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So… Did knuckle walking evolve twice?
January 11, 2010Posted by on
We had lots of clues that this was the case before Ardi, but now that we’ve got Ardi- the palmigrade extraordinaire, we know that humans did not go through a knuckle-walking phase, and that chimpanzee knuckle-walking has evolved since the split with our last common ancestor with them. Which would also means that it evolved after our split with the gorillas… which means that knuckle-walking evolved twice.
As we’ve discussed before, knuckle-walking is a pretty weird thing to do, which is why the idea that it evolved only once is hard to shake. But once you’ve got a particular body plan, there are only so many ways to accomplish a certain task. For example, the now-extinct Great Auk was a flightless sea bird that hunted fish underwater. It was white on its front, and black on the back, and had powerful rear feet and webbed toes. Sound familiar? The Great Auk was the Northern Hemisphere’s version of the Penguin, but the two were not particularly closely related. It’s simply that, once you’ve got the body plan of a bird and you want to start diving for fish at high latitudes, you’ve got to rework the wing a little bit so that it’s no longer any good for flying in the air- but man, will it be good for underwater flight! And then you can work on your body shape a little so that you’re like a little avian torpedo. And then you’ve got to put some body fat on so that you can withstand the frigid ocean temperatures. And voila! You’ve got two almost-identical ocean birds separated by an entire planet and a couple of hundred thousand years, if not more.
The apes have done a very similar thing. The general Miocene ape was an above-branch quadruped. The ape had a shoulder in which the socket of the shoulder joint (the glenoid fossa of the scapula) was deep and cup-like so that the ball of the humerus was relatively stable. In order to get a flexible shoulder, the ape had to move its scapulae so that the glenoid fossa was facing laterally (outward, toward the sides) instead of ventrally (forward). The glenoid fosssa also became a little shallower, so that it could accommodate a wider range of motion. When monkeys, or squirrels, or dogs land on the ground with their front feet, the forces are allowed to travel all the way up the humerus until they reach the glenoid fossa. But think… if you’re an ape with a fancy new flexible shoulder, and you try to land on your front feet, what happens?
Well, you’ll dislocate your shoulder. That’s a very bad thing for an ape to do!
Another thing that some of the apes have done is make their elbow more flexible, as well. If you look at a monkey’s (or squirrel’s, or dog’s) ulna at the elbow, they have a huge chunk of bone on the end of the olecranon process (the part that articulates with the distal humerus) that we don’t have. Also, the little U that cups the trochlea of the humerus points to the side in all of those quadrupeds- more like a C than a U. But in apes, it points straight up. All of that makes for an extremely flexible elbow- and gives us the same problem with dislocation that we had with the shoulder.
So, you’re a Miocene ape, and you’ve evolved this flexible shoulder and flexible arm, but you’re also starting to get quite large. So large, in fact, that sometimes you have to leave the trees because the terminal branches that connect one tree to another just aren’t large enough to support you. So, you come down out of the trees and walk on the ground. And man, are your muscles working overtime to keep those bones from dislocating! Each step you talk with your palms on the ground sends a huge amount of force up through your wrist to your elbow, and then up through your shoulder. Your muscles are concentrically contracting, trying to keep everything in its right place- but the problem with concentric contraction is that you’re already contracting as hard as you can, so one misplaced step, and your muscles can’t do anything to help you avoid injury.
But if you walk on your knuckles, it doesn’t do that! Because you are such an excellent climber, you have these massively strong flexor tendons in your fingers. If you flex them and put your weight on your knuckles, those tendons eccentrically contract and are able to absorb some of the force coming at you from the ground. That way, your arm muscles don’t have to work so hard simply to keep your bones from dislocating. Plus, you’ve got those long fingers for vertical tree climbing and suspension, and tucking them under your hand gets them out of the way.
Gorillas and chimps have both figured this out (anatomically speaking- I don’t know if any of them have put any thought into it!). And orangutans are fistwalkers- but I know that I’ve seen one on a really hard concrete surface using his knucles. It’s a classic case of convergent evolution- but you can’t really tell from the genetics or the living anatomy of the animals- we needed a fossil to finally be relatively certain. Much like the Auk and the Penguin, the Chimp and the Gorilla faced similar problems brought about by their environment and anatomy, and ended up solving them in similar ways.
Lovejoy, C., Simpson, S., White, T., Asfaw, B., & Suwa, G. (2009). Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive Science, 326 (5949), 70-70 DOI: 10.1126/science.1175827
Ward, Carol (2007). Postcranial and Locomotor Adaptations of the Hominoids. Handbook of Paleoanthropology. DOI: 10.1007/978-3-540-33761-4