Human evolution is closely related to the development of a larger brain. A new study suggests that high prenatal exposure to estrogen may have contributed to its expansion, and that traces of this hormone’s influence may be seen in the relative lengths of people’s fingers.
Professor John Manning, a member of Swansea University’s Applied Sport, Technology, Exercise and Medicine (A-STEM) research team, specializes in the study of digit ratios.
Finger proportions compare the length of the index finger, known as 2D, to the length of the ring finger, known as 4D. This measurement is called the 2D:4D ratio. Researchers use this as an indirect indicator of the balance of estrogen and testosterone that the fetus may have been exposed to during early pregnancy.
People who are thought to have relatively higher exposure to estrogen than testosterone typically have longer index fingers (2D) compared to ring fingers (4D). This results in a higher 2D:4D ratio.
In the latest study, Manning collaborated with researchers from Istanbul University’s Department of Anthropology. Their findings were published in the journal Early Human Development.
Comparison of newborn finger and head sizes
The team tested 225 newborns, including 100 boys and 125 girls. They measured each baby’s 2D:4D ratio and compared it to their head circumference.
Head circumference is often used as a general indicator of brain size in newborns. Many genetic, environmental, and developmental factors influence intelligence, but it is also associated with cognitive development and subsequent measurements of IQ.
Researchers found that boys with higher 2D:4D ratios (indicating higher prenatal estrogen) also tended to have larger head circumferences. A similar relationship was not observed in girls.
Possible clues to the evolution of the human brain
The results may support a theory known as the estrogenized ape hypothesis. This idea proposes that the evolution of larger human brains occurred in parallel with changes that made the human skeleton less robust and more physically feminine compared to those of earlier ancestors.
Professor Manning said: “This finding has implications for human evolution, as increased brain size coincides with skeletal feminization, known as the estrogenized ape hypothesis. Higher 2D:4D values in men have been found to be associated with higher rates of heart disease, lower sperm counts and a predisposition to schizophrenia.”
“However, increased brain size may offset these problems. Therefore, the evolutionary push toward larger brains in humans may be inevitably associated with reduced male viability, including the incidence of cardiovascular problems, infertility, and schizophrenia.”
The researchers suggest that larger brains may have conferred significant evolutionary advantages, even if the hormonal conditions associated with brain development had biological costs for men.
Evolution may involve trade-offs
The researchers say this finding adds to the evidence that prenatal estrogen may have played a positive role in the evolution of the human brain.
This study does not show that finger length directly affects brain size. Instead, researchers consider digit ratio as a possible marker of hormone exposure during early fetal development. The results identified a link that may provide clues about how prenatal hormones influenced human evolution.
Manning’s previous research looked at whether digit ratios could provide information about soccer players’ alcohol intake, post-COVID-19 outcomes, and oxygen consumption.
