Human and Chimpanzee Handedness

Of the many mysteries surrounding human evolution, the question of why humans, alone out of all the apes, display a strong tendency towards being right-handed is perhaps less well known than uncertainties about our locomotion, brain size and cultural capacity. Yet the fact remains, over 90% of humans are right handed, and strongly so - there are proportionally few left-handed individuals and very few ambidextrous ones. Handedness is a manifestation of laterality - having a behaviourally dominant side or limb - and may be related to the relative dominance of the two halves of the brain. In humans, who are (mostly) right-handed, the left side of the brain, which is the side associated with language, is therefore dominant.

In chimpanzees and other apes, though, the situation is different. Laterality is reduced (with many individuals being ambidextrous), and although the results of early research are inconclusive, there is no demonstrable preference for being right-handed over being left-handed (Braccini et al. 2010). In fact, these authors, publishing in the current issue of the Journal of Human Evolution, argue that much more research into handedness in the apes is needed to establish whether the human predominance of the right side of the body is an extension of a trait present in the last common ancestor or a uniquely human character.

Braccini et al. therefore set up an experiment in which a number of chimps (of whom 15 were ambidextrous, 15 right-handed and 16 left-handed according to previous research) were given sticks to access peanut butter in the middle of plastic tubes. Each chimp was tested in three different postures: for the first, they were allowed to hold the tube and all sat down to extract the food; for the second, the tube was suspended vertically above head-height but within a short distance of a wall, so they could support themselves with one hand while standing bipedally, and finally, the tube was suspended above head-height but away from the wall, so the chimps had to stand unsupported (Braccini et al. 2010). The research found that the degree of laterality (preference for one hand over the other) increased significantly as the chimps moved from seated to supported and unsupported bipedalism and from supported to unsupported postures, but the level of right-handedness in the group, interestingly, did not - in fact, although there was a slight increase in the proportion using their right hands to access food when standing bipedally, on the whole, the change of posture merely strengthened the chimps' earlier hand preferences.

The most interesting implication of this study, of course, is that while it does not disprove the widely-held hypothesis that tool use has driven human lateralization, it does require that an additional factor be invoked to explain the high proportion of right-handed people. In fact, Braccini et al.'s paper suggests that a change to bipedal locomotion, especially if associated with tool use and manipulation of the environment, might indeed have enhanced lateralization in early hominins substantially. The subsequent change which led to 90% being right-handed might, in fact, be the enhanced lateralization of the brain which accompanied the origins of language - a skill primarily located in the left half of the brain. While Braccini et al.'s paper does not provide any direct evidence for this change, it does support the existence of a second, directional, shift in lateralization which, furthermore, must have arisen after the human-chimpanzee split. However, this problem is of a chicken-and-egg nature - we cannot know whether cerebral lateralization occurred before, or was enabled as a result of, increases in right-handedness and left-brain dominance. In addition, it may be that there is no selective advantage to being right handed - because this is the result of selection for other features. It will be interesting to see how the debate turns out!



References

Braccini S, Lambeth S, Schapiro S, & Fitch WT (2010). Bipedal tool use strengthens chimpanzee hand preferences. Journal of human evolution, 58 (3), 234-241 PMID: 20089294