A new study challenges the idea that marsupials are more “primitive” than mammals by showing that their development has changed more than mammals since they last shared an ancestor.
“For a long time, people have treated marsupials as ‘small mammals,’ which represent the intermediate stage between placental mammals and egg-laying birds,” says evolutionary biologist Anjali Goswami of the Natural History Museum in the UK.
“It turns out that marsupials are the ones that are much more evolved compared to the ancestral form.”
This now outdated idea arose from the fact that marsupials, like kangaroos, give birth to young still in the early stages of development. This leaves their babies to do most of their development in pouches, apparently a more complicated stage than egg-laying monotremes like platypus do, but still simpler than the development of placental mammals.
Placental mammals – like us whales and hedgehogs – are born more developed after much longer gestation periods.
“As a placental mammal, we often have this prejudice that ours is the group that evolution is headed towards, but that’s not how evolution works,” Goswami says.
Evolution is often misrepresented as a direct path to greater complexity, where species with older traits are seen as simpler or “primitive”.
But genetics and a greater awareness of unusual traits demonstrate that complexity can be gained and lost many times over a species’ evolutionary journey. Moreover, larger evolutionary changes do not always lead to more diversity, because there is much more variation in placental mammals than in marsupials, the researchers point out.
Using specimens from the different developmental stages of 22 currently living species, Natural History Museum developmental biologist Heather White and her colleagues constructed a timeline of change that best explained what exists now.
The highly detailed set of 3D micro-scans revealed many other changes in the development of marsupial skulls. Specifically, skull development is slowed and lagged in marsupials, compared to placental mammals and our common Therian ancestor of 160 million years ago.
The results suggest that the extremely young age of marsupial births represents the most specialized type of development that requires greater trait changes from our common ancestor than our strategy of allowing the young to develop within us longer.
Other studies have hinted that this may be the case, with signs that placentals and marsupials significantly changed placentas from the yellow placenta of our common ancestor.
Additionally, a group of extinct mammals that split off from the rest of the mammalian branch of life before placentals and marsupials existed are no longer reproducing like us too.
“What we were able to show clearly is that the mode of development of marsupials is the one that has changed the most from the ancestor of marsupials and placentals,” says Goswami.
“The way marsupials reproduce is not an intermediate form between egg-laying and placental mammals. It’s just a completely different way of developing that marsupials evolved.”
You wouldn’t know it, but this endangered northern quoll is packing eight babies – hairless, pink “jellybeans.” She will wear them for a few more months before they leave for the great wilderness. #wildoz #quollpatrol pic.twitter.com/W3umKj0m1k
— Judy Dunlop (@Judy_Dunlop) October 9, 2018
However, since White and his colleagues were only able to include 22 species, more research is needed to support their conclusions. There were some limitations in estimating ancestral status due to the lack of examples of early platypus development that the team qualifies.
Either way, while comparing current traits with ancestral species allows us to draw evolutionary connections and timelines, it doesn’t indicate how “advanced” or “advanced” a species is. Instead, the changes most likely reflect environmental requirements or lack thereof, as is likely the case here.
“It has been suggested that the marsupial strategy is better if you live with a lot of environmental instability,” says Goswami. “Placental mammals have long gestation times, so if an animal goes through a period where resources dry up, both mother and offspring will likely die because it’s all internal.”
“With a marsupial, it’s a much lower risk 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.”
This may explain how marsupials made it all the way to Australia from their origins in North America while their placental neighbors did not, back when those continents were connected to landmasses.
“One idea is that marsupials were better equipped to make this journey due to their more flexible reproductive system,” Goswami speculates.
“So by stretching development and making it more external to the mother, marsupials might cope better with less stable environmental situations. But that’s really a guess and a hypothesis that needs to be tested.”
This research was published in Current biology.