Most humans think of the placenta as something that gets tossed out after childbirth. In fact, its appearance millions of years ago was a significant evolutionary development that gave rise to the vast majority of mammals alive today, from bats to whales to humans.
Until now, scientists believed that placental mammals first appeared some 125 million years ago. At that point, they branched off from the lineage that developed into modern marsupials, which nourish their young in their pouches instead of through placentas. Yet a recent fossil find backdates that divergence by about 35 million years, showing that mammals with placentas, known as eutherians, shared the earth with dinosaurs much longer than previously thought.
The fossil, described in August in the journal Nature (Scientific American is part of Nature Publishing Group), belongs to a tiny, shrewlike creature known as Juramaia sinensis that roamed China 160 million years ago. It appears to be the oldest known ancestor of placental mammals, according to a research group led by Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History.
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Placental nourishment allows a more rapid and efficient transfer of nutrients from mother to offspring, which can result in faster brain development, larger mature brains and increased metabolic rate—all of which have had broad implications for the evolution of the behavioral and social complexity observed among today’s mammals.
The Juramaia fossil also yields important clues about the life of early eutherians. This animal appears to have been an insectivore, judging from the shape of its teeth, and it had robust forelimbs, which could have helped it to climb trees. This ability may have enabled it to take over as yet unexploited territory for both safety from predators and access to insects among the foliage. Any opportunity to reduce competition with other Jurassic mammals by staking out higher ground may have helped strengthen
Juramaia’s divergence from marsupials, leading it to become the ancestor of an extremely diverse group of animals.
The discovery of this fossil corroborates previous molecular studies, which estimated the divergence between eutherians and other mammals to have occurred around 160 million years ago—yet such estimations are often taken with a grain of salt until they are bolstered by hard physical evidence in the form of fossils. This is a prime example of the way in which multiple lines of scientific investigation can work together to draw robust conclusions and make well-supported hypotheses, thereby highlighting the value of integrative approaches to answering scientific questions.