The more advanced mammals are possibly marsupials.

Mammalian evolution has been turned on its head, according to new research that indicates that marsupials are the most evolved mammals.

By estimating how the common ancestor of mammals reproduced and evolved, scientists are changing the long-held belief that marsupials are more primitive than placental animals.

Marsupials have long been considered the intermediate step in evolution between egg-laying and placental mammals, because they give birth to very underdeveloped young that resemble the embryonic state of the placenta.

However, new research has revealed that the ancestors of both groups were closer to the placenta than to the marsupials, meaning that the marsupials modified their way of reproduction more than the placental ones did.

The study published in Current Biologyanalyzed skulls during different stages of development in 22 species of living mammals. Delicate CT scans of 165 specimens helped the research team reconstruct changes in the skull of these species during this early stage.

Using this data, they estimated how the common ancestor of marsupials and placentals might have evolved and compared it with both groups to see which was the most similar.

Professor Anjali Goswami, Research Lead at the Museum and senior author of the study, says, “Using this large comparative dataset generated from the Museum’s historical collections, we have been able to turn on its head what we know about mammalian evolution.”

Using a massive dataset of CT scans of marsupials and placental samples ranging from fetuses to adults, the team measured how their skull shape changed through evolution and reconstructed how their ancestors evolved. This showed that marsupials had changed more from an ancestor than from placentals.

“For a long time, people have treated marsupials as ‘lower mammals,’ which represent the intermediate stage between placental mammals and egg layers,” Anjali explains. “It turns out that marsupials are the most evolved from the ancestral form.”

“As a member of the placental mammals, we often have this bias because our group is the group towards which evolution is directed, but that is not how evolution works.”

How did marsupial mammals and placentals evolve?

All mammals alive today can be divided into three groups: placental mammals, marsupials, and monotremes. They are easily distinguished by their breeding methods.

The largest group is the placental mammals, which give birth to live, well-matured young and make up about 95% of all living mammals, including humans.

Marsupials also give birth to young but a very short gestation period and so the offspring are very underdeveloped and must therefore be cared for by a parent in the pouch.

Monotremes are egg-laying mammals and are the smallest of the three groups. It contains only five species alive today in two families: platypus and echidnas.

All living mammals are believed to have descended from an egg-laying common ancestor that lived approximately 180 million years ago. Oxen, the group that includes both marsupials and placental mammals, is thought to have diverged from each other shortly after, around 160 million years ago.

Since therans evolved away from young birth via the ovum, live birth of an underdeveloped infant as in modern marsupials was originally thought to be the natural intermediate stage. However, this study found that this is not the case.

“What we can clearly show is that the marsupial method of evolution is the one that changed the most from the marsupial and placental ancestors,” says Anjali.

“How marsupials reproduce is not an intermediate form between egg-laying and placental mammals. It’s just a completely different way of evolution that marsupials have evolved.”

Why do marsupials give birth to underdeveloped offspring?

Placental mammals are born with four limbs and skulls already well formed, which grow in size as the animal gets older. The gestation period varies according to body size but can be up to 22 months in African elephants.

In contrast, marsupials are mainly born in an embryonic state. For example, red kangaroos give birth to a baby the size of a jillibean after only one month’s gestation before nursing their young for up to six months.

The hind limbs and skull of these children are not fully formed, but the forelimbs and oral bones are slightly more developed. Effectively, the animal has all the parts it needs to crawl through its mother’s fur to reach the milk-giving nipples and suckles, but no more than that. Many marsupials have a pouch that helps protect the underdeveloped young while they are in this vulnerable stage

But why marsupials evolved this reproductive strategy is still not fully understood.

“It has been suggested that surgery is a better strategy if you live with a lot of environmental instability,” Anjali explains. “Placental mammals have long gestation times, so if an animal goes through a period where resources dry up, both the mother and the offspring are likely to die because they are all inbreeding.”

“With marsupials, it’s a much less risky strategy because the mother can easily abandon them at a very early stage of development, so at least the mother can survive and try again later.”

While today about two-thirds of living marsupials are found in Australia, the oldest marsupials are believed to have originated in North America. From there, they spread across South America and eventually made the journey to Australia via Antarctica using overland connections.

However, several placental mammals also lived in South America during this time but did not seem to have managed to make the journey.

“One idea is that marsupials were better equipped to make this journey because of their more flexible reproductive system,” explains Anjali.

“So by extending evolution and making it more external to the mother, marsupials may be able to deal better with less stable environmental situations. But this is a guess and a hypothesis that needs to be tested.”

This story is republished with permission from the Natural History Museum. Read the original story here.