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The length of the finger points to promiscuity of early humans

3 November 2010

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. (The skeleton nicknamed ‘Lucy’, possibly the world’s most famous early hominin, belonged to Australopithecus afarensis.) However, 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.

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.

The research is supported by the British Academy Centenary Research Project, Lucy to Language, a multidisciplinary project that aims to understand the complexities of human social evolution.

For more information or for photographs, please contact the University of Oxford Press Office on +44 (0)1865 280534 or email press.office@admin.ox.ac.uk

Alternatively, please contact the University of Liverpool Press Office on 0151 794 2248, mobile 07970 247836 or email samantha.martin@liv.ac.uk

Notes for Editors:

  • The paper ‘Digit ratios and hominid social systems’ will be published by Proceedings of the Royal Society B on  Wednesday 3 November.
  • The University of Oxford’s Institute of Cognitive and Evolutionary Anthropology ICEA is a degree-granting component of the School of Anthropology and Museum Ethnography. The School is part of the Division of Social Sciences at the University of Oxford. ICEA is directed by Professor Robin Dunbar and is housed in its own building at 64 Banbury Road. ICEA supports two degree programs: a one-year MSc in Cognitive and Evolutionary Anthropology, and a (normally three-year) DPhil in Anthropology. ICEA also houses the Centre for Anthropology and Mind, a research centre under the direction of Professor Harvey Whitehouse with a mission to generate ground-breaking scholarship in the cognitive scientific study of culture. www.icea.ox.ac.uk
  • The University of Liverpool
    The University of Liverpool is a member of the Russell Group of leading research-intensive institutions in the UK. It attracts collaborative and contract research commissions from a wide range of national and international organisations valued at more than £98 million annually.