Exposure to in utero hormones could affect the growth of the human brain
Images Peter Dazeley / Getty
The human brain is one of the most complex objects in the universe – and this complexity can be due to a wave of hormones released by the placenta during pregnancy.
Although many ideas have been proposed to explain the evolution of the human brain, it remains one of our greatest scientific mysteries. An explanation, known as the social brain hypothesis, suggests that our big brain has evolved to manage complex social relationships. It postulates that the navigation of dynamics in large group requires a certain degree of cognitive capacity, pushing social species to develop a larger brain. For example, other highly sociable animals, such as dolphins and elephants, also have relatively important brains. But the biological mechanism underlying this link has remained clear.
Now Alex Tsompanidis at the University of Cambridge and his colleagues say that the answer can be placental sex hormones. During pregnancy, the placenta – a temporary organ which acts as an intermediary between the fetus and the mother – produces crucial hormones for fetal development. These include sex hormones such as estrogens and androgens.
“I know that seems to be a jump – thinking of human evolution, then ending in the placenta,” explains Tsompaanidis. “But the reason is that we have examined the fluctuations and variations in the levels of these hormones in the uterus and seeing that they predict things such as language development and social development.”
Emerging research also shows that these hormones influence the development brain. For example, a 2022 study revealed that administration of androgens, such as testosterone, with brain organoids – from simplified miniature versions of the brain made from human stem cells – during a period of critical development increased the number of cells in the cortex, a crucial brain region for memory, learning and thought. Other studies on brain organoids have shown that estrogens are important to train and stabilize connections between neurons.
There is also limited evidence that humans are exposed to higher levels of these hormones during pregnancy than non -human primates. A 1983 study revealed that gorillas and chimpanzees have four to five times less estrogen in their urine than humans during pregnancy. The placenta also has more activity in genes that produce aromatase – an enzyme that converts androgens into estrogens – in humans than in macaque monkeys.
“These hormones have become very important for brain development, and if we look at it relatively with other primates and other species, there seems to be evidence that these hormones are very high in humans [during pregnancy]Said tsompanidis.
This influx could also help explain why humans form such large social groups. Some evolutionary biologists believe that we are able to build large social networks because the differences in the sexes are more subtle in humans than in other primates. For example, men and women are more similar in body size than male and female Neanderthals, explains Tsompanidis. This is probably due to higher estrogen levels in utero, he said.
“If you have a lot of estrogens, not only are you a little less masculinized, but you are also more likely to have an interconnected brain,” explains Tsompanidis. “So the push to increase estrogens, the push to make everyone social and get along, is actually what makes the human brain larger and more connected.”
“I agree that placental genes influence the development of the human brain and probably the evolution of the brain of hominines,” explains David Geary at the University of Missouri. “However, I think they underestimate the influence of male-massive competition on the brain and cognitive evolution.”
If it is true that male humans within the same social group tend to be more cooperative and less aggressive towards each other than in other primates, this can have evolved following conflicts between groups, he said. After all, greater coordination and teamwork would be an advantage in a fatal confrontation, he underlines.
Our knowledge of placental differences between primates is also limited. Many non -human primates, such as chimpanzees, eat the placenta after childbirth, which makes the study difficult, explains Tsompanidis.
Identifying the factors in the form of the evolution of the human brain is more than a simple intellectual pursuit: it could also shed light on neurodiversity.
“Not all humans are social or do not have incredible linguistic skills – and that’s good. This does not make them less human, ”explains Tsompanidis. Understanding how the brain has evolved could give an overview of the question of whether certain cognitive features come with compromises, he said.
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