, sweat glands, or sebaceous glands, with the exception of some species having whiskers on their heads. Hippos also have thick skin, and unlike cetaceans, they have sparse hairs on their fur and sweat glands. The hippopotamus, which spends the day in the water and then comes to the ground at sunset to graze, gives birth and breastfeeding in the water, and hears sounds and senses its direction in the water, showing signs of adaptation to life in the water in common with cetaceans. There is this. Researchers believe that their common ancestor was semiaquatic, like hippos.
If this were true, the researchers hypothesized that cetaceans and hippos had a common skin evolution. To confirm this, the researchers found the skin tissue of two species of hippopotamus, including the blue dolphin (Tursiops truncatus) and the ghost whale (Eschrichtius robustus), two species of cetacean, and the hippopotamus we know and a relative called the pygmy hippo (Choeropsis liberiensis). On the other hand, their genomes were comparatively analyzed.
Whales have thicker skin
First, they compared their skin tissue with that of several land mammals, including humans, and found, as expected, that the skins of cetaceans and hippopotamuses were very thick and free of sebaceous glands. In the structure of the skin layer, differences between cetaceans and hippopotamus species were also observed. The skin of cetaceans is much thicker and the entire skin layer is thick from face to tail, whereas hippos have thick facial skin, but the skin on the tail and ears is thicker. It was thin and showed different patterns depending on the body part.
Meanwhile, by comparing the genomes of cetaceans and hippos with the genomes of 63 other mammals, the researchers analyzed the genes that were inactivated in cetaceans and hippos. These included the genomes of the northern minke whale (Balaenoptera acutorostrata), sperm whale (Physeter macrocephalus) and killer whale (Orcinus orca), in addition to hippos and large dolphins.
As a result of the analysis, the researchers identified 38 genes that were either inactivated or completely disappeared from cetaceans and hippos, nine of which were related to the skin or skin appendages such as sweat glands and hair roots. These are genes such as ALOX15, AWAT1, KPRP, and KRT2. In addition, the researchers also identified the gene ABCC11, which is inactivated only in cetaceans and is known to be associated with sweat glands in humans.
The skin characteristics of whales and hippos do not come from a common ancestor
The researchers confirmed whether the common inactivated skin-related genes in cetaceans and hippos had the same genetic mutation. This is to determine whether skin-related evolution occurred in the common ancestor of these species, and the analysis revealed that the gene inactivation was caused by different genetic mutations.
To determine when each of these genes was inactivated, the researchers used modeling. As a result, they predicted that this genetic change would have occurred around 37 million years ago in the hippocampus and 49 million years ago in cetaceans. When the inactivated genes were analyzed by dividing them into skin, hair root, and sweat glands, it was also confirmed that the timing of each genetic change was slightly different. The research team interpreted this to mean that cetaceans and hippopotamuses each went through genetic changes to adapt to life in the water, and the changes progressed in stages.
This is a study that explores changes in the skin histology and genetics that occurred when mammals that had adapted from water and evolved on land went back into the water and presented microscopic changes related to macroscopically revealed evolutionary features as evidence. The researchers emphasized.