Aimed throwing is surprisingly uncommon in the animal kingdom. Humans do it par excellence, and otherwise it only shows up occasionally, even in our closest relatives. Chimpanzees will throw things (often faeces) but unlike humans don't throw things when hunting or trying to get food; when non-human animals throw things, it's usually part of a social encounter.Throwing is a fascinating task for many reasons; I hope to blog some about the perception-action aspects of this task in the future as I prepare a couple of papers on the topic with my colleagues Qin Zhu and Geoff Bingham (who have previously done some excellent work on throwing to a maximum distance and the size-weight illusion; various papers available here). There are many fascinating questions about the perception of the affordances of throwing and distances to targets which we're starting to tackle empirically.
Biomechanically, throwing an object accurately over any distance requires the precise transmission of force from the large trunk muscles along a kinetic chain formed by the segments of the arm. The large trunk muscles generate forces the arm cannot, and this force is then transmitted by the motion of the arm; each segment weighs progressively less and so the force accelerates each one faster than the last. The end result is a hand moving at high speed. This requires careful timing; if the motion of the segments aren't coordinated carefully you will waste energy moving the limbs in ways that aren't helping the throw.There has been some speculation for a while now that the neural mechanisms that help support this fine tuned coordination and control for throwing might also be just the kind of resources that could support the development of spoken language. Speech is a complex action that requires exquisite control over the coordination and timing of numerous elements, just like throwing. One hypothesis is that our ancestors began to develop the ability to throw long distances (this being favoured by natural selection processes because it enabled us to hunt and kill huge prey with much less physical risk to ourselves; e.g. Calvin, 1983). Evolution selected for neural resources that supported this activity, and this then opened the door to the possibility of complex spoken language. So do we speak the way we do because we throw the way we do?
Zhu and Bingham (2011) speculated along these lines while discussing the relationship between throwing and the size-weight illusion. In that paper they showed that the size-weight illusion is not a misperception of weight due to changes in size, but instead reflects the correct perception of the throwability of the object, which depends on both size and weight. They suggested that the illusion shows up so robustly because evolution has built humans who are prepared to be able to throw; we are innately biased to perceive throwing related properties of objects, making us born 'ready to throw'. Zhu & Bingham argue this as an analogy to the way the human nervous system seems to come 'ready for language' (Kuhl, 2000) and they cite some of the research discussing a potential relationship between language and throwing.
Communication, throwing, and chimp brains
Hopkins, Russell & Schaeffer (2011) presented some interesting data in a special issue of the Philosophical Transactions of the Royal Society on language, 'From action to language: comparative perspectives on primate tool use, gesture and the evolution of human language'. Not all chimps throw reliably, but they were able to test 38 chimpanzee throwers from the Yerkes National Primate Center in Atlanta,GA as well as 38 non-throwers. They took a variety of cognitive measures, measuring physical and social cognition; things like spatial memory, object permanence vs communication comprehension and production. They also took detailed structural MRI scans of the chimp's brains, specifically the inferior frontal gyrus (IFG) and the motor-hand area of the precentral gyrus (KNOB, for some reason). IFG was of interest because it is equivalent to Broca's area in humans. They hypothesised that there would be increased white matter in these regions for throwing chimps and this this would be strongly lateralised to the left hemisphere in right handed chimps, relating throwing to the neural correlates of language production,
The results were straight forward: chimps who throw have a higher proportion of white matter in these regions compared to chimps who do not throw. This effect was lateralised to the left hemisphere for right handed chimps (lefties were less lateralised, a fairly typical finding in people too). Finally, the two groups of chimps (throwers vs non-throwers) were only different in one cognitive measure; throwers out-scored non-throwers on a measure of communication skill from the modified Primate Cognition Test Battery (Herrmann et al, 2007). Throwing, handedness and communication skill were all related to greater connectivity in brain regions analogous to those likely involved in the production of spoken language in people. Throwing may have been a preadaptation that formed part of the scaffolding which allowed the development of language skills.
Conclusions
This is obviously preliminary and correlational data; it's not yet clear which factors are causing which effects. But it's an interesting piece to the puzzle of the evolution of that human trick, language, and ties its development to a perception-action task that only humans have genuinely specialised in. There are many follow-up questions; can you match variation in ability to learn to throw and communication in humans? Is Broca's area more about fine motor control than speech per se, and if so, is it involved in throwing? There's plenty more to do, although research on this topic is beginning to show up. For instance, a recent paper in PLoSOne by Meunier et al (2012) showed that baboons and human children show a similar pattern of handedness for communicative gestures that doesn't show up in simple prehension tasks, and baboons have been known to do a little throwing (Hamilton et al, 1975). Right handedness is a proxy measure for left hemisphere specialisation, and the suggestion here is that gestural communication is part of the basis for language itself.
I liked this paper for several reasons. First, the authors were extraordinarily disciplined in their conclusions and they way they discussed their results. Second, it works to directly connect communication skill and throwing which has been a tantalising prospect for a little while now, and given my recent interest in throwing as a task it's something I'd like to spend some time working on. Finally, it's a great set of data; hypothesis driven, carefully collected and they looked not just for confirming evidence (finding differences in communication skills between throwers and non-throwers) but also the dissociation results that helped bolster their argument (no differences in non-language related tasks).
It all works to place language in an evolutionary context where it's development is less of a mystery and more connected to the development and natural selection of related skills. We are (slowly!) working on an analysis of language along these lines, and so research like this is of great interest.
Calvin, W. H. (1983). The throwing madonna: essays on the brain. New York: McGraw-Hill.Hamilton, W., Buskirk, R., & Buskirk, W. (1975). Defensive stoning by baboons Nature, 256 (5517), 488-489 DOI: 10.1038/256488a0
Herrmann, E., Call, J., Hernandez-Lloreda, M., Hare, B., & Tomasello, M. (2007). Humans Have Evolved Specialized Skills of Social Cognition: The Cultural Intelligence Hypothesis Science, 317 (5843), 1360-1366 DOI: 10.1126/science.1146282
Hopkins, W., Russell, J., & Schaeffer, J. (2011). The neural and cognitive correlates of aimed throwing in chimpanzees: a magnetic resonance image and behavioural study on a unique form of social tool use Philosophical Transactions of the Royal Society B: Biological Sciences, 367 (1585), 37-47 DOI: 10.1098/rstb.2011.0195
Kuhl, P. K. (2000). A new view of language acquisition. Proceedings of the National Academy of Science USA, 97, 11850–11857 (Download)
Meunier, H., Vauclair, J., & Fagard, J. (2012). Human Infants and Baboons Show the Same Pattern of Handedness for a Communicative Gesture PLoS ONE, 7 (3) DOI: 10.1371/journal.pone.0033959
Zhu, Q., & Bingham, G. (2011). Human readiness to throw: the size–weight illusion is not an illusion when picking the best objects to throw Evolution and Human Behavior, 32 (4), 288-293 DOI: 10.1016/j.evolhumbehav.2010.11.005
