(Australopithecus afarensis recreation; Wikimedia Commons image)
Nelson and her team discovered that the fossil finger ratios of Neanderthals and early humans were lower than those of most living people. This suggests they had been exposed to high levels of prenatal androgens, indicating they were likely to be more competitive and promiscuous than people are today.
The results, published in the Proceedings of the Royal Society B, also suggest that Australopithecus, which lived approximately three to four million years ago, was likely monogamous, while the earlier Ardipithecus appears to have been highly promiscuous and more similar to living great apes.
Nelson explained, "It is believed that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system. We have recently shown that promiscuous primate species have low index to ring finger ratios, while monogamous species have high ratios."
She added, "We used this information to estimate the social behaviour of extinct apes and hominins. Although the fossil record is limited for this period, and more fossils are needed to confirm our findings, this method could prove to be an exciting new way of understanding how our social behavior has evolved."
Susanne Shultz from the Institute of Cognitive and Evolutionary Anthropology at the University of Oxford also worked on the study.
Shultz concluded, "Social behaviors are notoriously difficult to identify in the fossil record. Developing novel approaches, such as finger ratios, can help inform the current debate surrounding the social systems of the earliest human ancestors."