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The Placental Steroid Hypothesis of Human Brain Evolution
Alex Tsompanidis1, Graham J Burton2, Simon Baron-Cohen1
1Autism Research Centre, Department of Psychiatry, University of Cambridge, Cambridge, UK.
Evolutionary Anthropology
|June 20, 2025
Summary
Human brain evolution is influenced by sex steroid hormones produced by the placenta during pregnancy. These hormones play a key role in regulating physiology, fertility, and cognition in both sexes.
Area of Science:
- Evolutionary biology
- Neuroscience
- Endocrinology
Background:
- Human brain evolution has traditionally focused on sexual selection and minor sex-based volumetric differences.
- Recent research highlights molecular, genetic, and clinical findings in neurodevelopment and sex steroid hormone production.
Purpose of the Study:
- To review current findings on sex differences in human brain evolution.
- To propose a novel hypothesis emphasizing the role of prenatal sex steroid hormones in human brain development and evolution.
Main Methods:
- Review of molecular, genetic, and clinical findings.
- Analysis of sex steroid hormone production during pregnancy.
- Consideration of evolutionary pressures in social groups.
- Synthesis of evidence to form a new hypothesis.
Main Results:
- On-average sex differences are relevant to human evolution, with significant overlap between sexes.
- Female physiology shows more adaptations than male physiology.
- Sex steroid hormones (testosterone, oestradiol) produced by the placenta are crucial during prenatal development.
Conclusions:
- A novel hypothesis is presented for human brain evolution, centering on placental sex steroid hormones.
- Prenatal hormone exposure significantly impacts physiology, fertility, and cognition.
- This perspective shifts focus from simple volumetric differences to hormonal regulation in brain evolution.
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