Many manuals on natural history indicate that, based on known fossils, it is estimated that marsupials originated approximately 130 million years ago, while placental mammals did approximately 110 million years ago. The discovery of a 160-million-year-old fossil in China in 2011, known as Juramaia, changed the history of the separation between marsupials and placental mammals. The most established scientific theory to date holds that all living mammals descended from a common ancestor that lived approximately 180 million years ago and reproduced by laying eggs. Therians, the first non-egg-laying mammals, a group that includes marsupials and placental mammals, arose about 160 million years ago. However, this segment of history is still open and subject to discussion, how could it be otherwise.
A team led by researcher Anjali Goswani from the Natural History Museum in London (United Kingdom) and first coauthor Professor Heather E. White has now studied this dilemma from another perspective: mammalian paedomorphosis and marsupials. With this really bad sounding word (paedomorphosis or paedomorphosis) specialized scientists in developmental biology define the study of the characteristics of individuals that live from the juvenile to adult stage.
Specifically, Goswani and White’s team analyzed the skull size and volume of 165 specimens from 22 species of mammals and consequently proposed the hypothesis that placental mammals are closer to the common ancestor of egg-laying mammals than marsupials. are close.
In other words, this team believes that marsupials may actually be a more recent form of mammal than placental (contrary to what the scientific community has thought to date).
The informative note of this study, published by the Museum of Natural History, is accompanied by an even more fascinating title: “Marsupials may be the most evolved mammals”. The scientific results are presented in an article published in the journal Current Biology, with a title befitting its scientific seriousness: “Pedomorphosis in the Ancestors of Marsupial Mammals.”
study of the skull in detail
Microcomputed tomography (micro-CT) scans of 165 specimens helped the research team reconstruct changes in the skull of these species during this early stage of development. Using this data, they estimated how the common ancestor of marsupials and placentals might have evolved and compared the two groups to see which was most similar. “Using this large set of comparative data generated from museum historical collections, we are able to propose changes to what we know about mammalian evolution,” he explains. Anjali Goswami In a note released by the museum where she works as a researcher and leader of the group.
The team measured how its skull shape changed throughout evolution and reconstructed how its ancestors might have evolved. This showed that marsupials have changed more from the placenta than from the common ancestor.
Anjali Goswami explains, ‘For a long time, people regarded marsupials as ‘minor mammals’, representing an intermediate stage between placental and egg-laying mammals. After new analysis, it has been proposed that “marsupials are those that are much more evolved than the ancestral form.”
“As a member of placental mammals, we often have [los humanos] The prejudice that we are the group that development is targeting, but that is not how development works,” recalls this expert.
real growth of our family
All mammals alive today can be divided into three groups: placental mammals, marsupials, and monotremes (egg-layers, like the platypus), which are easily distinguished by their methods of reproduction, the authors practically reminded .
The largest group (about 95% of all living mammals, including humans by current number of species) are placental mammals, which give birth to live, well-developed young.
Marsupials also give birth to live young, but their gestation period is very short, so the young are very underdeveloped and must be cared for by the parents in a pouch.
Monotremes are egg-laying mammals and are the smallest of the three groups. It has only five species alive today in two families: the platypus and the echidna.
As therians developed from puberty through an egg, the live birth of an undeveloped infant was originally thought to be a natural intermediate stage, as in modern marsupials. However, the study now being published suggests that this explanation may not be correct.
“We were able to clearly show that the form of evolution of marsupials changed most from the ancestor of marsupials and placentals,” says Professor Anjali Goswami.
“The way marsupials reproduce is not an intermediate method between egg-laying and placental mammals; It’s a completely different form of evolution that marsupials evolved.”
explanation of premature birth
Placental mammals are born with all four limbs and skull already well developed, which increase in size as the animal ages. The gestation period varies depending on body size, but can last up to 22 months in African elephants.
In contrast, marsupials are born essentially in an embryonic stage. For example, red kangaroos give birth to jelly bean-sized young just one month after conception and take care of their young for six months.
The back legs and skull of these babies are not fully formed, but the bones of the front legs and mouth are slightly more developed. In fact, the animal has all the organs it needs to crawl through its mother’s fur to reach and suck milk nipples, but not much else. Many marsupials have a pouch that helps protect the underdeveloped young when they are in this vulnerable state.
due to development
“It has been suggested that the marsupial strategy is preferable if you live with a lot of environmental instability,” explains Anjali in a note from the Natural History Museum. “Placental mammals have a long gestation period, so if an animal goes through a period where resources are depleted, both mother and offspring are likely to die because it’s all internal.” “With a marsupial, this is a very low-risk strategy because the mother can easily abandon it at a very early stage of development, so at least the mother can survive and try again later.” “So by extending development and making it more external to the mother, marsupials may be better able to cope with less stable environmental conditions. But this is largely conjecture and a hypothesis that needs to be tested,” he said. Acknowledges the leader of the published study.
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