News | Cambridge and Oxford Researchers Propose Placenta and Pregnancy Hormones May Have Driven Human Brain Evolution
Researchers from the University of Cambridge and the University of Oxford have proposed a new hypothesis that the placenta and the sex steroid hormones it produces during pregnancy, such as testosterone and estrogen, may have played a central role in human brain evolution. The study was published in Evolutionary Anthropology.
The study seeks to explain not only why humans have larger, more complex brains than other primates, but also how distinctively human social traits—such as extensive cooperation and stable, large-scale communities—gradually emerged through evolution.
The placenta: more than a nutrient pathway and possibly a key engine of brain evolution
“The uniqueness of the human brain did not appear from nowhere. Our hypothesis puts pregnancy at the center of the human evolutionary story,” said first author Dr. Alex Tsompanidis, a senior researcher at the University of Cambridge’s Autism Research Centre. “The placenta’s ability to regulate sex hormones may have been key to the evolution of our brains and even our behavior.”
The placenta connects the pregnant person and fetus, regulating nutrient supply and the duration of pregnancy. Previous research has shown that estrogen levels in the human placenta are higher than in other primates, and these hormones play an important role in fetal brain development.
Co-author Professor Graham Burton, founding director of the University of Cambridge’s Loke Centre of Trophoblast Research, added: “The placenta determines how long pregnancy lasts and how efficiently nutrients are delivered, both of which are crucial to uniquely human brain development. Here, we systematically connect this with brain evolution.”
Hormones and the brain: fetal sex hormone levels may shape neural structure and social ability
Drawing on recent findings from “mini-brain” experiments using brain organoids grown from human stem cells, the researchers noted that testosterone can increase brain volume while estrogen enhances neural connectivity. Even small differences in these hormone levels may predict the pace of an infant’s cognitive development and social abilities.
Professor Robin Dunbar, an evolutionary biologist at the University of Oxford and co-senior author, said: “We have long known that brain size is related to social complexity. The real challenge is identifying the mechanism that allowed us to evolve both larger brains and more complex social abilities.”
From Neanderthals to modern humans: subtle shifts in sex hormone balance may have favored cooperation over competition
The study also proposes that human sex-differentiated traits are more “neutral” than those of other primates or extinct human groups such as Neanderthals. Relatively lower androgen levels may have reduced sex-based conflict and competition within groups, strengthening cooperation and overall fertility and providing a biological basis for larger, more stable human communities.
The researchers also found much higher levels of aromatase—the enzyme that converts testosterone to estrogen—in the human placenta than in macaques. Males may even have higher aromatase expression than females, further highlighting the distinctive regulation of hormones during human fetal development.
Conclusion: hormone regulation during pregnancy may have jointly driven brain evolution and human social behavior
Professor Simon Baron-Cohen, director of the University of Cambridge’s Autism Research Centre and a co-author, said: “We have studied the effects of prenatal sex hormones on neurodevelopment for 20 years. This study further proposes that these hormones not only shape individual differences but may also have driven brain evolution at the species level.”
Dr. Tsompanidis concluded: “This hypothesis places pregnancy and the placenta at the beginning of the human story. Our cognitive abilities, social traits, and even language may trace back to the subtle interaction between sex hormones and the placenta during fetal life.”
News | Cambridge and Oxford Researchers Propose Placenta and Pregnancy Hormones May Have Driven Human Brain Evolution
News | Cambridge and Oxford Researchers Propose Placenta and Pregnancy Hormones May Have Driven Human Brain Evolution
Researchers from the University of Cambridge and the University of Oxford have proposed a new hypothesis that the placenta and the sex steroid hormones it produces during pregnancy, such as testosterone and estrogen, may have played a central role in human brain evolution. The study was published in Evolutionary Anthropology.
The study seeks to explain not only why humans have larger, more complex brains than other primates, but also how distinctively human social traits—such as extensive cooperation and stable, large-scale communities—gradually emerged through evolution.
The placenta: more than a nutrient pathway and possibly a key engine of brain evolution
“The uniqueness of the human brain did not appear from nowhere. Our hypothesis puts pregnancy at the center of the human evolutionary story,” said first author Dr. Alex Tsompanidis, a senior researcher at the University of Cambridge’s Autism Research Centre. “The placenta’s ability to regulate sex hormones may have been key to the evolution of our brains and even our behavior.”
The placenta connects the pregnant person and fetus, regulating nutrient supply and the duration of pregnancy. Previous research has shown that estrogen levels in the human placenta are higher than in other primates, and these hormones play an important role in fetal brain development.
Co-author Professor Graham Burton, founding director of the University of Cambridge’s Loke Centre of Trophoblast Research, added: “The placenta determines how long pregnancy lasts and how efficiently nutrients are delivered, both of which are crucial to uniquely human brain development. Here, we systematically connect this with brain evolution.”
Hormones and the brain: fetal sex hormone levels may shape neural structure and social ability
Drawing on recent findings from “mini-brain” experiments using brain organoids grown from human stem cells, the researchers noted that testosterone can increase brain volume while estrogen enhances neural connectivity. Even small differences in these hormone levels may predict the pace of an infant’s cognitive development and social abilities.
Professor Robin Dunbar, an evolutionary biologist at the University of Oxford and co-senior author, said: “We have long known that brain size is related to social complexity. The real challenge is identifying the mechanism that allowed us to evolve both larger brains and more complex social abilities.”
From Neanderthals to modern humans: subtle shifts in sex hormone balance may have favored cooperation over competition
The study also proposes that human sex-differentiated traits are more “neutral” than those of other primates or extinct human groups such as Neanderthals. Relatively lower androgen levels may have reduced sex-based conflict and competition within groups, strengthening cooperation and overall fertility and providing a biological basis for larger, more stable human communities.
The researchers also found much higher levels of aromatase—the enzyme that converts testosterone to estrogen—in the human placenta than in macaques. Males may even have higher aromatase expression than females, further highlighting the distinctive regulation of hormones during human fetal development.
Conclusion: hormone regulation during pregnancy may have jointly driven brain evolution and human social behavior
Professor Simon Baron-Cohen, director of the University of Cambridge’s Autism Research Centre and a co-author, said: “We have studied the effects of prenatal sex hormones on neurodevelopment for 20 years. This study further proposes that these hormones not only shape individual differences but may also have driven brain evolution at the species level.”
Dr. Tsompanidis concluded: “This hypothesis places pregnancy and the placenta at the beginning of the human story. Our cognitive abilities, social traits, and even language may trace back to the subtle interaction between sex hormones and the placenta during fetal life.”
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