What makes us human? Most would say our minds. The ability to think, empathize, and build complex societies sets us apart from every other species on Earth. But where did this begin?
A new hypothesis from researchers at Cambridge and Oxford suggests a surprising factor in human brain evolution: the placenta.
This temporary organ that nurtures unborn babies may have driven the development of our large, social human brains.
The placenta is a dynamic, intelligent system that nurtures and safeguards new life from conception to delivery. It attaches to the lining of the uterus and connects to the fetus through the umbilical cord, acting as a vital lifeline.
Through this connection, the placenta delivers oxygen and nutrients from the mother’s bloodstream to the fetus while removing carbon dioxide and waste products.
It also produces hormones – such as progesterone, estrogen, and human chorionic gonadotropin – that help maintain the pregnancy and support fetal development.
Beyond its physiological functions, the placenta serves as a protective barrier, filtering out many harmful substances, though not all.
Certain viruses, bacteria, and drugs can still cross into fetal circulation, which underscores the importance of maternal health and prenatal care.
Dr. Alex Tsompanidis, senior researcher at Cambridge’s Autism Research Centre, is the study’s lead author.
“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 prompted us to consider their relevance for human evolution,” noted Dr. Tsompanidis.
He and his co-authors argue that these hormones influenced not just individuals but helped shape human social traits across millennia.
“We’ve known for a long time that living in larger, more complex social groups is associated with increases in the size of the brain. But we still don’t know what mechanisms may link these behavioral and physical adaptations in humans,” said Professor Robin Dunbar.
This study suggests the missing piece may be sex steroid hormones like testosterone and oestrogens. These affect brain structure and connectivity even before birth.
To explore this, the researchers used “mini-brains” grown from stem cells in petri dishes. These clusters allowed them to observe how hormones shape developing neurons.
The team found that testosterone increases brain volume, while oestrogens enhance neural connectivity.
Humans and great apes share a powerful hormonal connection between mother and fetus via the placenta. But humans seem to have a unique advantage.
“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. But the advantage of human placentas over those of other primates has been less clear,” noted Professor Graham Burton.
That advantage may lie in hormone levels. Studies show that oestrogen levels in pregnant humans are higher than in other primates.
One human hallmark is our ability to maintain very large, stable groups. To manage this, our ancestors had to evolve both high fertility and reduced male competition. Sex steroid hormones may have supported both changes.
Hormones like testosterone drive male-like traits such as strength and competitiveness. In humans, these traits exist but appear less extreme than in our close relatives or extinct cousins like Neanderthals.
In contrast, traits linked to oestrogens, like reduced body hair and certain finger ratios, became more common.
Why? Possibly because the placenta helps convert testosterone into oestrogens using an enzyme called aromatase. Humans have more of this enzyme than other primates, and interestingly, males may have slightly more than females.
By combining all these pieces, the researchers argue that stronger placental function and elevated prenatal hormone levels together expanded brain size and connectivity.
A lower ratio of testosterone to oestrogen could have reduced male aggression and boosted fertility in females. This allowed humans to live together in larger, more cooperative groups.
“We have been studying the effects of prenatal sex steroids on neurodevelopment for the past 20 years. This has led to the discovery that prenatal sex steroids are important for neurodiversity in human populations,” said Professor Simon Baron-Cohen.
“This new hypothesis takes this further in arguing that these hormones may have also shaped the evolution of the human brain.”
According to Dr. Tsompanidis, the team’s 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,” Dr. Tsompanidis concluded.
The study is published in the journal Evolutionary Anthropology Issues News and Reviews.
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