Biology Magazine

Human Hands Are Still Evolving; but Not for the Reason You Might Think

Posted on the 22 October 2015 by Reprieve @EvoAnth
Human hands are still evolving; but not for the reason you might think

Evolution takes place at a very slow rate. This can make it very difficult to identify if any part of us is still evolving. Which - as an egocentric species - we obviously want to know about. Fortunately for our egos, new research has found that at least one part of our body is still evolving after all: our hands. However, the cause for this isn't what you might expect since it doesn't involve anything we actually do with our hands. Instead, it all comes down to climate.

The evolution of our hands has long been influenced by what we do with them. Our early, ape-like relatives (like Lucy) had long, curved fingers. These served as (if you'll pardon the pun) handy "hooks" at the end of our arms. This made it much easier to grab onto branches and climb tress; something they often seemed to do.

However, as our relatives spent more time using and making tools (and less time in the trees) our hands began to change. The fingers got short and stubby whilst the thumb became relatively long. This allowed us to easily form a "pincer grip". This sort of grip is very precise yet still quite powerful. The perfect (again, more puns need forgiving) tool to help us do fiddly things. Like make and use early stone tools.

So when considering whether our hands are continuing to evolve; it makes sense that what we do with them would be a big influence on this. In fact, speculative studies on how our hand could improve have identified several bits that might change to help us adapt to modern life. Like improving the strength of our little finger to aid in typing. Yet this new research did not identify any functional reason for the ongoing evolution of the hand.

The study examined the handbones from populations around the world. Their goal was to test whether or not environment influenced the size and shape of them. There's a biological "rule" (called Allen's rule) that populations of mammals in colder environments have shorter limbs. This reduces their surface area, minimising heat loss. It's already been established that humans have been influenced by Allen's rule; but these researchers wanted to see if our hands were being altered as well.

Well - as the introduction to this post gave away - it turns out we are. Humans living in colder climates had shorter, stockier first metacarpals (bones in the palm of the hand). This would help reduce the overall surface area of the hand; minimising heat loss and perhaps even frostbite. Given that hands are one of the few parts of the body that are routinely naked, this could provide a fairly big advantage.

And it would seem that this advantage is strong enough to drive ongoing evolution of the human body. That said, some people disagree. They believe that Allen's rule is the bi-product of the fact that populations living in colder climates wind up with less food. Thus, the smaller limbs are the result of nutritional deficits rather than an adaptation.

However, this study also examined the foot. They found no such changes in size there; which would be expected if evolution was at work. Reducing its size would make locomotion less efficient, negating any benefits from reducing heat loss. However, if nutritional deficits really were behind it surely the foot would also be affected by the poor diet.

So the next time you wind up outside in the cold without gloves remember: evolution has helped make that experience a bit less unpleasent for you. Praise Darwin.

References

Betti, L., Lycett, S. J., von Cramon‐Taubadel, N., & Pearson, O. M. (2015). Are human hands and feet affected by climate? A test of Allen's rule. American journal of physical anthropology, 158(1), 132-140.


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