Pregnancy May Be the Hidden Force Behind Human Intelligence, Study Finds

A new study from the University of Cambridge and the University of Oxford suggests that the placenta and the hormones it makes during pregnancy may have helped human brains grow larger. These hormones may also have helped people become more social, which made it easier for humans to live and work in large groups.

The study was published in the journal Evolutionary Anthropology.

The researchers say that small changes in hormones before birth, like testosterone and estrogen, can affect how babies grow, learn, and behave. These hormones might even be linked to conditions like autism.

“Small variations in the prenatal levels of steroid hormones, like testosterone and estrogen, can predict the rate of social and cognitive learning in infants and even the likelihood of conditions such as autism,” said Dr. Alex Tsompanidis, the lead author from Cambridge.

Professor Robin Dunbar from Oxford added, “We’ve known for a long time that living in larger, more complex social groups is associated with increases in the size of the brain. But we still don’t know what mechanisms may link these behavioral and physical changes in humans.”

To learn more, scientists looked at tiny brain-like cell groups called “mini-brains” grown from human stem cells. They found that testosterone can help the brain grow bigger, while estrogen helps brain cells connect better.

The placenta is an organ that connects a mother to her baby during pregnancy. It helps send hormones and nutrients to the baby and controls how long the pregnancy lasts. This is important for growing bigger and smarter brains in humans.

“The placenta regulates the duration of the pregnancy and the supply of nutrients to the fetus, both of which are crucial for the development of our species' characteristically large brains,” said Professor Graham Burton from Cambridge.

Past studies show that humans have more estrogen during pregnancy than other primates. Humans also have more of an enzyme called aromatase, which turns testosterone into estrogen. This may help explain why the differences between men and women are smaller in humans compared to other species.

“Our hypothesis puts pregnancy at the heart of our story as a species,” said Dr. Tsompanidis. “Adaptations in the placenta and the way it produces sex steroid hormones may have been crucial for our brain’s evolution and for the emergence of the cognitive and social traits that make us human.”

Professor Simon Baron-Cohen, co-author and director of the Autism Research Centre, said, “We’ve been studying the effects of prenatal sex steroids on neurodevelopment for the past 20 years. This new hypothesis takes it further, suggesting these hormones may have shaped how the human brain evolved.”