Washington DC [US], February 11 : Human evolution has long been tied to growing brain size, and new research suggests prenatal hormones may have played a surprising role. By studying the relative lengths of the index and ring fingers, a marker of prenatal exposure to oestrogen and testosterone, researchers found that higher prenatal oestrogen exposure was associated with larger head size in newborn boys.
Human evolution is closely tied to a steady increase in brain size. New research suggests that this expansion may be partly influenced by pre-birth oestrogen levels, with an unexpected indicator in the length of our fingers.
Professor John Manning, part of Swansea's Applied Sports, Technology, Exercise and Med-icine (A-STEM) research team, has spent years studying what is known as digit ratio.
This measure compares the length of the index finger to the ring finger, a relationship called the 2D:4D ratio. Scientists have shown that this ratio reflects the balance of estrogen and testosterone a fetus is exposed to during the first trimester of pregnancy.
When prenatal oestrogen levels are higher relative to testosterone, the index finger tends to be longer than the ring finger. In these cases, the 2D:4D value is considered high.
In his latest study, Professor Manning worked with researchers from Istanbul University's Department of Anthropology. Their findings were recently published in the journal Early Human Development.
Because head circumference in newborns is strongly linked to brain size and later measures of intelligence, the research team examined both finger ratios and head size in a group of 225 newborns. The group included 100 boys and 125 girls.
The results showed a clear pattern in boys. Higher 2D:4D ratios, which point to greater prenatal estrogen exposure, were associated with larger head circumference. This relationship did not appear in girls.
Professor Manning explained the broader significance of the findings. "This finding is relevant to human evolution because increases in brain size are found alongside feminisation of the skeleton, what is known as the estrogenized ape hypothesis."
High values of 2D:4D in males have been found to be related to elevated rates of heart problems, poor sperm counts and predisposition to schizophrenia.
"However, increases in brain size may offset these problems. Thus, the evolutionary drive for larger brains in humans may inevitably be linked to reductions in male viability including cardiovascular problems, infertility and rates of schizophrenia," added Manning.
The researchers say the study adds to growing evidence that prenatal estrogen has played a positive role in shaping the evolution of the human brain, even if it came with biological costs.
Professor Manning's earlier work has also linked digit ratio to a wide range of outcomes. Past studies have explored connections between finger length and alcohol consumption, recovery after Covid-19 infection, and oxygen use in football players. Together, this research highlights how a simple anatomical trait may reflect powerful influences during early human development.
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