Although certainly not the most invested-in of questions in the quest for uncovering primate origins, the understanding of manual lateralization has certainly proved to be interesting (and, arguably, important). As are many topics in primate origins research, the evolutionary path of manual lateralization (handedness, or right versus left handed) is not clear. Many attempts have been made to establish evolutionary paths of manual lateralization in a variety of vertebrates, yet as it can only be tested in extant species the evidence on which the conclusions are drawn is shaky at best. Of course, this doesn’t stop them. It never does…

The most energetic attempt at answering this question of questions was by MacNeilage et al. (1987). That’s right – 1987. Their elderly assertion is known as the “postural origins theory,” and states a left hand bias existed for the ancestral primate species for visually guided reaching, coupled with right hand side asymmetry for for manipulation. Subsequently, in recently evolved terrestrial primates the right hand became used as the preferred hand for manipulation rather than support.

Not only does this ‘theory’ appear to be pure speculation, but the last assertion is plainly false and completely contradicted by the evidence. In fact, there is an overarching lack of a specific manual lateralization pattern not only within one species, but also between species within the same genus and family in the order Primates.

With many studies finding manual lateralization affected by sex, age, task experience, and inheritance, there has been a bit of confusion on what actually qualifies as handedness. There are two popular definitions: (1) hand preference and (2) successful hand usage. What Leliveld et al. are doing here is testing the second definition on non-human primates, specifically the mouse lemur (qualified by many as representing the ancestral primate condition, which is an entirely different topic of debate).

The subjects were 44 gray mouse lemurs (Microcebus murinus) and 19 Goodman’s mouse lemurs (Microcebus lehilahytsara) at the Institute of Zoology, University of Veterinary Medicine Hanover. During the experiment the subjects were forced to grab with one hand an object, while the other hand was needed for support. What the researchers found was that, although all subjects showed a significant individual hand preference, there was no population level preference found. The effects of task experience and parental influence, but not sex or age, were observed. Comparing species, the Goodman’s mouse lemurs showed a stronger lateralization than the grey mouse lemurs.

As a result of the observation of no population level manual lateralization the evidence for simple genetic features as an explanation is weakened substantially. Couple this with a negative mother-offspring correlation of hand preference for the grey mouse lemurs (as compared to similar studies which found a positive mother-offspring correlation in chimpanzees and no correlation at all in bonnet monkeys), the complexity of the handedness question becomes one with no answer in sight.

So, what exactly is it about this question that makes it important? Well, of course understanding the evolution of any mechanism is worthwhile in and of itself, as it aids in the understanding of evolution as a process. However, with respect to primates I believe it’s merely our inherent nature to want to know more about ourselves – our natural history – and nothing more.

I mean, how cool would it be just to know why your write with your right instead of your left hand (or vice versa)?

From this study, we see it is unlikely that handedness evolved early in primate evolution. And, while the conclusion certainly isn’t worthy of a newspaper headline, it’s worth a blog post. It brings us that much closer to understanding primate evolution, and evolution as a whole.

(Obligate Image)

(References)

Leliveld, L, Scheumann, M & Zimmermann, E, 2008, 'Manual lateralization in early primates: A comparison of two mouse lemur species', American Journal of Physical Anthropology, vol. 137, no. 2, pp. 156-163. 10.1002/ajpa.20852