A baby’s fingers might hold a clue about how the human brain got bigger.
A study of 225 newborns suggests prenatal estrogen may have played a part in the evolution of larger human brains. Researchers found that boys with finger-length patterns linked to higher estrogen exposure before birth tended to have larger head circumferences, a measure commonly used as a general indicator of brain size in newborns.
The research was led by Professor John Manning from Swansea University’s Applied Sports, Technology, Exercise and Medicine research team and colleagues from Istanbul University’s Department of Anthropology. The findings were published in Early Human Development.
Manning studies digit ratio, which compares the length of the index finger, or 2D, with the length of the ring finger, or 4D. The measurement is called the 2D:4D ratio and researchers use it as an indirect indicator of the balance of estrogen and testosterone a fetus may have been exposed to during the first trimester of pregnancy.
People thought to have had relatively higher exposure to estrogen than testosterone generally have a longer index finger compared with their ring finger. That produces a higher 2D:4D ratio.
The team measured the 2D:4D ratio of 100 boys and 125 girls, then compared it with head circumference.
They found that boys with a higher 2D:4D ratio, indicating high prenatal estrogen, also tended to have a larger head circumference. The same relationship was not observed in girls.
The results may support the estrogenized ape hypothesis, which proposes that the evolution of larger human brains happened alongside changes that made the human skeleton less robust and more physically feminine than those of earlier ancestors.
Professor Manning said: “This finding is relevant to human evolution because increases in brain size are found alongside feminization 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.”
The study does not show that finger length directly determines brain size. The researchers said digit ratio may instead act as a marker of hormonal exposure during early fetal development.
The paper is titled Digit ratio (2D:4D) and neonatal head circumference: Evidence for oestrogenization in human brain evolution? by Barış Özener, John T. Manning, Berna Ertuğrul and Fatih Aydık.
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