Pouched mammals, like kangaroos and koalas, have a radical evolutionary history that suggests they’re “more evolved” than previously thought, a new study says.
According to the new study, marsupials were once thought to be an evolutionary stepping stone between egg-laying mammals, the monotremes, like platypuses, and placental mammals, like humans. While modern science now recognizes that marsupials and placentals evolved from a common ancestor around 160 million years ago, the authors argue that marsupials retain a slight stigma from when they were classified as intermediaries.
By scanning the skulls of placental mammals and marsupials at different developmental stages, the researchers concluded that the developmental strategy of placental mammals — and not marsupials — is closer to that of their common ancestor, suggesting that marsupials are more likely to have evolved have evolved as placenta mammals since cleavage.
“They have a much more extreme evolutionary history compared to placentals, so the idea that they’re this half-animal or half-mammal is wrong,” says the study’s co-author Anjali Goswami (opens in new tab), a life sciences research director at the Natural History Museum in London, told Live Science. “In a way, they’re the more evolved or divergent group.”
The researchers published their findings in the journal on April 28 Current Biology (opens in new tab).
Related: Ancient saber-toothed marsupials had eyes unlike any other mammalian predator
Placental mammals have a variety of developmental strategies. For example, human babies are virtually helpless at birth, unable to walk, while zebra foals are mobile within hours, according to the book “Encyclopedia of Child Behavior and Development (opens in new tab)(Springer, 2011). However, no placental newborns are as immature as marsupial offspring. Marsupials give birth to fetus-like embryos, which climb from the birth canal to their mothers’ pouches to complete development, according to the San Diego Zoo (opens in new tab).
For the new study, researchers created 3D images of 165 mammalian skulls, from fetus to adult, across 22 species. Then they placed points on the images that served as 3D coordinates to capture the general skull shape and determine how the skulls evolved in each species. Finally, they compared this evolution between marsupials and placental mammals to what they estimated for their hypothetical common ancestor.
The evolution of the skull of the placenta was more similar to that of the predicted ancestral mammal than the evolution of the marsupial skull. This led the authors to hypothesize that the common ancestor evolved like placenta and that the extreme marsupial strategy of terminating pregnancy in a mother’s pouch came later.
First Author heather white (opens in new tab), a postdoctoral fellow at the Natural History Museum, told Live Science in an email that marsupials have experienced a slowdown in skull growth compared to placental mammals and the ancestral mammal; Hence, it is the marsupial strategy that has changed more from the ancestral state. “It really puts marsupials in a new light, which is very exciting,” added White.
Gregory Funston (opens in new tab)a paleontology postdoctoral fellow at the Royal Ontario Museum in Canada, who was not involved with the study, told Live Science in an email that the new research leads to a major misconception that historically has shaped much research involving marsupials were considered as less successful intermediates.
“I’m really impressed with the study and hope it will help change the way we think about marsupials as much as I think,” said Funston. “Of course, we’ve known for a long time that they aren’t intermediates, but the study by White and colleagues convincingly argues that marsupials do have a highly specialized developmental pattern.”
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