Hormones produced in pregnancy may have been crucial to human brain evolution, researchers say

New Delhi: Sex hormones produced during pregnancy may have played a crucial role in human brain development, possibly by leading to behaviours that enabled societies to thrive and expand, researchers say.

Sex hormones, such as testosterone or oestrogen, are steroid-based and help with reproduction and developing body features specific to a gender. During pregnancy, sex hormones are produced in the placenta.

"The placenta regulates the duration of the pregnancy and the supply of nutrients to the foetus, both of which are crucial for the development of our species' characteristically large brains. But the advantage of human placentas over those of other primates has been less clear," co-author Graham Burton, from the University of Cambridge, UK, said.

Levels of sex hormones during pregnancy are known to be able to predict early development in infants, including chances of autism, prompting the team to examine how the hormones influence human evolution, according to lead author Alex Tsompanidis, a senior researcher at the University of Cambridge.

In a paper, published in the journal Evolutionary Anthropology, the authors argued that high levels of sex hormones in the womb during pregnancy, combined with a naturally improved function of the placenta, may have made human brains larger and more interconnected.

At the same time, lower levels of male sex hormones, including testosterone, to oestrogens may have led to reduced competition between males while also improving fertility in females, thereby allowing humans to form larger, more cohesive social groups, the team said.

Co-senior author Simon Baron-Cohen, director of the Autism Research Centre at the University of Cambridge, said that sex hormones in the mother during pregnancy are known to be important for neurodiversity in human populations.

The team's paper takes it further in arguing that these hormones may have also shaped how the human brain evolved, added Baron-Cohen.

"Our hypothesis puts pregnancy at the heart of our story as a species. The human brain is remarkable and unique, but it does not develop in a vacuum," Tsompanidis said.

"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," the lead author said.

The authors wrote, "(we) present a novel hypothesis for understanding human brain development and evolution, which emphasises the role of sex steroid hormones, their prenatal production by the placenta and their roles in regulating physiology, fertility and cognition."