Our big brains may have evolved due to placental sex hormones

The human brain is one of the most complex objects in the universe – and that complexity can be to a wave of released by placenta during pregnancy.

While several ideas have been suggested to explain human brain development, it is still one of our greatest scientific mysteries. An explanation, known as the social brain hypothesis, suggests that our great brains evolved to control complex social conditions. It suggests that navigating large group dynamics requires a certain degree of cognitive ability and pushes social species to develop larger brains. For example, other very sociable animals, such as dolphins and elephants, also have relatively large brains. But the biological mechanism underlying this link has remained unclear.

Now Alex Tespanidis at the University of Cambridge and his colleagues says the answer may be placental sex hormones. During pregnancy, the placenta – a temporary organ that acts as an intermediary between the fetus and the mother – produces hormones that are essential for fetal development. These include sex hormones such as estrogens and androgens.

“I know it seems like a jump – thinking about human development and then ending up on the mork cake,” says Tsomanidis. “But the reason for it is because we have been seen on fluctuations and variations in the levels of these hormones in the wit and see that they predict things like long development and social development.

Emerging research also shows that these hormones affect the developmental heart. For example, a study from 2022 found that adist androgens, such as testosterone, to brain organoids – simplified, miniature versions of the brain made of human stem cells – during a critical development period increased the number of cells in cortex, brain area decisive for memory, learning and thinking. Other studies in brain organoids have shown that estrogens are important for forming and stabilizing connections between neurons.

There are also some limited that it is exposed to high levels of these hormones bold pregnancy than non-human primates are. A 1983 study found that gorillas and slopes 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 transforms androgens into estrogens – in humans than in makak monkeys.

“These hormones have become very important for brain development, and if we look at it comparable to other primates and other species, it seems to be evidence, there are hormones that are very high in humans [during pregnancy]”Says Tersomanidis.

This influx can also help explain that WHYNS forms such large social groups. Some evolutionary biologists believe that we are able to build extensive social networks because the difference between the sexes is more subtle in humans than in other primates. For example, men and women are more similar in body size than male and female Neanderthals, says Tespanidis. This is probably due to high estrogen levels in Utero, he says.

“If you have a lot of estrogen, you are not only a little male man, but you are also like a interconnected brain,” says Tespanidis. “So pushing to increase estrogen, pushed to make everyone social and get together is actually what makes the human brain bigger and more connected.”

“I agree that the placenta genes affect the development of the human brain and probably hominin -brain development,” says David Geary at the University of Missouri. “Howwe, I think they underestimate the influence of male-male competition on brain and cognitive development.”

Although it is true that male people without the same social group tend to be cooperative and less aggressive towards someone other than seen in other primates, this may have evolved as a result of intermediate group conflicts, he says. After all, greater coordination and teamwork would be an advantage in a deadly confrontation, he points out.

Our knowledge of placental difference between primates is also limited. Many non-human primates, such as chimpanzees, eat placenta after birth, making it difficult to study, says Tsomanidis.

Identification of which factors shaped human brain development are more than just an intellectual persecution: it can also shed light on neurodiversity.

“Not all people are social or have incredible language skills – and that’s fine. It doesn’t make them less human,” says Tsomanidis. Understanding how the brain developed could give insight into whether certain cognitive traits come with trade -offs, he says.