Pregnancy Hormones May Have Shaped Human Brain Evolution

What if the secret to humanity’s extraordinary social intelligence lies not in ancient battlefields where males competed for mates, but in the quiet biochemical drama unfolding inside pregnant women?

A provocative new hypothesis suggests that the placenta—that temporary organ connecting mother and developing child—may have been the unlikely architect of our species’ cognitive abilities.

Researchers from Cambridge and Oxford universities are challenging Darwin’s original idea that human brain evolution was driven primarily by male competition. Instead, they propose that hormonal changes during pregnancy, particularly the placenta’s production of sex steroid hormones like testosterone and estrogen, fundamentally reshaped both our brains and our social behavior.

The Placental Powerhouse

The human placenta does something remarkable that sets us apart from our primate relatives—it produces exceptionally high levels of sex steroid hormones during pregnancy. Recent studies using “mini-brains” grown from human stem cells reveal that testosterone can increase brain size, while estrogens enhance connectivity between neurons.

“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,” explains Professor Graham Burton from Cambridge’s Loke Centre of Trophoblast Research.

But here’s where the story gets intriguing: the human placenta contains higher levels of aromatase, an enzyme that rapidly converts testosterone into estrogens, compared to other primates. This biochemical balancing act may have been evolution’s solution to a complex puzzle.

The Social Brain Paradox

Humans excel at something most primates struggle with—forming and maintaining large, cohesive social groups. Yet this ability presents an evolutionary paradox. Larger groups typically mean more competition for resources and mates, which should increase male aggression and reduce female fertility through stress and harassment.

Instead, humans show the opposite pattern. Compared to our closest relatives, human males are less aggressive, show reduced physical sexual dimorphism, and engage in less infanticide. Meanwhile, human females have evolved permanent secondary sexual characteristics and concealed ovulation—traits that signal long-term reproductive potential rather than immediate fertility.

The researchers propose this apparent contradiction resolves when we consider the placenta’s hormonal symphony:

• High prenatal steroid levels promote brain growth and neural connectivity
• Efficient conversion of testosterone to estrogen reduces male-typical aggression
• Enhanced estrogen signaling improves social cognition and bonding behaviors
• Modified hormonal patterns support stable, long-term reproductive relationships

From Womb to Social World

Dr. Alex Tsompanidis, who led this research at Cambridge’s Autism Research Centre, notes that “small variations in the prenatal levels of steroid hormones, like testosterone and oestrogen, can predict the rate of social and cognitive learning in infants and even the likelihood of conditions such as autism.”

This connection between prenatal hormones and social development isn’t merely correlational. The placenta’s steroidogenic system directly influences the developing hypothalamic-pituitary-gonadal axis, potentially creating transgenerational effects. Women with certain hormonal conditions can pass modified endocrine patterns to their daughters, suggesting that successful social adaptations could accumulate across generations.

Recent work with human brain organoids demonstrates that these hormonal effects are profound and specific. Testosterone administration increases cortical progenitor proliferation, expanding the neurogenic pool during critical developmental windows. Meanwhile, estrogens promote synapse formation, spine density, and astrocyte specialization—features that distinguish human neurons from those of other primates.

Rewriting Human Evolution

This placental hypothesis offers a strikingly different narrative from traditional evolutionary accounts focused on male competition and sexual selection. Instead of emphasizing differences between sexes, it highlights how hormonal systems evolved to benefit both males and females while enabling unprecedented social cooperation.

Professor Simon Baron-Cohen, who co-authored the study, reflects on two decades of research: “We have been studying the effects of prenatal sex steroids on neurodevelopment for the past 20 years. This new hypothesis takes this further in arguing that these hormones may have also shaped the evolution of the human brain.”

The implications extend beyond academic theory. If correct, this framework suggests that human cognitive abilities—our capacity for empathy, language, theory of mind, and complex social reasoning—emerged not despite our biological constraints, but because of sophisticated biochemical adaptations in pregnancy itself.

The Maternal Foundation

Perhaps most remarkably, this hypothesis places female biology at the center of human evolution. Rather than viewing pregnancy as a constraint on female participation in evolutionary progress, it positions the maternal-placental-fetal interface as the primary driver of our species’ most distinctive characteristics.

As Dr. Tsompanidis concludes, “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. 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.”

This reframing doesn’t diminish human achievement—it simply relocates its biological foundation from the competitive arena to the collaborative chemistry of human reproduction. Sometimes the most profound changes happen not in dramatic battles, but in the quiet, transformative work of creating the next generation.