We tend to look at evolution as linear. No wonder when the image of it is often simplified this way: a fish comes out of the water, turns into a reptile, then a mammal, a primate, and finally a human. This image is inaccurate and kangaroos may be proof of that.
More evolved. A study has just questioned the idea (quite widespread among experts) that marsupials (Marsupialia) are less evolved mammals than other animals in their class, closer to oviparous animals. The most recent evidence, however, indicates otherwise.
“For a long time, people have treated marsupials as ‘lower mammals,’ representing an intermediate stage between placental mammals and egg-laying mammals,” said Anjali Goswami, a researcher at the Natural History Museum in London and author of a new study on the matter. “It turns out that marsupials are the most evolved from the ancestral form.”
The most striking characteristic of marsupials has to do with their early development and it is precisely this that made biologists see this subclass of mammals as less evolved animals, somewhat similar to oviparous mammals, the monotremes (Monotremata), like the platypus.
A gestation in two acts. Marsupials have short gestation periods, after delivery, the newborn is housed in a marsupial bag where it has access to the mammary glands and which acts as an incubator for a while until the offspring reaches a certain stage of development, although in each species It may present certain variations in the length of this period.
Until now it was believed that marsupials were a species of intermediate stage between placental mammals, the group that includes most of the species of the family, including humans; and monotremes, the oviparous minority group, which includes platypuses and echidnas (Tachyglossidae).
All mammals would have descended from a last common, oviparous ancestor, who lived about 180 million years ago. There the monotremes would have separated from the terios (Theria). The division between these theria into placental and marsupial mammals would have occurred about 160 million years ago. In this common ancestry is where the key to the new study lies.
Evolution in 22 species. The new study carried out by Goswami and his team studied 165 specimens of 22 different species of mammals to better understand the evolution of pregnancy and childbirth of these animals. Details of the study were published in an article in the journal Current Biology.
They performed computerized microtomographies on specimens of different ages and species and thanks to this process, the researchers were able to get an idea of the characteristics of the common ancestors of the therians.
According to the research, the common ancestor would have been more similar in its gestational and vital development to placental mammals than to marsupials. This would imply that marsupials would have seen more radical changes in their evolution compared to placental mammals.
evolutionary advantage. The question of what is the advantage that this system offers remains to be answered. The researchers speculate that the marsupial system is effective in contexts of scarcity. A pregnancy is uninterrupted for placental pregnancies, while marsupials could shed their young earlier in hard times. That is, it would allow the cubs to be sacrificed to save the mothers in such contexts.
If this hypothesis is correct, this advantage could have been decisive in allowing marsupials to reach areas of Oceania where no other mammal was able to reach.
This is not my last form. There are two issues that tend to be misunderstood when talking about evolution. The first is to associate it with a linear process, when species disintegrate rather like branches on a tree. It is also common to see successive changes as improvements or advances rather than adaptations. It is not the best that survives, but the one that best adapts. That’s why it exists
A clear example is that of carcinization, an evolutionary tendency among crustaceans to evolve towards the classic shape of a crab. This evolutionary trend could invite us to think that this form has some absolute advantage in evolutionary terms when it is simply an adaptation to the environment. When these changes no longer make evolutionary sense, species can reverse them or evolve in a new direction.
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Image | carles rabada
