How Placental Mammals, Like Whales and Humans, Evolved Bigger Brains and an Evolutionary Advantage

• Placental mammals have a placenta, an organ that develops during gestation and bridges mother and fetus.

• The placenta provides nutrients to the fetus to develop in the womb, protecting the baby from the outside world until they are fully developed.

• While we don’t know the first placental mammal for sure because of their low preservation potential, we do know that around 55 million years ago, 10 million years after the extinction of the dinosaurs, placental mammals exploded in numbers while marsupials decreased.

Placental mammals have, you guessed it, a placenta, the important organ that develops during gestation and acts as a bridge between mother and fetus. It provides the nutrients that the fetus needs to survive in the womb and allows a species’ young to live inside their mother until such time as they are ready to be born. This means that babies are protected from the outside world until they are fully developed.

Humans, like most mammals, have a placenta that they use to grow their young. Animals, including mice, dogs, cows, bats, and orcas, all have placentas.

This is different from marsupial mammals, which include opossums, kangaroos, wombats, and koalas. They are provided a safe place to grow inside their mother’s pouch after birth until they are fully developed.

Read More: When the Dinos Died, Mammals Were Already Adopting Terrestrial Lifestyles

It’s difficult to know the first placental mammal because placentas are soft tissue and don’t fossilize. The bones, which do preserve, don’t tell us if an animal held its young inside the body.

“Placentas are features that are difficult to know from the fossil record,” says mammalian paleontologist Ornella C. Bertrand.

To discern whether a mammal had a placenta, a pouch, or something else entirely, researchers have to look at other placental mammals and compare features that do fossilize, for example, the teeth or the morphology of the body. This can also help scientists to decide which mammals were somewhere in between.

The first group of evolving placental mammals was a stem group in the midst of evolving into what would become a placental mammal. Eomaia was the first of this group, and according to a study published in Nature, the eutherian specimen had limb and foot features that would be useful for climbing and living in the trees, in contrast with the running features of the earlier non-placental Cretaceous eutherians. 

Another early candidate, Juramaia, lived during the early Jurassic in China. It had similar dental morphology to Eomaia, however, the actual age of the specimen has been questioned in newer research.

While we don’t know the first placental mammal for sure because of their low preservation potential, we do know that around 55 million years ago, 10 million years after the extinction of the dinosaurs, placental mammals exploded in numbers while marsupials decreased.

Carnivores, rodents, and herbivores, all with placentas, became the most important forms of mammals. Most of the initial groups have gone extinct, with the exception of primates, whales, and rodents.

After their numbers exploded, they grew larger, and eventually, their brains also expanded. Bertrand wrote in a 2022 Science study that their body size came before brain size around the  Eocene Epoch.

It was only later, “likely driven by a need for greater cognition in increasingly complex environments,” says Bertrand, that these creatures needed bigger brains to control their bodies.

The benefit of a placenta is shown by the fact that today there are around 5,000 placental mammals versus 334 marsupials. 

“There must have been an advantage to having a placenta,” says Bertrand.

It could also be why some placental mammals have such complex brains. They have more time to gestate and form the big brains needed to operate their bigger and more complex bodies.

We don’t know for sure, but it could be that placental mammals had more of a fighting chance because they didn’t have to compete with so many species of dinosaurs. However, it’s worth noting that marsupial mammals were doing well before the asteroid hit and then not so well afterward.

It could also be that after the devastation that followed the asteroid's hit 66 million years ago, placental mammals were not only still tiny and potentially better able to burrow underground, they also had months to form the next generation inside the womb, rather than deal with the nuclear winter that was occurring on their doorstep.

Read More: Was the First Mammal to Live on Earth the Morganucodon or Brasilodon? Experts Still Debate

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Britannica. placental mammal

• Nature. The earliest known eutherian mammal

• Molecular Phylogenetics and Evolution. Timing and biogeography of the eutherian radiation: fossils and molecules compared

• Science. Brawn before brains in placental mammals after the end-Cretaceous extinction

• Wiley. Eutheria (Placental Mammals)

Sara Novak is a science journalist based in South Carolina. In addition to writing for Discover, her work appears in Scientific American, Popular Science, New Scientist, Sierra Magazine, Astronomy Magazine, and many more. She graduated with a bachelor’s degree in Journalism from the Grady School of Journalism at the University of Georgia. She's also a candidate for a master’s degree in science writing from Johns Hopkins University.