A new study has shown that marsupial mammals, such as kangaroos and koalas, have a longer evolutionary history in terms of the mechanism of pupping.
Heather White from the Natural History Museum in London and colleagues found that marsupials have evolved further than mammals with placentas, such as humans.
Modern science recognizes that marsupials and placentals evolved from a common ancestor around 160 million years ago.
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But the new findings, published in the peer-reviewed scientific journal Current Biology, show that the development of mammals with placentas is closer to that of their common ancestor.
Mammals with placentas have a range of different developmental tactics. For example, human babies are almost helpless at birth, while zebra foals can start walking within hours.
But none of these mammals is as immature at birth as a marsupial. This is because marsupials actually give birth to fetus-like embryos. These embryos climb up the birth canal into their mother’s pouch and complete development there.
In the new study, White and his colleagues wondered which offspring development tactic has gone through more evolutionary processes.
To find the answer, the researchers examined the skulls of placentate mammals and marsupials at various stages of development. In total, three-dimensional images of 165 mammal skulls were created.
The animals studied included 22 species. Their maturity levels ranged from the fetal stage to adulthood.
As a result of this analysis, the developmental differences between marsupials and mammals with placentas were compared with previously established scientific models for their hypothetical common ancestor.
The cranial development of placentals more closely resembled that of the ancestor of mammals. This led the researchers to speculate that the common ancestor’s offspring developed like mammals with placentas, and that the marsupial developmental tactic came later.
In other words, the research team concluded that marsupials had evolved more than mammals with placentas since the time they diverged from the common ancestor.
“The research sheds more light on marsupials, which is very exciting,” White told Livescience.
