The Evolution of Our Large Brains: The Role of Placental Sex Hormones

The human brain stands as one of the universe’s most intricate structures, potentially shaped by the surge of hormones released by the placenta during pregnancy.

Numerous theories have emerged regarding the evolution of the human brain, yet it remains one of science’s greatest enigmas. The social brain hypothesis posits that our expansive brains evolved to navigate complicated social interactions. This suggests that managing dynamics in larger groups necessitates enhanced cognitive abilities, and that species with strong social inclinations require increased brain development. Comparable highly social animals, like dolphins and elephants, possess significant brain sizes too; however, the biological mechanisms linking these features are still unclear.

Recently, Alex Tsompanidis from Cambridge University and his team propose that a placental sex hormone might be the key. The placenta, a temporary organ bridging the fetus and the mother, releases hormones crucial for fetal development, including sex hormones like estrogens and androgens.

“It may sound like a stretch, linking human evolution to the placenta,” notes Tsompanidis. “However, we’ve observed fluctuations in these hormone levels in utero and predicted outcomes regarding language and social development, among other areas.”

Recent studies indicate these hormones significantly impact brain development. For instance, a 2022 study revealed that administering androgens like testosterone to brain organoids—a simplified brain model derived from human stem cells—during crucial developmental stages led to an increased number of cortical cells and expansion in regions vital for memory and cognition. Other investigations involving brain organoids have highlighted the importance of estrogens in forming and solidifying neural connections.

Limited evidence suggests that humans experience greater exposure to these hormones during pregnancy compared to non-human primates. A 1983 study indicated that gorillas and chimpanzees excrete 4-5 times less estrogen than pregnant humans. Additionally, human placentas exhibit greater gene activity associated with aromatase—an enzyme converting androgens to estrogens—compared to macaques.

“These hormones appear crucial for brain development. Evidence indicates significantly elevated levels in humans, especially during pregnancy,” asserts Tsompanidis.

This influx of hormones may also clarify why humans form larger social networks. Some evolutionary biologists theorize that differences between sexes are subtler in humans than in other primates, fostering broader social connections. For instance, men and women exhibit greater size similarity in comparison to male and female Neanderthals, suggests Tsompanidis, likely a result of elevated estrogen levels in utero.

“High estrogen levels not only reduce masculinization but may also foster a more interconnected brain,” Tsompanidis explains. “Thus, the drive to elevate estrogen levels promotes social cohesion and interconnectedness, integral to human brain development.”

David Geary from the University of Missouri agrees that placental genes influence human brain development and its evolutionary path. However, he believes the significance of male-male competition in brain and cognitive evolution is often underestimated.

He notes that human males within the same groups tend to exhibit more coordination and less aggression compared to other primates—a trait that may have evolved due to intergroup conflicts. Enhanced teamwork and coordination could significantly benefit survival during life-threatening confrontations.

Our understanding of placental differences among primates remains limited. Many non-human primates, such as chimpanzees, consume their placenta post-birth, complicating research efforts, as Tsompanidis highlights.

Unraveling the factors that influenced human brain evolution is not merely an academic endeavor; it also brings insights into human nature.

“Not every human possesses extensive social or linguistic skills, and that’s perfectly acceptable—these traits don’t define humanity,” Tsompanidis remarks. Understanding the brain’s evolutionary journey can illuminate whether certain cognitive attributes come with trade-offs.