SBoardman.co.uk

From my previous post on research (link) I mentioned that I was interested in the phenomenon of human handedness.  To recap I want to know why there is such a total dominance of right handedness that is ubiquitous to human civilization.  All extant human society has a proportion of right handers around 0.9, and studies of the tool factories in East Africa or the cave art in France by prehistoric Homo sapiens suggests that right handedness may have been dominant for a long time.

There are several key questions surrounding the prehistoric notion of handedness.

1. Is handedness unique to humans?
2. Why are we predominantly right handed?
3. Why does left handedness persist?

Looking at the evidence from our closest relatives (Chimpanzees and Orang-utans) it is clear that whilst there may be individual level biases the population is found in an antisymmetry with no direction having a majority.  To help explain what an antisymmetry is we need to imagine a spectrum of numerical values to represent different levels of handedness.  0 is totally left handed, 1 is totally right handed and 0.5 is no preference either way, so chimps and other apes have a population level average of about 0.5.  This occurs when there are lots of very strongly lateralised individuals which have values of 0.9 or 0.1 but they are found in equal numbers of right and left biased individuals giving the population an average of 0.5.  Modern humans have a distribution of handedness which is described as directional asymmetry where the majority of individuals have handedness values greater than 0.5.  Perhaps this is most easily visualised in the following graphs.

So our closest relatives don’t have a strong handedness bias, but we do.  Therefore this extreme level of handedness is highly likely to have evolved since our split from chimpanzees.  Its difficult to predict when and where it did evolve, as fossils, paintings or anything else that allows an individual’s handedness to be assessed are hard to come by the further back in time we go.  We can however postulate why we have this extreme skew.

If we are assuming handedness is determined by some genetic means, which the evidence suggests it does, then handedness itself must be advantageous for it to have persisted at such a high frequency in the population.  Furthermore being right handed must therefore confer greater fitness than being left handed for it be at a higher frequency, but what possible advantage can being right handed have over being left handed?  All sorts of theories have been suggested as to how this might be the case.  The most interesting, and perhaps the most incorrect, is the theory that left handedness is pathological.  There are several cases where left handedness is thought to be due to lesions in the brain however to suggest that all left handedness is due to pathological causes is most definitely not the case.  Various studies have shown that in groups of mentally handicapped individuals have a higher incidence rate of left handers (in the region of 18% compared to 9% in the total population) but to state that all left handed individuals have brain legions is mistaking cause and effect.

Why we are predominantly right handed has not been answered sufficiently by science, so far.  It may be that a genetic mutation caused right handedness and it’s advantage over left handedness or no handedness meant it swept through the population.  The fact that it was right handedness rather than left handedness may just be due to chance.  From research it is clear that lateralisation of the brain is advantageous as it prevents both hemispheres (half a brain) from trying to control the same action.  Perhaps handedness, which is strongly linked to cerebral lateralisation, is advantageous through a similar means.  However the advantage of one side of the brain controlling different aspects of our bodies, and thus making one hand dominant, doesn’t explain why right is superior to left.  Hence we still don’t know why right handedness is so common compared to left handedness.

So right handedness is better than left handedness for whatever means but left handedness still persists.  This is odd and is what geneticists refer to as a balanced polymorphism where two or more variants of the same trait are found in the population at constant frequencies.  If right handedness is more advantageous than left handedness it begs the question why left handedness hasn’t been selected out of the population.  Its thought that left handedness may possess some advantages relating to language or other traits strongly linked to cerebral lateralisation which may account for its persistence.  Many studies have investigated whether left handers leave fewer offspring or are worse at certain other traits than right handers but there is no evidence to suggest that left handed indivdiuals are actually “inferior” to right handers in any way.

So the mystery remains why human handedness has such an odd skew and where it came from in the first place.

Whilst this won’t be the last post on handedness I will suggest a couple of excellent sources if you want to investigate a bit more for yourself.

Right Hand, Left Hand by Professor Chris McManus.

A great popular science book by the world’s leading expert on handedness. Covers asymmetry from molecules to hands.

Check out his website too for more info.

Always a good place to start, but don’t limit yourself to wiki, follow their sources and a greater understanding will definitely present itself.