Hold up a hand. If you're female your ring and index fingers should be roughly the same length. If you're male, the ring finger should be a bit longer, in general. This is the 2D:4D digit ratio and is one of the more famous examples of sexual dimorphism in humans.
So what? This is a website about human evolution, not just listing random human facts. So what does this have to do with our evolution? After all, we aren't aye-ayes, who've evolved a creepy long finger they actually use.
Well, it's not the digit ratio itself that's interesting but what it's linked too. See, there's a correlation between this ratio and hormones during development. This, in turn, has a correlation with key attributes like how we mate. So by tracking changes in this ratio, we can track the evolution of these features. Right?
Well, maybe not. For the first time, researchers have found a group without sexual dimorphism in this digit ratio.
Don't Yali
This unusually fingered group are the Yali of Indonesia. These are horticulturalists living in the mountainous Paupa region of Indonesia. This is so difficult to navigate it wasn't fully explored the 1950s. As such, most of their contact is with each other and other nearby groups. Although they're often in conflict with the latter so avoid them as well.
Living off pigs and yams in the mountains of Indonesia is tough work. Particularly when they're so isolated. Even bigger villages only get a dozen visitors every year. As such, they still seem to retain many traditional aspects of life.
For example, initiated men live in a communal hut. Although they may be married in a polygenous relationship, the women don't get to move into that hut. Instead, the husband will build their wives their own hut (each woman often getting an individual hut, even if part of the same marriage). Despite this inequality in huts, both sexes have to work very hard to survive in these conditions. Men normally build and plant crops, whilst women tend to and harvest them. As such, women are often the ones responsible for most of the food of the family.
And of course, their digit ratio doesn't differ. Or rather, the difference is statistically insignificant.
Now, at this point, you might expect me to launch into my usual tirade about how this is what happens when you only study Westerners. You wind up thinking a trait (like the digit ratio) is universal when really it only happens in certain circumstances. But this isn't the case here. The digit ratio has been studied in many cultures and - whilst its severity does vary - it always exists.
Tha Yali people are a genuine anomaly.
Evolving digit ratios
Like I said, the significance here isn't the digit ratio itself. Rather, what it's linked to is what's interesting. Specifically, to the hormones you're exposed to in the womb. This, in turn, is interesting from an evolutionary point of view because it seems like those hormones have been changing with our evolution, with interesting results.
See, one of the big hormones involved in the digit ratio is testosterone. The more of that you get, the longer the ring finger and the more typically "male" the resulting ratio. This tracks across species too, with the more testosterone filled and aggressive chimps having a manlier digit ratio than bonobos. And when you look at fossil hominins, you see a shift from a more chimp-like ratio towards the bonobo-like human one.
In other words, it seems that over our evolution we've been evolving lower testosterone levels. Some have hypothesied this is responsible for our lower aggression compared to chimps, and a shift away from a chimp-like mating system towards our pair-bonding strategy. In it's extreme, most low-testosterone version we have monogamy.
The implication
So how much of this do we have to chuck out based on the Yali? Less than you might think. Research shows that the hormones they experience in the womb are much more similar between the sexes than in other groups. This may stem from their unusual living conditions, where both men and women have to work exceptionally hard to survive.
In other words, the relationship between hormones and digit ratio still stands.However, despite this it's worth remembering the Yali are polygamous. That's not how it's supposed to work, with more male testosterone being the one theoretically linked to polygamy. So it looks like that part might be out the window.
Long story short, our species has had changing hormones over our evolution. This is true. But what that means is a bit more debatable than we thought.
References
Marczak, M., Misiak, M., Sorokowska, A. and Sorokowski, P., 2017. No sex difference in digit ratios (2D: 4D) in the traditional Yali of Papua and its meaning for the previous hypotheses on the inter‐populational variability in 2D: 4D. American Journal of Human Biology.
McFadden, D. and Shubel, E., 2002. Relative lengths of fingers and toes in human males and females. Hormones and Behavior, 42(4), pp.492-500.
McIntyre, M.H., Herrmann, E., Wobber, V., Halbwax, M., Mohamba, C., de Sousa, N., Atencia, R., Cox, D. and Hare, B., 2009. Bonobos have a more human-like second-to-fourth finger length ratio (2D: 4D) than chimpanzees: a hypothesized indication of lower prenatal androgens. Journal of Human Evolution, 56(4), pp.361-365.
Nelson E, Rolian C, Cashmore L, & Shultz S (2011). Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus. Proceedings. Biological sciences / The Royal Society, 278 (1711), 1556-63 PMID: 21047863
Sorokowski, P., Sorokowska, A. and Danel, D.P., 2013. Why pigs are important in Papua? Wealth, height and reproductive success among the Yali tribe of West Papua. Economics & Human Biology, 11(3), pp.382-390.
