Neanderthals outperformed their human counterparts when it cae to sexual relations (Source: erix/Flickr)
Neanderthals may have been underdeveloped mentally compared to modern humans, but in one respect they outperformed us: In the number of sex partners.That's the conclusion of a study published by the Proceedings of the Royal Society B, which suggests finger length can indicate promiscuity among hominins, as the ancient family of humans is known.
Researchers led by Emma Nelson of Liverpool University, northwestern England, looked at fossilised fingers from four hominin species.
They comprised Ardipithecus ramidus, a hominid who lived around 4.4 million years ago; Australopithecus afarensis around three to four million years ago; Neanderthals, who disappeared around 28,000 years ago; and a fossil of an early Homo sapiens, as anatomically modern humans are known, from around 90,000 years ago.
Nelson's theory is based on the ratio between the length of the index finger and that of the ring finger.
Competitiveness and promiscuityPrevious research by her group concluded that exposure in the womb to key sex hormones known as androgens, which includes testosterone, affects finger length - and future behaviour.
High levels of in-utero androgens increase the length of the fourth finger in relation to the second finger, which thus lowers the ratio.
They are also linked with competitiveness and promiscuity, according to this work.
So how did the primates line up?
A low finger ratio showed Ardipithecus ramidus was likely to "play the field", while a high finger ratio indicated Australopithecus afarensis was more likely to stay at home.
Meanwhile, low ratios from the Neanderthal and the early human "suggest that both groups may have been more promiscuous than most living human populations," say the authors.
The scientists admit that their approach is novel, and further evidence is needed to shed light on the social behaviour of ancient humans.
"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," says Nelson.
The study's conclusions add a new element of debate over human lineage. More promiscuous species of hominins would theoretically have an advantage over monogamous ones, both in terms of more offspring and a more varied gene pool.