Exposure to hormones in the uterus can affect human brain growth
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The human brain is one of the most complex objects in the universe. The complexity may be due to the surge in hormones released by the placenta during pregnancy.
Many ideas have been proposed to explain the evolution of the human brain, but it continues to be one of our biggest scientific mysteries. One explanation known as the social brain hypothesis suggests that our large brains have evolved to manage complex social relationships. We hypothesize that navigating large group dynamics requires some degree of cognitive ability and that social species require greater brain development. Other very sociable animals, such as dolphins and elephants, also have relatively large brains. However, the biological mechanisms underlying this link remain unknown.
now, Alex Tsompanidis Cambridge University and his colleagues say the answer may be a placental sex hormone. During pregnancy, the placenta, a temporary organ that acts as a mediator between the fetus and the mother, produces hormones essential for fetal development. These include sex hormones such as estrogens and androgens.
“I know it sounds like a jump. Think about human evolution and then it ends in the placenta,” says Tsompanidis. “But the reason is because we’ve seen fluctuations and variations in the levels of these hormones in the uterus and predicted language development, social development, and more.”
Emerging research also shows that these hormones affect the developing brain. For example, a 2022 Research To administer androgens such as testosterone to brain organoids – a simplified miniature version of the brain made from human stem cells – during critical development, the number of cells in the cortex increased, and the number of brain regions important for memory, learning, and thinking. Other studies of brain organoids have shown that estrogens are important for forming and stabilizing connections between neurons.
There is also some limited evidence that humans are exposed to higher levels of these hormones during pregnancy than non-human primates. a Research from 1983 We found that gorillas and chimpanzees have 4-5 times less estrogen in their urine than pregnant humans. I also have a placenta More activity of genes It produces aromatase – an enzyme that converts androgens to estrogens – in humans than macaque monkeys.
“These hormones have become extremely important for brain development. Compared to other primates and other species, there appears to be evidence that these hormones are very high in humans. [during pregnancy]says Tsompanidis.
This influx also helps explain why humans form such large social groups. Some evolutionary biologists believe that gender differences are more subtle than other primates, allowing for broad social networks to be built. For example, men and women are more similar in size than male and female Neanderthals, says Tsompanidis. This is probably due to high levels of estrogen in the uterus, he says.
“If you have a lot of estrogens, you’re not only a bit less masculinized, but you’re also more likely to have an interconnected brain,” says Tsompanidis. “So the push to increase estrogen, the push to make everyone social and get along is actually something that’s bigger and more connected to the human brain.”
“I agree that placental genes affect human brain development and possibly the evolution of human brains.” David Geary At the University of Missouri. “But I think they underestimate the impact of male-male competition on brain and cognitive evolution.”
Male humans within the same social group tend to be more coordinated and less aggressive than seen in other primates, and are less aggressive with each other, but this may have evolved as a result of intergroup conflicts, he says. After all, greater coordination and teamwork will be beneficial in fatal conflicts, he points out.
Our knowledge of placenta differences among primates is also limited. Many inhuman primates, such as chimpanzees, eat the placenta after giving birth, making studying difficult, says Tsompanidis.
Identifying the factors that shaped the evolution of the human brain is more than just intellectual pursuits. It also sheds light on the nerve body.
“Not all humans are social or have incredible language skills. That’s fine. It doesn’t make them human,” says Tsompanidis. Understanding how the brain evolved can provide insight into whether certain cognitive traits entail trade-offs, he says.
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