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A provocative new hypothesis suggests that the placenta (the temporary organ associated with mothers and grown children) may be an impossible architect of our species’ cognitive abilities.
Researchers from Cambridge and Oxford University challenged Darwin’s initial idea that the evolution of the human brain was driven primarily by male competition. Instead, they suggest that hormone changes during pregnancy, especially sexual steroid hormones produced by the placenta, such as testosterone and estrogen, fundamentally reshape our brain and social behavior.
Placenta power plant
The human placenta does something compelling to distinguish us from relatives of primates – it produces abnormally high sex steroid hormones during pregnancy. Recent studies using the “mini brain” that grows human stem cells show that testosterone can increase brain size, while estrogen can enhance connectivity between neurons.
“The placenta regulates the duration of pregnancy and the feeding of the fetus, both of which are crucial for the development of the characteristic brain of our species,” explains Professor Graham Burton, a trophoblast research center at Professor Graham Burton, Cambridge.
But here’s what makes the story fascinating: compared to other primates, the human placenta contains higher levels of aromatase, an enzyme that quickly converts testosterone into estrogen. This biochemical balancing behavior may be an evolutionary solution to the puzzle.
Social Brain Paradox
Humans are good at what most primates struggle with – forming and maintaining large, cohesive social groups. However, this ability presents an evolutionary paradox. Larger groups often mean more resources and competing with partners, which should increase male aggression and reduce female fertility through stress and harassment.
Instead, humans show the opposite pattern. Compared with our closest relatives, human males are less aggressive, exhibit reduced body dimorphism, and engage in less infanticide. Meanwhile, human women develop permanent secondary traits and hidden ovulation, which suggests long-term reproductive potential rather than immediate fertility.
The researchers suggest that this obvious contradiction can be resolved when we consider the hormonal symphony of the placenta:
- High prenatal steroid levels promote brain growth and nerve connectivity
- The effective conversion of testosterone to estrogen can reduce typical aggression in men
- Enhanced estrogen signaling improves social cognition and bonding behavior
- Modified hormone patterns support stable long-term reproductive relationships
From the uterus to the social world
Dr. Alex Tsompanidis, who led the study at the Cambridge Autism Research Center, noted that “small changes in prenatal levels of steroid hormones such as testosterone and estrogen can predict the rate of social and cognitive learning in babies and may even predict the likelihood of conditions such as autism.”
This connection between prenatal hormones and social development is not just a correlation. The steroid-generating system of the placenta directly affects the developing hypothalamic-pituitary-axial axis, which may have a reincarnation effect. Women with certain hormonal conditions can pass on improved endocrine patterns to their daughters, suggesting that successful social adaptation can accumulate over generations.
Latest research with human brain organs shows that these hormones act far-reaching and specific. Testosterone administration increases proliferation of cortical progenitor cells, thereby enlarging nerve sources during critical developmental windows. Meanwhile, estrogen promotes synaptic formation, spinal density and astrocyte specialization – distinguishing human neurons from those of other primates.
Rewrite human evolution
This placental hypothesis provides a narrative with traditional evolutionary theory, involving male competition and sexual choice. Rather than highlighting the differences between genders, it emphasizes how the hormonal system develops to benefit men and women while enabling unprecedented social cooperation.
Co-author Professor Simon Baron-Cohen reflects on two decades of research: “For the past 20 years, we have been studying the effects of prenatal steroids on neurodevelopment. This new hypothesis further proposes this, suggesting that these hormones may also shape the development of the human brain.”
The meaning goes beyond academic theory. If correct, the framework suggests human cognitive abilities—our empathy, language, thought theory, and complex social reasoning—though our biological limitations, but because pregnancy itself has complex biochemical adaptability.
Maternity Foundation
Perhaps most striking is that this hypothesis places female biology at the center of human evolution. Rather than viewing pregnancy as a limitation on female participation in evolutionary progress, it positioned the maternal-local interface as the main driver of the most unique traits of our species.
As Dr. Tsompanidis summarizes: “Our hypothesis makes your story the core of a species. The human brain is significant and unique, but does not develop in a vacuum. The adaptation of the placenta and its way of producing sexual steroid hormones may be crucial to the evolution of our brains, and for the development of humans and the emergence of social humans, this makes it both humans and humans.
This remark does not reduce human achievements, it simply transfers its biological basis from the competitive stage to the cooperative chemistry of human reproduction. Sometimes the deepest changes are not happening in dramatic battles, but in the quiet, transformative work that creates the next generation.
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