Can you throw with half a brain?

Homo erectus had half the brains that we have. This guy's got even less. (source)

This being at least my third post about the evolution of throwing here on the MT ("A prehistory of throwing things" and "Hurling words and turds, an evolutionary link"), I think I should give a little background about my interest. Even before I started graduate school my doctoral advisor got me (a lifelong athlete) interested in it, we teamed up with a biomechanist, did some modeling, wrote about what throwing might have been like for extinct hominins like Lucy and the Nariokotome Boy ("Throwing and bipedalism: A new look at an old idea" will email the pdf, just ask), and I was awarded an NSF graduate fellowship for the proposal to come out of that project.

So obviously I earnestly thought I was going to study hominin throwing for my dissertation, but when I started to try to pull a comparative experimental study together and realized that everything I wanted to do would require that I go somewhere(s) else and work with someone(s) else, it didn't make much sense. I needed a biomechanics lab and I needed lots of practice in one too. And, I couldn't even begin to figure out where to find chimps or other primates that I could work with, let alone that I could get to visit me in that biomechanics lab. Or I'd have to learn ways to study kinematics of animals without much control. And I'd have to go somewhere else to do most dissections too. And let's not forget how much smaller the fossil hominin sample was 12 years ago. It wasn't feasible for my dissertation.

It was okay though because while I was doing what eventually became my dissertation research on Proconsul feet in Kenya I met Neil Roach who was just starting his studies at Harvard and was working on the evolution throwing! If it wasn't going to be me, at least someone was going to figure this out for us, and that someone had the resources to do so. And that someone's research made the cover of Nature this week.

The paper's called "Elastic energy storage in the shoulder and the evolution of high-speed throwing in Homo." [link to paper]

Here's a promotional video that Harvard posted:



In the paper, the authors ask, Why can we throw like we do and why can't chimpanzees? and they look to the musculoskeletal differences between us to find out.

In this paper, it's all about the whip-like motion of the human body during a throw and the slingshot-like energy storage in tendons, ligaments and muscles that makes it work. They elegantly describe this process and write about three key differences between humans and chimpanzees that they say are critical to why we throw like pros:

(1) our flexible torsos that allow more rotation at the waist

Human (a) and Chimpanzee (b)
(Source: Figure 3 from Bramble and Lieberman, 2004)

(2) our lower amount of humeral torsion in our throwing arms

from Roach et al., 2012: "Fig. 1 Humeral torsion (in blue) is determined by measuring the angle

between the orientation of the humeral head and the distal condyle

of the humerus. In the clinical literature, the same angle is referred to

as humeral retroversion (in yellow) and is measured in the opposite

direction."

(3) the lateral, not cranial, orientation of our shoulder joints

From the study's website: "This image shows differences in the position of the shoulder between chimpanzees (left) and humans (right). These differences can be seen in both the muscular anatomy and in the bony anatomy of the scapula (shoulder blade). (Image credit: Brian Roach/Neil Roach)" 

Then the meat of the paper is devoted to their experimental analyses of a group of skilled human throwers that appear to support the importance of these traits in throwing, particularly for storing elastic energy for fast throws. I say "appear to" because I wasn't given any of the supplemental materials and what's in the main text (which is all I have) doesn't quite pull the analyses and those traits together for me, but that could just be my very rusty biomechanics training rearing its rusty head. Or it could be my incredibly high and probably impossible expectations about what we can take to the fossil record with only a study of human throwers upon which to rest our assumptions of functional anatomy.

For example, humeral torsion is often asymmetrical in humans, being lower in the dominant arm, adding support for the assumption that humeral torsion changes depending on activity during an individual's lifetime. Is it fair to compare that trait to chimpanzees when it's not necessarily an in-born difference? Yes. But that makes explaining selection on that trait a little bit more difficult. I think it's fair to consider it, at least its asymmetry, a marker that an individual may have been habitually throwing during its life. So in that regard, it's worth looking for in the fossil record.

But then there's another snafu: How important could it be in reality and across space and time when there is so much overlap in the quite acceptable (but relatively small compared to reality) sample here?

from the study (with my blue box added to highlight overlap): "d, Humans and chimpanzees show comparable degrees of torsion5, although throwing athletes show reduced dominant-arm torsion2 consistent with low torsion in Australopithecus and Homo erectus5. Chimpanzees are a combined sample as they do not show arm dominance, whereas human values are split between dominant and non-dominant arms as they show arm dominance. In d, the black and grey boxes and whiskers show torsion-angle quartiles, and the red dot shows the torsion angle mean."

