A groundbreaking hypothesis emerging from researchers at the Universities of Cambridge and Oxford suggests that the placenta, and specifically the steroid hormones it generates during pregnancy, may have played a decisive role in shaping the evolution of the human brain. This theory not only sheds light on the biological foundations of our cognitive capacities but also provides a fresh perspective on the emergence of social behaviors that have enabled human societies to expand and thrive. The synthesis of endocrinology, evolutionary biology, and neurodevelopment composes an intricate narrative of how prenatal hormonal environments sculpt both brain architecture and social dynamics, forming a cornerstone of what makes us distinctly human.
Dr. Alex Tsompanidis, a senior investigator at the Autism Research Centre at Cambridge, spearheaded this inquiry into the intricate relationship between prenatal steroid hormones and neurodevelopmental trajectories. Variability in prenatal levels of key steroid hormones like testosterone and estrogens appears to predict the pace at which infants acquire social and cognitive skills, as well as their susceptibility to neurodevelopmental disorders such as autism spectrum conditions. These initial findings propelled the consideration that such hormonal patterns could have evolutionary significance, particularly when viewed through the lens of human brain development and behavioral specializations.
One fascinating dimension of this hypothesis stems from human adaptation to complex sociality. Professor Robin Dunbar, an Evolutionary Biologist from Oxford, has long posited a connection between social group size and brain enlargement, forming the basis of theories such as the social brain hypothesis. However, the underlying mechanisms bridging behavioral complexity and physical brain changes had remained elusive. The new research proposes that prenatal sex steroid hormones act as biochemical mediators, intricately influencing neural growth and connectivity, thus establishing a developmental link between social living and brain architecture.
Central to this investigation is the role of sex steroid hormones produced in the womb, particularly testosterone and estrogens. These hormones are derived not only from the fetus but significantly through the function of the placenta, which acts as an intermediary endocrine organ connecting maternal and fetal systems. Through the use of innovative ‘mini-brains’—lab-grown clusters of human neuronal cells derived from stem cells—scientists have begun to unravel how these hormones directly impact brain tissue at the cellular level. Evidence shows that testosterone can stimulate increased brain volume, while estrogens enhance synaptic connectivity, underscoring the dual yet complementary roles these hormones serve in cerebral development.
Comparative studies with non-human primates such as chimpanzees and gorillas reveal that while placental steroidogenesis is a shared trait, humans exhibit distinctive patterns in hormone production. Specifically, human pregnancies are characterized by elevated levels of estrogens compared to other primates, a finding supported by two recent independent studies. This hormonal divergence may be integral to the pronounced brain enlargement and complex social behaviors unique to Homo sapiens, suggesting an evolutionary recalibration of placental endocrine function as a driver of human-specific traits.
The human capacity to maintain larger and more cohesive social groups distinguishes us from our closest relatives and extinct hominins like Neanderthals. Achieving large, stable social structures presents evolutionary challenges, including maintaining fertility and curbing intra-group competition over mates and resources. Prenatal sex steroids, with their powerful modulatory effects on sex differentiation and behavior, may have been crucial in negotiating these challenges. Alterations in hormonal ratios could reduce aggressive competition among males while promoting reproductive efficiency in females, facilitating social cohesion essential for group survival.
Sex differentiation driven by hormonal milieu profoundly influences anatomical and behavioral traits, with a typical pattern involving higher testosterone leading to more pronounced male phenotypes such as increased body size and competitive behaviors. Intriguingly, humans display attenuated sex differences relative to other primates and extinct relatives, with female-specific traits and estrogenic effects appearing more prominent. This includes characteristics such as reduced body hair and distinct digit ratios, which have genetic and developmental underpinnings linked to estrogenic activity during prenatal development.
At the biochemical heart of this modulation is the enzyme aromatase, which converts testosterone to estrogens within the placenta. Comparative analyses reveal that humans possess elevated levels of placental aromatase compared to macaques, while male human fetuses may exhibit slightly higher enzymatic activity than females. This enzymatic adaptation suggests a finely tuned mechanism by which humans regulate the balance of androgens and estrogens during critical phases of brain and body development, contributing to both the evolution of cognitive traits and reduction in overt sexual dimorphism.
Synthesizing diverse lines of evidence, the researchers advance the placental steroid hypothesis: the synergy of heightened prenatal sex steroid production and enhanced placental enzymatic function may have catalyzed the evolutionary enlargement and increased connectivity of the human brain. Concurrently, a lower androgen-to-estrogen ratio may have played a pivotal role in tempering male competitive behaviors while bolstering female fertility, creating a socio-biological environment conducive to the emergence of large social units.
Professor Simon Baron-Cohen, Director of the Autism Research Centre at Cambridge and a senior author of the study, highlighted that decades of research into prenatal sex steroid influences on neurodevelopment have elucidated aspects of human neurodiversity. This new hypothesis extends that understanding by contending that these very hormonal processes, traditionally studied in the context of individual development, have also shaped the macroevolutionary trajectory of the human brain, underscoring the interplay between developmental biology and evolutionary change.
Dr. Tsompanidis emphasized the importance of reframing pregnancy as a central focus in the story of human evolution. The intimate biochemical dialogue orchestrated by the placenta—modulating the fetal brain’s hormonal environment—may have been a pivotal mechanism underpinning the emergence of cognitive complexity and social sophistication that define humanity. Such placental adaptations represent a crucial evolutionary nexus where biology and cognition intersect, offering profound insights into how our species came to inhabit its unique ecological and social niche.
While the placental steroid hypothesis is compelling, it invites further experimental and comparative studies to unravel the molecular and developmental intricacies behind hormone-mediated brain evolution. Ongoing advancements in organoid technology, genetic analysis, and cross-species endocrinology promise to enrich our understanding of how pregnancy’s subtle biochemical variations have sculpted the neural circuits foundational to human thought, culture, and social life.
This pioneering work opens up exciting avenues in the broader field of evolutionary anthropology and neuroendocrinology, compelling a reevaluation of how reproductive biology influences species-level behavioral traits. It underscores the placenta’s critical role beyond nutrient delivery, positioning it as a dynamic driver of evolutionary innovation—a biological incubator shaping not only individual development but the very fabric of human evolution itself.
Subject of Research: Human brain evolution and the role of prenatal sex steroid hormones produced by the placenta
Article Title: The placental steroid hypothesis of human brain evolution
News Publication Date: 20 June 2025
Web References:
http://dx.doi.org/10.1002/evan.70003
References:
Tsompanidis, A et al. The placental steroid hypothesis of human brain evolution. Evolutionary Anthropology; 20 June 2025; DOI: 10.1002/evan.70003
Keywords: Brain evolution, Evolutionary biology, Human evolution, Endocrinology, Estrogen, Testosterone
