Ring finger indicates early humans were promiscuous

Science

03 Nov 10

A new study indicates that Neanderthals and early humans were more promiscuous than most human populations today. The research team reached their conclusion by measuring the fossilised finger bones of early extinct apes and extinct members of our own lineage – hominins.

The ratio of the second to the fourth finger can be used as an indicator of mating behaviour in primate species, the study says. This novel approach to understanding how our ancestors behaved was conducted by a team from Liverpool, Oxford, Southampton and Calgary Universities, and is published in the latest issue of Proceedings of the Royal Society.

Previous studies by the research team have shown that promiscuous species have low index to ring finger ratios while monogamous species have high finger ratios. High levels of androgens, such as testosterone, increase the length of the fourth finger in comparison to the second finger. It is thought that prenatal androgens affect finger length during development in the womb, which in turn is linked to adult behaviour. High levels of prenatal androgens are linked with competitiveness and promiscuity.

In this latest study, the finger ratios of early apes were low suggesting they were highly promiscuous, similar to the living great apes. However, hominins were found to be split: Ardipithecus ramidus (around 4.4 million years old) appeared to be highly promiscuous, while Australopithecus afarensis (around 3-4 million years ago) was more likely to be monogamous. Neanderthal ratios and those of an early human from Israel (dated to 90 thousand years old) suggest that both groups may have been more promiscuous than most living human populations. This suggests that hominin mating systems may have been more variable across species than currently recognised.

Senior author Dr Susanne Shultz from the University of Oxford said: ‘Social behaviours 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.’

Lead author Emma Nelson from the University of Liverpool said: ‘Although finger ratios provide some really exciting suggestions about hominin behaviour, we do accept that the evidence is limited and to confirm these findings we really need more fossils.’

Dr Campbell Rolian from the University of Calgary adds: ‘Evidence for the social systems of our ancestors has always been ambiguous, and no single approach will likely ever give a definitive answer.’