Geneticists have found fragments of genes responsible for the assimilation of chitin in the DNA of humans and all other mammals, according to an article published in the journal Science Advances.
According to experts, this unequivocally convinces us that our most ancient ancestors, who lived at the end of the era of dinosaurs, were insectivorous creatures.
“Only insectivorous mammals that feed exclusively on invertebrates have all five genes associated with chitinase production. Our study shows that the first placental mammals also possessed this feature. All of this indicates that beetles and other invertebrates formed the basis of the diet of our early ancestors,”said Christopher Emerling of the University of California at Berkeley (USA).
At the dawn of evolution
The timing of the first mammals is still a mystery to paleontologists. Most scientists believe that the first warm-blooded animals appeared already 220 million years ago, in the middle of the Triassic period, simultaneously with the first primitive dinosaurs. It is not entirely clear whether mammals lived in this era on all continents or whether they spread across the Earth after the extinction of the dinosaurs 65 million years ago.
The first primitive mammals, judging by the structure of their teeth, were relatively small in size and ate uniformly, mainly insects and fruits. Therefore, scientists believed for a long time that the boom in their evolution began only after the dinosaurs became extinct, which did not allow them to occupy new ecological niches.
These animals, as Emerling notes, were not the direct ancestors of man - almost all of them either died out along with the dinosaurs or were pushed to the "margins of evolution" immediately after the beginning of the modern geological era. Their place was taken by the first placental mammals, which appeared much later, about 160-145 million years ago, as indicated by the remains of two ancient animals found in China in 2002 and 2011.
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Most scientists believe that the first placental animals, like their marsupial counterparts, ate mainly insects, but today there is a lot of controversy on this score. Some paleontologists suggest that the transition to intrauterine development could have allowed our ancestors to develop new niches even before the extinction of dinosaurs and the beginning of the Cenozoic boom in mammalian evolution.
Genetic records
Emerling and colleagues tested these theories. They suggested that if all mammals descended from one insectivorous ancestor, then at least traces of genes responsible for the digestion of chitin, the main component of the shell of all invertebrates, should have been preserved in their DNA.
Taking as a sample the CHIA gene, which plays a similar role in the body of aardvarks and other insectivorous mammals, scientists tried to find its analogues in the DNA of over a hundred species of modern animals, including humans, bears, elephants and other creatures that do not feed on invertebrates.
As it turned out, the genomes of all placental mammals contain not one, but five copies of this gene. In most animals, almost all of these DNA regions are damaged, and only in armadillos, aardvarks, and some insect-eating lemurs do all five genes work.
Such a picture, as noted by Emerling, confirms the current views of paleontologists and allows us to calculate the approximate time of the appearance of the first herbivorous and carnivorous mammals. They, as shown by a comparison of "scraps" CHIA, appeared about 67, 56 and 53 million years ago, about 10 million years after the extinction of the dinosaurs.
This, again, is consistent with the excavation data and suggests that other "fossil genes" hidden in the DNA of animals and humans will allow us to learn a lot about the evolution of life or to resolve other disputes related to the origin of humans and other inhabitants of the Earth.