Neanderthals, early apes and other ancient human species may have been much more promiscuous and competitive than we are today, according to an unusual new study based on finger fossils.

The fingers weren't found in compromising positions, although that would've made for some interesting photos. Instead, researchers used finger ratios from fossilized skeletal remains to make the determination.

It's a reworking of a technique that has been applied to everything from studies on sexual orientation to research on why some people are superstitious.

(Neanderthal skeleton at the American Museum of Natural History. Note the fingers. Credit: Claire Houck)

The gist is that androgens — a group of hormones important in the development of masculine characteristics — are thought to affect finger length. High levels of the hormones are believed to increase the length of the fourth finger in comparison to the second finger, resulting in a low index to ring finger ratio.

Emma Nelson, from the University of Liverpool's School of Archaeology, Classics and Egyptology and her colleagues applied the digit ratio theory to fossilized finger bones from early apes, Neanderthals, and two other early hominids: Ardipithecus ramidus and Australopithecus afarensis.

(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."