When it comes to love, we Homo sapiens are a peculiar breed: We thrill at the thought of torrid affairs while dreaming about the perfect someone with whom we can spend the rest of our lives.
Some of this never-ending tug-of-war for our hearts is certainly cultural, but according to a new study it's also encoded in the finger bones of Neanderthals and the upright walking primate Australopithecus.
Emma Nelson of the University of Liverpool and a team of researchers combed through literature on early human-like primates in search of fossils that contained hands with intact index and ring fingers (the second and fourth digits).
In humans and primates, the ratio between the index and ring fingers is thought to be a telltale marker for how much of the androgen class of hormones — and specifically, testosterone —someone was exposed to while in the womb. Extra androgen leads to longer ring fingers, the thinking goes, and therefore a lower index-to-ring finger ratio.
Though highly contentious, studies indicate that men who receive high levels of androgen before birth are more likely to be stronger, faster, and more sexually competitive. Women who receive high levels of androgen may have similar traits.
Nelson's work suggests the same holds true for most primates living today, but the team wanted to see how our ancient relatives stacked up. They found two Neanderthals and one Australopithecus afarensis skeleton with the first bones of the index and ring fingers intact —enough detail to do the job.
The Neanderthals had long ring fingers, suggesting they were a promiscuous bunch — like many primates alive today they probably lived in groups. Males may have likely either kept harems of female mates, or males and females each mated with multiple partners.
A. afarensis, made famous by the popular "Lucy" skeleton, lived between 4 and 3 million years ago, long before modern humans. Its short ring finger hints that it was faithful to a single mate, but Nelson says that doesn't sit well.
"These were small creatures that probably lived in groups and were being eaten by predators." she said. "How do you keep from mating with different members of the group?"
Nelson is presenting the team's work today at the Society for Vertebrate Paleontology meeting in Bristol, United Kingdom.
"What they're seeing is very interesting," Dean Snow of Pennsylvania State University in University Park. "The difference between being pair-bonded and non pair-bonded mating is a major watershed within primates. If a distinction is that Neanderthals weren't pair-bonded and modern humans were, that would be a major consideration in trying to figure out why modern humans out-competed Neanderthals in Europe."
Pair-bonded males help feed and look after females while they're pregnant, while females and males both forage equally in non-pair bonded social structures, Snow added.
But because the work involved such a tiny sample size, it is highly speculative, Nelson noted. She stressed that firm conclusions about the sex lives of our ancestors can't be made until the team examines many more fossil hominids, not least of which should be skeletons of H. sapiens that lived during the same period as Neanderthals.
If successful though, her results could shine light on why the modern human animal displays such an array of of sexual behavior.