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TMillions of years ago, whales were a little different from what they are today. Instead of sneaking into the water, they had their paws and hooves and walked on dry land – though some sometimes took a water trip. But the transition from the peasants on the ground to the inhabitants of the water did not happen overnight. The order of marine mammals called cetaceans, which includes whales, dolphins and porpoises, have evolved over millions of years to lose their legs and swim full time.
Cetaceans have had to undergo serious body changes to survive in deep dives under the waves. Progress of science shows that these changes are appearing in the DNA of modern cetaceans, offering new clues as to what they had to trade for a life spent at sea.
In a study published Wednesday, a team of researchers showed that when the ancestors of modern cetaceans separated from the ancestors of hippos, 85 different genes were disabled. These genes supported all sorts of crucial functions for life on Earth, such as wound healing and saliva production.
But for cetaceans to leave, they released these genes in favor of more appropriate adaptations.
"To store and retain oxygen effectively during extended snorkeling, cetaceans have developed various adaptations," write the study's authors, led by evolutionary genomics Michael Hiller, Ph.D., of Max Planck Institute of Cell Biology and Genetics. in Germany.
Among these adaptations, there were larger oxygen stores – result of increased blood volume and more concentrated blood – as well as a breathing system that allows animals to breathe out quickly when they surface, and soft ribs that allow the lungs to collapse.
For anyone familiar with human anatomy, this last adaptation may seem extremely serious and potentially life threatening. But for cetaceans, folding lungs are essential for deep dive and do not rise to the surface.
Moreover, while ancestors of cetaceans produced saliva, this was not necessary for animals that swallow huge amounts of water with their meals. So they just do not produce it anymore.
They have also disabled a gene involved in blood clotting, which prevents dangerous blood clots that could cause strokes when whales dive and resurface quickly.
For each of these adaptations, the researchers identified inactivated genes during the evolution of cetacean evolution. In total, they found 85, of which 62 had never been reported before.
The team explains that these genetic losses seem to have two functions: first, they are associated with hyper-specific adaptations – like the pulmonary thing – that help aquatic animals to develop and compete. And secondly, they seem to be crucial elements of the transition from land to sea.
Thus, on the one hand, these genetic losses make cetaceans more effective as swimmers and inhabitants of the water, while, on the other hand, these genetic losses are only real cetaceans.
"More generally, our study highlights the significant genomic changes that occurred during the transition from land to sea in the cetacean lineage and thus helps to understand the molecular determinants of their remarkable adaptations," they write.
After all, there are not only complex evolutionary stories. They are the result of survival, mutation, sexual reproduction and all the environmental factors that influence these things.
In short, whales have not just decided to jump into the water and drop their hooves. They have evolved slowly, over millions of years, to become the consecrated marine mammals that they are today.
And now, we know that this story is written in their genes.
Abstract: The transition from land to water in whales and dolphins (cetaceans) is accompanied by remarkable adaptations. To reveal the genomic changes that occurred during this transition, we selected genes coding for inactivated proteins in the ancestral lineage of cetaceans. We found 85 gene losses. Some of them were probably beneficial to cetaceans, for example, by reducing the risk of thrombus formation during the dive (F12 and KLKB1), faulty DNA damage repair (polm), and pulmonary inflammation induced by oxidative stress (MAP3K19). Additional genetic losses may reflect other adaptations related to diving, such as increased vasoconstriction during the response to diving (induced by SLC6A18) and a modified pulmonary surfactant composition (SEC14L3), while the loss of SLC4A9 concerns a reduced need for saliva. Finally, the loss of melatonin synthesis and receptor genes (AANAT, ASMT, and MTNR1A / B) may have been a prerequisite for adopting hemispheric sleep. Our results suggest that some genes lost in ancestral cetaceans have probably been implicated in adapting to a fully aquatic lifestyle.
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