Neil Roach and colleagues have written a paper in Nature this week about the role of elastic energy storage in human throwing
Throws are powered by rapid, sequential activation of many muscles, starting in the legs and progressing through the hips, torso, shoulder, elbow and wrist. Torques generated at each joint accelerate segmental masses, creating rapid angular movements that accumulate kinetic energy in the projectile until its release. It has been shown that internal (medial) rotation around the long axis of the humerus makes the largest contribution to projectile velocity. This rotation, which occurs in a few milliseconds and can exceed 9,000 per s (ref. 13), is the fastest motion that the human body produces.
That's just lovely biomechanics-speak. The paper demonstrates some new details of how throwing functions in humans. If I were going to criticize it, I would say it doesn't really provide a comparative basis for its conclusions. Humans with lots of practice (like the subjects in the paper) tend to throw in a particular way, but underhand pitchers in fast pitch softball show that it is possible to attain nearly equivalent throwing performance with a very different method, using very different muscle groups than typical for baseball. The fact that we can learn to do both things very well does not prove that we evolved to do either of them.
What I don't really understand is where all the hype in the press has come from about this paper. Many stories have attempted the "evolution of baseball pitchers" hook, or something similar. The paper itself makes essentially no claims about the evolution of throwing, other than a general claim that it may have been important to human hunting in the last 2 million years. Here is what the paper includes about the evolutionary history of features relevant to throwing:
It is difficult to establish when high-speed throwing first evolved because the first projectiles were probably rocks and untipped wooden spears (Supplementary Notes 7?and?13). However, many of the derived morphological features that help human throwers to store elastic energy can be assessed in the fossil record (Supplementary Note 14). These features evolved in a mosaic fashion, some pre-dating the evolution of Homo. Tall, decoupled waists first appear in Australopithecus as adaptations for locomotion. Low humeral torsion also appears in Australopithecus, probably resulting from the release of the forelimbs from weight-bearing during quadrupedal locomotion, and is present in early Homo (Fig. 4d). Although variation in glenoid orientation exists within Australopithecus, a fully lateral glenoid position is first definitively present in Homo erectus (Supplementary Notes 15 and 16). Such laterally oriented shoulders probably decreased the mechanical advantage of the scapular rotator muscles during climbing, and probably had little or no effect on stone-tool production. Throwing performance may also have benefited from low, wide shoulders, long legs, and hyperextendable wrists, which are all present in H. erectus. Although some of these features were probably selected for functions other than throwing, their combined configuration, first present in H. erectus, would have benefited throwing performance by enabling elastic energy storage in the shoulder, providing a selective advantage during hunting (Supplementary Note 1). Furthermore, high-speed throwing was probably a critical component of a suite of hunting behaviours that enabled early members of the genus Homo to thrive in new and varied habitats both in and out of Africa.
As you can see, it is very nonspecific and demonstrates that different features appeared at different times in human evolution. There is no single moment where the mechanics of throwing suddenly changed, as far as these features can demonstrate. Furthermore, most of these features have influence on other behaviors besides throwing, meaning that it is hard to argue that throwing itself determined the pattern of natural selection on them. Some of them were likely incidental side effects of bipedality -- "spandrels" that did not appear for the purpose of throwing at all. At least one -- a longer waist with more rotational freedom of motion -- may have been shared with other Miocene apes and not characteristic specifically of hominins at all. I think the paper treats the subject with appropriate caution. What they are observing in modern, practiced overhand throwers may have some relevance to the evolution of the shoulder, but is not the whole story.
I find the idea of practice and learning much more interesting. It takes a lot of training to become very skilled at throwing. This was the topic of a recent MythBusters episode, which tested the myth that you can "throw like a girl", by comparing men and women's throwing directly with both dominant (practiced) and non-dominant hands. The difference in throwing style went down to training. And indeed, if you compare unpracticed humans to practiced chimpanzees, you see the importance of practice and training to throwing form.
On the subject of learning, practice, and whether Homo erectus was a good thrower, Holly Dunsworth's discussion is well worth reading: "Can you throw with half a brain?"
This isn't to say chimpanzees are klutzes. Far from it. It's to say that evolutionarily new and/or simply more motorneuronal matter in the brain and throughout the body is probably required to throw like a human, and to do all the things we do with our dexterous and powerful fingers, hands, and arms, and to do new-fangled, fast, sequential coordinated movements with our bodies. And it's probably a big part of why chimpanzees cannot or do not. (But trust also that it's highly likely that they have neurologically based abilities that we do not!)
Unfortunately, in this department, all we really have is brain size for early hominins and that's just not going to cut it for parsing apart these details we need to reconstruct throwing in extinct hominins even if we knew exactly what our brains were doing to aid in throwing.
That's sort of where I put the problem. We do lots of amazing things today because we can learn them. We use the anatomical equipment we have at hand -- and different groups use the same equipment with different styles as a result of practice. Is it "natural" to throw overhand? Probably as natural as beating things with sticks, and yet we find so many different ways to do both of these motions in sports from baseball and softball, to tennis and golf.