Biology Magazine

Our Hands Are an Evolutionary Accident

Posted on the 27 February 2014 by Reprieve @EvoAnth
The big difference is that foot bones are called tarsals but hand bones are called carpals.

The big difference is that foot bones are called tarsals but hand bones are called carpals.

Our hands and legs are very similar, sharing many of the same bones and muscles. They’re just a different size and shape in each limb. In fact, pretty much every bone in your hand has a matching partner in your feet. And these limbs don’t just share physical similarities; recent research has revealed that they share a lot of the same genes and developmental pathways1.

Both our hands and feet appear to be highly adapted to fulfill their different roles. Large and robust, the footbones are great at supporting our body weight whilst our small and stubby fingers make it really easy for us to touch out thumb to our fingers; creating the precision grip we use to make tools and all that fun stuff2. But the shared genes and developmental pathways raise the fascinating possibility that some of these features might be the result of coincidence. If evolution altered the foot, the changes to the genes might also result in changes to the hand (or vice versa); changes which turned out to be useful by chance.

Back in the distant past of 2010 three researchers wanted to investigate this possibility. They measured the finger and toes of nearly 300 apes and humans, looking to see if there really was a correlation between them. Which they found, suggesting that the two sets of digits were co-evolving after all. They were able to extrapolate out from this correlation, creating a model that simulated how changes in one limb would influence the other1.

This computer hokus pokus provided very strong evidence that the two digits were co-evolving, limiting their evolutionary options. Although being short and stubby is useful for the fingers, they can’t get too short or it would compromise the toes ability to walk. Our digits are not optimally evolved, but the result of an evolutionary compromise1.

They also ran a second set of simulations, in which they took chimp toes and fingers and “evolved” them into human toes and fingers. This simulated evolution showed that natural selection was operating on our ancestors toes with almost twice the force it was influencing the fingers. As such, the evolution of our toes was likely overwhelming the signal from the fingers and this was the primary driving force behind the evolution of our digits. In other words: evolution changed our toes, which coincidentally changed our fingers1.

Of course, they were only examining one aspect of our hands (the size and shape of fingers) and it’s not like our hands and feet share every gene. This is also just a simulation, so take it with as many pinches of salt as you think that mandates. Our hand has done a fair bit of evolving on its own, specialising as a highly dexterous, tool making instrument. But the initial change which kicked off (if you’ll pardon the pun) this evolutionary trajectory appears to be changes to the size of the fingers; which are the result of co-evolution with the foot2.

Our success as a species can be traced back to our tool use, which stems from our powerful precision grip. A grip that appears to have originally been an evolutionary accident. In the 4.5 billion years of our plants history, I think we would have to take the award for “species with the best dumb luck.”

References

  1. Rolian, C., Lieberman, D. E., & Hallgrímsson, B. (2010). The coevolution of human hands and feet. Evolution64(6), 1558-1568.
  2. Ambrose, S. H. (2001). Paleolithic technology and human evolution. Science,291(5509), 1748-1753.

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