Before birth, beyond the brain: Researchers trace human evolution to the womb

(CN) — Hormones in the womb may hold hidden clues to why humans evolved with bigger brains and better social skills than our ancient ancestors.

Scientists from the Universities of Cambridge and Oxford say hormones produced by the placenta during pregnancy may have helped shape the way the human brain develops, laying the groundwork for both our cognitive complexity and social nature.

Their new hypothesis, published Thursday in the journal Evolutionary Anthropology, ties together several lines of evidence, from primate pregnancy differences to new studies using lab-grown “mini-brains.”

What shapes the brain ?

For years, scientists have linked bigger brains to larger, more complex social groups. But this new work explores how such a transformation might have happened. The researchers propose that prenatal sex steroid hormones, which are produced with help from the placenta, may have shaped not just brain size, but also behavior, social development and fertility in ways that allowed early human societies to flourish.

Recent discoveries using lab-grown brain organoids, or “mini-brains,” suggest that testosterone increases brain size while estrogens improve connectivity between neurons.

Humans, unlike most other primates, also appear to have higher levels of aromatase, which is an enzyme that converts testosterone into estrogen within the placenta.

According to researchers, that means our developing brains may be exposed to different hormonal environments, possibly encouraging both growth and social wiring.

“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,” said Professor Graham Burton, founding director of the Loke Centre of Trophoblast Research at Cambridge, in the press release. “But the advantage of human placentas over those of other primates has been less clear.”

Humans vs. Neanderthals

The researchers say this hormone-driven theory could also help explain some differences between modern humans and Neanderthals. Fossil evidence suggests Neanderthals had greater physical differences between sexes, such as bigger bodies and more pronounced competition traits. While modern humans have smaller sexual dimorphism and traits associated with cooperation and social bonding.

“While Neanderthals had large body size differences between adult men and women, humans have adapted to reduce those differences,” Dr. Alex Tsompanidis, senior researcher at the Autism Research Centre at Cambridge and lead author of the study, said in an email. “In our article, we make the case that this is linked to the amount of estrogen in the womb and perhaps throughout life, which may have, in turn, enabled human societies to increase their size and maintain fertility.”

That shift in hormonal balance, the team argues, may have made humans less combative, more fertile and more capable of living in bigger groups — traits that likely gave our species an edge.

The study builds on more than two decades of work by co-author Simon Baron-Cohen, director of Cambridge’s Autism Research Centre, who has studied the effects of prenatal sex steroids on neurodevelopment. “This new hypothesis takes this further in arguing that these hormones may have also shaped the evolution of the human brain,” he said in the press release.

Testing the theory

To test this theory, Tsompanidis and his team are planning studies that focus on genes regulated by estrogens to see whether they show signs of recent evolutionary change. They’re also hoping to collaborate with zoologists to examine primate placentas more closely.

“We have proposed a series of studies that can help test this hypothesis further,” Tsompanidis said in an email. “For example, we plan to zoom-in on the genes that are regulated by estrogens and see if they show evidence of recent adaptations in humans. In addition, we are seeking collaborations with zoologists around the world, in order to examine the placentas of our primate relatives more closely than ever before.”

He said the team is also interested in how testosterone and estrogen may create trade-offs in brain development.

“Testosterone is important for brain size and cellular proliferation, while estrogen is important for regulating aspects of synapse formation and connectivity,” Tsompanidis said in an email. “Our bodies also make sure the rates between the two are well-regulated… It’s plausible that the physical process of expanding the brain’s cortex and making sure the neurons communicate with each other may involve trade-offs between the two, which contribute to the wonderful neurodiversity of human populations.”

This theory, he said, might also shed light on what makes human social behavior and cognition so unique.

“Humans are a social species, and our pro-sociality is one of the most important aspects that have helped our groups thrive and expand around the world,” Tsompanidis said in an email. “In these large groups, neurodiversity may have been an important asset.”

Ultimately, Tsompanidis said the story of how we became human may begin in the womb.

“The evolutionary story of our brain is perhaps the most beautiful mystery in science,” he said. “It can help us understand our journey as a species and what makes our cognition and societies unique relative to other animals. We may never know for sure exactly what aspects, mechanisms and events have led to its evolution, but that does not mean we should stop thinking about it.”