DNA reveals surprising supergroups of mammals – 05/13/2023 – Reinaldo José Lopes

Deep time weirdness is one of those things that should make all of us perpetually drop-jaw. It is the most natural thing in the world, for example, to take a look at the animals around us—a cat here, a dog there, cows grazing, each monkey on its branch—and to imagine that there is something immutable about this variety of shapes and sizes. . Cats, dogs, cows and monkeys have been around “since the world began”, and certainly since there have been human beings capable of designating them by those names, in our language or any other.

Nothing, I repeat, is more natural—or more factually wrong. And not just because each of these modern animals has its own history of anatomical and behavioral transformations over millions of years (like dogs, whose great-great-grandparents were all wolves relatively recently).

The true source of astonishment lies in understanding that, going back into deep time, these infinite forms merge with each other, like grandchildren who disappear and go back to their grandparents if the film of human generations is rewound. The miracle of common ancestry, one of the cornerstones of evolution, means that when you rewind the film to a certain point, the distinction between “dogs” and “cats” (or between “humans” and “monkeys” no longer makes sense). ) because there was a time when they were the same thing, members of the same species, perfectly capable of mating with each other and producing offspring.

And here, again, just go back in time long enough for the thing to take on a science-fiction feel. That’s what came to mind when reading an article recently published in the specialized journal Science. In it, the team of researchers led by Nicole Foley of Texas A&M University used DNA data from 241 species of mammals alive today to reconstruct how their ancestors evolved from the Cretaceous period. That is, more than 100 million years ago, in the middle of the Age of Dinosaurs.

The study, which is one of the largest of its kind to date, confirms the primal kinship between mammals that, viewed side by side today, would look like a mere bag of cats (pardon the pun). In the depths of the Cretaceous, for example, the separation of the lineages of the so-called Euarchontoglires had not yet occurred — a group that today includes both primates, such as capuchin monkeys and humans, as well as rodents and rabbits (well, they are not rodents per se ; yes, it is confusing indeed).

Even more wildly eclectic is the large group of Scrotifera, which includes bats, carnivores and ungulates (cows, for example, but also whales), among others. My favorite, however, is the Atlantogenata group, which gave rise, on this side of the Atlantic (hence the name of the group), to armadillos and anteaters, while on the other side, in Africa and neighboring regions, it generated elephants, manatees and even more peculiar animals, such as the anteater.

It should be remembered that probably none of the characteristics we associate with these mammals today existed in the Age of Dinosaurs. These ancestors were all small in size and relatively general in anatomy, perhaps somewhat resembling rodents or shrews. DNA data suggest that these large groups initially separated from each other by geological processes, such as the one that made South America “detach” from Africa in the Cretaceous period.

The fragmentation of these supergroups into smaller groups, which, little by little, took the form of mammals more similar to those we know, seems to have been the result of the mass extinction that wiped out the dinosaurs 66 million years ago.

Without needing to compete with the giant reptiles, the surviving mammals inherited the world and gradually adapted to the most different ecosystems and lifestyles, from tree branches colonized by primates to the predatory lifestyle of the ancestors of wolves and felines. In addition to common ancestry, this is another constant in the evolution movie script: life’s ability to find a way out.

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