Modern humans have a lot of "strengths." We're smart, innovative and make awesome tools. Although ironically, strength isn't one of them. What's up with that? Chimpanzees, our closest living relatives, are physically stronger than us in almost every way. Even Neanderthals - who are so similar to us we interbred- have significantly stronger bones and muscles. Why are humans the runt of the ape litter? Why are humans weak?
Our bones in particular are especially fragile. Trabecular (or spongy) bone in joints is less dense than standard bone, acting as a shock-absorber. However, humans don't have much of this type of bone in our joints (sometimes as little as half the trabecular bone of chimps) making our limbs a lot more vulnerable to damage. Why did we evolve such an obvious weakness?
Popular explanations
The fact humans are weak is hardly a new discovery. Scientists have been studying this question for years and have developed several popular explanations. Arguably the most popular is that humans don't need to be strong any more. Our tools and technology have take over for us. We don't need the muscles to hurl a spear across a field, we can use a bow and arrow instead.
Another popular explanation is that it's an adaptation to walking upright. All of our body weight is supported by only two legs, compared to the four legs that prop up most other apes. As such, our legs have to work a lot harder to keep us upright; so anything which reduces the weight they have to support would be useful. Even if that lost weight also means we loose some muscle and strength.
Unfortunatley Neanderthals undermine both of these explanations. As I just mentioned, they were a lot stronger than us. Yet their technology wasn't particularly primitive and they didn't have much trouble walking either. In short, Homo neanderthalensis seems to have been able to balance being strong with advanced technology and walking upright. So why did humans fail to walk that tightrope? Why are humans weak?
When did human bones become weak?
So, the Neandethals have made many popular arguments for why humans are weak, extinct (haha, get it. Because all the Neanderthals died). However, a recent review of fossil humans' trabecular bone strength may have uncovered the real reason as to why are human bones are weak. A reason not even the Neanderthals can dispute.
This review examined the limb bones of dozens of modern and fossil humans; along with the limbs of other members of the human family (like Neanderthals and Homo erectus) and our close relatives (like chimps). One surprising trend emerged out of all this data: our weak bones are "new."
We're living in a period called the Holocene; which started around 12,000 years ago. Human fossils - and, you know, the people still living today - were found to have relatively little trabecular bone. Yet if you wind the clock back to the period before the Holocene; suddenly we start finding humans with levels of trabecular bone similar to chimps, Neanderthals and the other strong boned species.
So why are humans weak?
So what happened in the Holocene that might explain our evolution of weak bones? Well, the Holocene marks the first human experiments with domestication; eventually leading to the invention of farming. The adoption of farming represents the biggest lifestyle change in our family's history and it seems likely that this might be the source of our weak bones.
Farming is hardly an easy life. Any feudal peasent will tell you that. However, it is a less active lifestyle than being a hunter-gatherer; who often have to travel significant distances to find food. On top of that farming tends to produce a food surplus, allowing people to spend a lot of time doing other things. Things which might not involve that much physical activity.
Nothing in life is free and our bodies are no exception. They require energy to grow and sustain. If evolution can cut a few corners without harming our chance for survival, it will. It's also worth noting that this is actually a genuine case of evolution in action. The change in trabecular bone levels is greater than the variation within our species. Something new was going on.
tl;dr
Why are humans weak? Well, we physically suck in a lot of ways. One key weakeness is that our joints have less spongy bone than many other species; which normally acts as a shock absorber. Why did humans loose some? With the invention of farming our lifestyle became a lot more sedentary and such physical adaptations were no longer needed.
Reference
Chirchir, H., Kivell, T. L., Ruff, C. B., Hublin, J. J., Carlson, K. J., Zipfel, B., & Richmond, B. G. (2014). Recent origin of low trabecular bone density in modern humans. Proceedings of the National Academy of Sciences, 201411696.
