More than half a million Americans break their wrist every year. It's one of the most common fractures out there, accounting for 25% of the broken bones in kids. What makes it so vulnerable? Something we've all probably done. As you fall you stretch out your arms to try and land on your hands. The result is all that force hitting your wrists. Whilst that might be preferable to landing on your face, it puts your arms in a vulnerable position.
Why hasn't evolution done something about this? It should have given us a more robust hand. Or maybe an extra limb for catching us as we fall. Well, it turns out we have evolved to deal with the risks of walking upright. Our wrists evolved a degree of redundancy, allowing them to happily break.
Well, as happily as any broken bone can be.
The harms of bipedalism
Walking upright has given our family many benefits. It exposes less of our body to heat, allowing us to survive in hotter environments than our quadrupedal ape relatives. Some even think that we can trace our ability to speak to walking upright. It means that breathing and stepping aren't tied together anymore, allowing us to breath (and thus speak) at our leisure.
However, arguably the critical benefit we gained by walking upright is that it means our hands are useless. At least, mostly useless for walking. So we gave them another use. We can carry babies, reach for fruit, and make and use tools. As we became more committed to our bipedal lifestyle our hands became increasingly specialised at using our hands for manipulation, tools, and more.
Even before we specialised in walking on the ground, our wrists had an important job. We were never knuckle-walkers like chimps. Sahelanthropus tchadensis and the fossils of our other early relatives show we were always specialised at moving upright; we just used to do in the trees. Our hands and wrists used to be specialised for climbing. Long fingers allowed them to hook onto branches and swing through the trees with ease.
Unfortunately, this makes us quite vulnerable as it places a lot of pressure on our hands. Both literally and metaphorically. It means that if we damage our hands then we lose one of our most valuable attributes. And without our tools humans are basically weak, naked apes. Our earlier ancestors also lost out on the trees if they were injured. So they'd be weak apes stuck on the ground without tools. Easy work for a lion.
Redundant wrists
For at least 7 million years our wrist has been crucial to our survival. So you would think that there'd be loads of fossils and other archaeological remains of people who hurt their wrist and promptly died. But whilst there is a lot of evidence for the former, the subsequent death seems surprisingly rare. Almost everyone survived and their fractures showed signs of healing.
As important as our wrists are, it seems that we can break them with relatively little consequence. This was first identified more than 200 years ago before evolution and Neanderthals had been discovered. In 1814 an Irish surgeon noted:
"The limb will at some remote period again enjoy perfect freedom in all of its motions and be completely exempt from pain: the deformity, however, will remain undiminished through life."
Wrists break, they remain deformed, but function returned. Researchers have discovered this is because our wrist has an extensive degree of redundancy built in. For example, the wrist itself can tolerate about 6 mm of shortening before its function is hampered. This is almost twice the average shortening that occurs during a fracture (3.5 mm for reference). In fact, there is a similar level of tolerance built into almost every aspect of wrist movement that's reduced by breakage. The average tilt in the joint due to damage is 11 degrees, whilst the joint can tolerate 20 degrees before it loses function.
Redundancy like this is unique to humans, made possible by a unique wrist morphology that evolved within the last 4.4 million years. Why would we develop such an overbuilt wrist? Perhaps to deal with the fact we're clumsy bipeds to break it a lot.
Summary
The wrist has a lot of redundancy built in, allowing it to retain function after it's broken. Such a development is unique to our family and seems to be an adaptation to the fact we break our wrist a lot.
References
Nellans, K.W., Kowalski, E. and Chung, K.C., 2012. The epidemiology of distal radius fractures. Hand clinics, 28(2), pp.113-125.
Provine, R.R., 2016. Laughter as an approach to vocal evolution: The bipedal theory. Psychonomic Bulletin & Review, pp.1-7.
Uzoigwe, C. and Johnson, N., 2016. Wrist function in malunion: Is the distal radius designed to retain function in the face of fracture?. The Annals of The Royal College of Surgeons of England, (0), pp.1-4.
