Promiscuity riddle

Although Neanderthals have not lived in Europe for 30,000 years, they continue to fascinate scientists around the world.

A new study from the United Kingdom suggests that Neanderthals had more partners than modern humans. They reached this conclusion by comparing the ratio of different finger lengths that allow scientists to determine some kinds of behavior, including sexual promiscuity.

Emma Nelson, a doctoral student at the University of Liverpool, had shown in previous research that high levels of in-utero androgens increase the length of the fourth, or ring, finger when compared to the second, or index finger. The team found that a low ratio indicates higher levels of competitiveness, aggressiveness and more sexual partners, while a high ratio indicates more monogamy.

Comparisons amongst four species

In a paper published today in the British journal Proceedings of the Royal Society B, the team compared the finger-length ratios amongst various ancient hominin species, including Ardipithecus ramidus, a hominid who lived around 4.4 million years ago; Australopithecus afarensis, which lived around three to four million years ago; Neanderthals, who disappeared around 28,000 years ago; and early humans dating back to 95,000 years ago.

"It is believed that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system," said Nelson in a statement. "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 behavior 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."

The study found that modern humans have an average of a 0.957 index-to-ring finger ratio, and are generally a more monogamous species, while chimps', gorillas' and orangutans' average ratio is at 0.91, and are generally more promiscuous. The researchers also found that an early modern human found in an Israeli cave, dating back to approximately 95,000 years ago, had a ratio of 0.935, which would be more promiscuous than modern humans as well. Neanderthals, meanwhile, had a ratio of 0.928.

Further study needed

In an interview with the Agence France Presse, Nelson added that the research team would like to be able to confirm their findings through other means.

Anthropologists still do not agree when or why humans and their ancestors began to migrate from a more promiscuous mating system to a more monogamous one, given that the former group would hold an evolutionary advantage in terms of being able to spread their genes more quickly and more widely.

"Pair-bonding, in a broad sense, is universal among humans, but it is not known when the transition from a promiscuous mating system to a stable bonded one occurred," wrote the scientists in their scientific paper.

Disagreement amongst anthropologists

However, not all scientists agree with these findings.

In an interview with the American news website, MSNBC.com, another anthropologist who was not part of the study said that this correlation of finger ratios is not a "good predictor of any social behaviors."

"In addition to the problem of using a poor predictor, this study has another problem that we often face with fossils -- there are very few of them, and it's not obvious which sample of living primates we should be comparing with them," said John Hawks, an anthropology professor at the University of Wisconsin, Madison.

Author: Cyrus Farivar (AFP)
Editor: Stuart Tiffen