The Placenta Could Be The Hidden Force Behind Human Brain Evolution, Scientists Propose

CAMBRIDGE, England — New research suggests that the unique way human placentas produce sex hormones during pregnancy may have been a key driver behind our species’ extraordinary brain development and social behavior.

A team of researchers from the University of Cambridge and the University of Oxford argues that instead of male competition driving human brain evolution, adaptations in female reproductive physiology may have powered our cognitive advancement. Their review, published in Evolutionary Anthropology, reveals that human placentas produce exceptionally high levels of sex steroid hormones compared to other primates, and these hormones appear to directly influence brain growth and social development.

The proposed “Placental Steroid Hypothesis” could explain why humans developed larger, more connected brains while simultaneously becoming less sexually dimorphic than our primate relatives. Rather than males evolving bigger brains to compete for mates, both sexes may have benefited from hormonal changes that emerged to support large social groups and reduce male aggression.

“Our hypothesis puts pregnancy at the heart of our story as a species,” says the paper’s lead author Dr. Alex Tsompanidis, a senior researcher at the Autism Research Centre at Cambridge, in a statement. “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.”

Human Evolution Defied Expectations

Contrary to popular assumptions about human evolution, modern humans actually show reduced sex differences compared to our closest relatives. Men and women are more similar in body size, muscle mass, and facial features than males and females in most other primate species.

The research team highlights that anatomically modern humans are “characterised by reduced male‐type dimorphism and increased female-type dimorphism, compared to other hominids.”

Human males show less reactive aggression compared to other primates like chimpanzees. Meanwhile, human females evolved more permanent secondary characteristics, like enlarged breasts, that signal reproductive maturity independently of fertility cycles.

The Placental Hormone Factory

Human placentas are highly specialized among primates in their ability to produce massive quantities of sex steroid hormones during pregnancy. The paper notes that human pregnancies produce substantially higher gestational estrogen peaks compared to other apes, with placental hormone production reaching “unprecedented concentrations.” These hormones don’t just support pregnancy—they directly influence fetal brain development.

During pregnancy, the placenta “has evolved into a specialized steroidogenic hub,” creating a unique hormonal environment for the fetus. These hormones cross into the developing baby’s brain, where they can influence everything from neuron proliferation to the formation of social behavior circuits.

Both male and female fetuses are exposed to these high hormone levels, though males experience an additional brief surge from their own developing testes. This largely shared hormonal environment may help explain why human cognitive abilities show significant overlap between the sexes.

“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,” notes co-author Graham Burton, Founding Director of the Loke Centre of Trophoblast Research at Cambridge. “But the advantage of human placentas over those of other primates has been less clear.”

Brain Building and Social Evolution

Recent experiments using human brain organoids (lab-grown mini-brains developed from stem cells) have shed light on how these pregnancy hormones may influence brain development. When researchers administered testosterone to developing brain tissue, it increased the proliferation of cortical progenitors, expanding the overall neurogenic pool.

The authors note that human male neonates average 3.5% more whole brain volume than females, even when controlling for body size or weight. However, estrogens appear to have more multifaceted roles in brain development, with studies indicating they enhance synapse formation and neural connectivity.

The high levels of estrogens produced during human pregnancy may help explain why human neurons show increased spine number and more specialized glial cells compared to other primates — features associated with enhanced cognitive abilities.

The researchers connect their hormonal findings to the “social brain hypothesis,” which proposes that large brains evolved to manage complex social relationships in big groups. Maintaining cohesion in large groups requires sophisticated cognitive abilities for tracking relationships, resolving conflicts, and coordinating activities.

Large groups can also create an “infertility trap” where increased male competition leads to stress, infanticide, and reduced female reproductive success. The researchers argue that evolutionary pressures favored hormonal changes that could maintain group size while minimizing male aggression and maximizing female fertility.

Reframing Human Evolution

This research proposes a reframing of human evolution as being driven by female reproductive innovations as a key factor, rather than solely by male competition. The extraordinary hormone-producing capacity of human placentas may represent one of our species’ most important, yet overlooked, evolutionary adaptations.

Sex steroid hormones, particularly estrogens, directly influence the oxytocin system that regulates social bonding. Higher prenatal hormone exposure could have promoted pro-social behaviors while reducing reactive aggression, exactly what large human societies needed to function.

The placental steroid hypothesis suggests that natural selection could have favored women whose placentas produced higher hormone levels, leading to offspring with larger, more socially-oriented brains. Over generations, this could create a positive feedback loop where hormonal, cognitive, and social traits co-evolved.

Rather than viewing human intelligence only as the result of male brains competing for reproductive success, this research suggests our cognitive abilities emerged from the remarkable evolutionary innovation of pregnancy itself, highlighting the critical role of the matriline in human evolution.

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