There are many unique things about the human body. We've got a large brain, move around upright, and are relatively hairless. Our body is also pretty big. In fact, we're the second largest primate out there; beaten only by gorillas. And since they could actually physically beat us up; I'm happy to let them win that competition.
Many explanations for why we evolved such large bodies have been proposed. One of the more popular is that it gives us very long legs. These make our strides longer; lowering the relative energy needed to walk. Since our ancestors wound up walking all the way around the world,this seems to make sense.
However, there is growing evidence that the human body might not have evolved for any specific reason. It turns out that it's not a completely independent trait. Changes to other bits of our ancestors might have "accidentally" driven the development of a big human body.
Co-evolution of the human body
Genetics is hard work. A single gene can be involved in the development of many different characteristics. Each characteristic, in turn, could be governed by dozens of genes.
All of this means that evolution rarely causes a single trait to change; even if only one trait is beneficial. As long as the other changes aren't detrimental; they'll tag along for the ride. This phenomena is called "correlated evolution" which is exactly what it sounds like. Biologists are nothing if not blunt.
Still I'd rather have a boring name than a stupid one like in physics. Charm quarks can go suck a lemon.
Of course, just because two traits are correlated doesn't mean that one evolved "accidentally". Changes to one could force the other to evolve. For instance, some think Neanderthals had to develop a larger gut to deal with all the meat they ate. This would have forced their body to change size and shape.
Where things start to get interesting is when one gene influences several different characteristics. Evolutionary forces might drive it to change because of how it influences one trait; "accidentally" dragging other traits along with it. The phenomena of genes doing lots of stuff is called " pleiotropy ".
Which might sounds fancy, but is just Greek for "many changes". Again, biologists are blunt.
The brain did it!
Biologists have found many traits whose evolution seems to be correlated with body size. Like the changes to gut seen in Neanderthals. But a more prominent one is brain size. Animals with larger bodies also have larger brains. So the real question is whether this shift is a case of genetic correlated evolution or actual pleiotropy.
For a long time it was thought that it was the former because humans have ruddy massive brains. If our body followed the same correlation seen in other animals our body should be upwards of 7 times larger than it actually is. Now, whilst it might be great to be that big (we could finally stomp those smug gorillas) we're clearly not. So the same genes can't be responsible for both developments. Or can they?
A thorough review of many different datasets revealed that there was actually a pleiotropic correlation between the human body and the human brain. Examination of modern humans, primates, fossil hominins, and statistics all revealed a link between brain and body size in our species.
They aren't completely intertwined. After all, the exact same genes don't code for both body and brain. That's why our body isn't absolutely massive. It also means our large bodies aren't completely accidental. But a significant chunk of our size is simply a side-effect of having big brain. In fact, it's around 1/3. Which just so happens to be how much bigger we are compared to a chimp.
So yeah, our body isn't totally accidental. But it seems that a good chunk of it is.
Accidental bodies?
The relationship between brain and body size seen here is also seen in the fossil record of our family. As our ancestors' brains got bigger, so did their bodies.
Around 2 million years ago Homo erectus first appeared, with a brain 70% larger than earlier species. Their body was 30% larger; matching the model. Similar leaps in body size were seen as our brain continued to grow. Up to a point.
It turns out the human body isn't completely linked to the human brain. As the results of this study show in modern humans natural selection was still pushing for a big brain; but also a smaller body. It seems that we were getting too big for our boots, literally. Perhaps our ancestors just couldn't find enough food to fuel a big brain and body, so natural selection was trying to hold body size back.
Clearly the human body was not completely accidental.
The author of this research even identifies some benefits to large bodies (in the context of large brains) that could explain why it took so long for evolution to try and separate the two. The larger bodies could have improved our hunting capability; allowing us to get enough meat to fuel our big brains. After all, they take about a Big Mac's worth of calories a day to fuel. We have to fight with the lions for that meat.
We evolved big brains. This led to big bodies. Natural selection was all "this is great; lets stick with it". And modern humans were born.
tl;dr
Because some genes code for lots of different things, evolving a bigger brain results in a bigger body. So it turns out the human body might not have evolved for anything in particular.
References
Aiello, Leslie C., and Peter Wheeler. 1995. The expensive-tissue hypothesis: the brain and the digestive system in human and primate evolution. Current Anthropology 36(2):199-221.
Bramble, Dennis M., and Daniel E. Lieberman. 2004. Endurance running and the evolution of Homo. Nature 432(7015):345-352. doi:10.1038/nature03052.
Grabowski, M., Costa, B., Rossoni, D., Marroig, G., DeSilva, J., Herculano-Houzel, S., Neubauer, S. and Grabowski, M., 2016. From Bigger Brains to Bigger Bodies: The Correlated Evolution of Human Brain and Body Size.Current Anthropology, 57(2), pp.000-000.
