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Caveman were randier than people today, announces the headline in a story in Britain's Mirror newspaper, "because they had long ring fingers".
Huh? would be a reasonable reaction.
According to an article by Emma Nelson, Campbell Rolian, Lisa Cashmore, and Susanne Schultz published yesterday in the Proceedings of the Royal Society B (Biological Sciences), the proportions of the fingers of the hand are tell tale signs of a promiscuous past, at least for Ardipithecus, early modern humans, and Neanderthals.
The researchers argue that
the second-to-fourth digit ratio (2D : 4D), a putative biomarker for prenatal androgen effects (PAEs), covaries with intra-sexual competition and social systems across haplorrhines; non-pair-bonded polygynous taxa have significantly lower 2D : 4D ratios (high PAE) than pair-bonded monogamous species. Here, we use proximal phalanx ratios of extant and fossil specimens to reconstruct the social systems of extinct hominoids.
What this means in layman's terms: when infants in the womb are exposed to high levels of androgens- steroid hormones that contribute to formation of male sexual anatomy- the lengths of fingers are affected, so that finger length ratios is a proposed ("putative") sign of this exposure.
Since we have no access to actual measures of androgen levels in the bodies of ancient human ancestors (let alone to their fetal levels), the durable finger bones provide an indirect (hence "putative") measure of relative levels of androgen exposure.
In studies of living monkeys and apes (haplorrhines), the ratio of forefinger (2D) to ring finger (4D) is correlated with forms of sexual pairing: species that form durable bonds between a single male and female have higher 2nd to 4th digit ratios as a result of having lower exposure to androgens in the uterus. The reverse is true of species where males mate with multiple females: they have lower 2nd to 4th digit ratios, indicating they were exposed to higher androgen levels.
Or, to simplify: androgens affect the fetal body in ways written in the bone, ways that may also lead to different sexual behavioral patterns in adulthood.
Emphasize may.
Studies of so-called "digit ratios" have a more than modest professional and popular currency. Willow Lawson, writing about the research of biologist John Manning, who pioneered this controversial line of research, warned that
You may be tempted to draw conclusions from your own fingers. But it's impossible to do so accurately in a vacuum... Fingers are an indication of the environment that molded the brain, but only if you know how you measure up to others....
Even as digit ratio research flourishes and more behavioral links are established, the relationships will remain mere statistical correlations until researchers fully understand how sex hormones physically affect the brain.
Apparently, no such caution is necessary when extrapolating the sexual behaviors of entire species from a few isolated finger bones.
Not that everyone is convinced. Anthropologist John Hawks of the University of Wisconsin sounds a cautious note in comments on MSNBC's Cosmic Log, noting that the digital ratio
"may be correlated with mating system in primates, but that doesn't mean it's a good predictor of mating system. ... As fossil hominins go, I wouldn't expect the story to go any further - there just aren't many hands, so there's never going to be a significantly predictive result" (my emphasis).
Correlations aren't causes; and in this case, a lot of basic work needs to be done to convince skeptical anthropologists:
"If you were going to do this study right, you would look far beyond the apes to take in many kinds of primates with different social systems. Then you could see whether closely related species have 2D:4D ratios that track their mating systems."
Meanwhile, brace yourself: within hours of the release of the research paper, Google news reported upwards of 40 news stories, most of them emphasizing how Neanderthals are now shown to have been promiscuous "sex-obsessed thugs" with "naughty" sex lives.
Projection, anyone?
