Early human ancestors were probably more sexually promiscuous than present-day societies if a study of the finger lengths of fossilised bones is to be believed.
Scientists from Liverpool University have already claimed to have found a link between the relative lengths of the second and third digits on the hands of various primate species and their tendency to be promiscuous.
Now the researchers claim they have found that the finger lengths of the Neanderthals and other early human species suggest that sexual promiscuity was rife in the Stone Age compared to modern-day Britain.
Their claims are based on the idea that variations in the levels of male sex hormones in the wombs of a pregnant woman influence the relative growth of the baby's second hand digit (the forefinger) and the fourth digit (the ring finger).
High levels of androgens result in a short forefinger and a longer ring finger, resulting in a low forefinger to ring finger ratio, whereas low androgen levels cause a higher ratio. The scientists believe that because prenatal androgen levels are linked to male characteristics such as aggression and competitiveness, finger length ratios can be used as a marker for a person's "maleness".
"It is believed that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system," said Emma Nelson, of Liverpool's school of archaeology, classics and Egyptology.
"We have recently shown that promiscuous primate species have low index to ring finger ratios, while monogamous species have high ratios. We used this information to estimate the social behaviour of extinct apes and hominins."
One ancient group of hominins that lived three to four million years ago, called Australopithecus, had a finger ratio indicating that it lived a monogamous lifestyle, whereas another even earlier group, called Ardipithecus, had a finger ratio suggesting it was highly promiscuous and had a sexual lifestyle more like modern primates, according to the study in the Proceedings of the Royal Society B.
Susanne Shultz, of the Institute of Cognitive and Evolutionary Anthropology at 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."