There's good reason to believe that low torsion is different. But there's also reason to wonder if it's necessarily due to throwing or to, say, using the dominant arm to do any number of things, one of which is throwing.

There's also good reason to curse the early hominin fossil record for providing us with only five individuals to include in this figure!

And there's also good reason to curse australopiths for not clearly fitting in the human or chimpanzee box.

And there's also good reason to curse chimpanzees for not being different enough from us so that our evolutionary histories are easier to reconstruct.

That these three traits listed above do exist in Homo erectus means Homo erectus could have thrown like us. (How else would they have experienced such a shift in foraging strategy?) And that these traits don't exist in australopiths (although the flexible waist issue is still up in the air) means that australopiths couldn't have thrown like us, not with the force that we can put behind a projectile.

And if that caused you to pause and recall that chimpanzees are stronger than us, and to wonder why such a powerful animal is such a weak thrower... then you're right here with me, in my head.

This paper provides a strong explanation: it's all the elastic energy storage and the coordinated movements of the body to maximize it and to maximize its transfer to the projectile.

But that may be only part of it.

And because it's probably like most things about living organisms are (and like most things in the universe are), human throwing is probably more complicated... even more complicated than a complicated sequence of elastic energy storage and use.

There's a large body of literature that explains human throwing with another major difference we see between chimpanzees and humans: Our big brains.

That coordinated fine motor control of the body, head to toe, during a forceful throw is brainy. Is it brainier than what a chimpanzee's working with? It's not yet known, but it's not a bad assumption.

That fine-tuned timing of release that's crucial for accuracy, particularly with forceful throws is also brainy. Is it brainier than what a chimpanzee's working with? This is also, I believe, not yet known, but it's not a bad assumption either.

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. But comparative work with living species will continue to be enlightening about the brainy differences among us, even if they don't enable us to nail down behavioral details about fossil hominins.

Even still, I think there's a lot we can still hope to figure out about australopith throwing. We have a biped without a modern body. It would be really useful to better understand how well australopiths could throw under these circumstances. There’s a lot of variation on the baseball field but none that comes close to that between us and the australopiths. If we knew about their throwing ability and style, we could better know how they foraged for food, thwarted predators, and socialized with one another since action-at-a-distance is key for all. It's also important to know because there's only a bit (as one would predict), but there's evidence for tool use among them  (for example). What if having all the built-in throwing skill of a chimp is good enough as long as you’re built to be upright, you’ve been practicing, and you’re hungry?

As our 2003 research suggested...the long australopith arm would have given them a larger release window for slow, lobbing throws, something their puny brains could work with and still hit a target. But as arms got shorter with the genus Homo, particularly the forearms, the release window shrank, particularly as throwing velocity increases, and that’s when their larger brain would help with accuracy, especially for fast, flinging throws.

But that's only if the brain matters.

But it probably does.

And if brains do matter, then Homo erectus, with its half to three quarters of ours, was either throwing like a pro without all our fancy gear (suggesting we’re over-equipped) since it was pretty good at hunting  ...or that Homo erectus overcame its lackluster equipment somehow and still survived and reproduced just fine. Fine enough so that they could, over time, leave much brainier descendants. 

And then there's of course the running issue. Aside from humeral torsion, all the skeletal traits for human throwing are also for running. And doesn't every little step you take in a run feel like a little throw?

And then, to add to that, throwing anatomy appears to be important for making and using tools of all kinds, not just projectiles. If that's not intuitive, think of the similarities between throwing a ball and swinging a bat. If we want to reconstruct hominin evolution as a sequence of origins of traits and selection "for" this or that trait...this then that then this then that then tadaaa: modern  humans... we'll have to parse apart adaptations for throwing and for running and for tool making and use. But if that makes you wince, like I'm doing, that's because maybe that's the wrong question, the wrong tack. The more we understand about things like throwing, the more obvious it becomes that things are complicated (even moreso than this) and things are interrelated and they always were and in varying ways back through time.

While we're figuring out whether Homo erectus could throw like us with only half the brains that we have (at least as far as we're limited by cranial capacity and reconstructions of spinal cord diameter), and while we're figuring out more about what the australopiths could do, let's also figure out why chimpanzees are so much stronger than us. Is it because of this hypothesis about trading off muscular strength for speed? If someone has solved this mystery and I just missed out, please leave a comment! I feel like I could ramble on and on and on, but I'm going to leave it right here for today.


Note: References are linked within.

And here's one I just found while writing that I haven't read yet: "The uniquely human capacity to throw evolved from a non-throwing primate: an evolutionary dissociation between action and perception."