I confess it! I am a leftie!
Only 5 to 20% of people in any given human population are left-handed. Neanderthals, and the purported human-Neanderthal ancestor, Homo heidelbergensis were mostly right-handed like us. But modern apes such as chimpanzees show no population bias when it comes to handedness. It seems that favouring the right hand for tool use is a human trait that has been with us for millennia. We also know from studying identical twins, that about 24% of handedness is genetic – the rest is probably environmental.
Why are humans right-handed?
There are numerous theories that propose to explain why we are right-handed animal. One of the more convincing ideas concerns the connection with language development and handedness.
The brain is divided into two hemispheres – left and right. When it comes to movement, the right hemisphere of the brain controls the left side of the body, and the left hemisphere of the brain controls the right side of the body. The hemisphere that processes language is typically the left hemisphere. In fact 96% of right-handers have their language focused on the left hemisphere, while only 73% of left-handers follow suit. This bias toward the left-side of the brain in handedness and language could indicate a link between the development of language and right-hand dominance in humans. The direct biological relationship between handedness and language is from speculation that our early ancestors (Homo heidelbergensis) used gestures and sounds for language, so in line with this theory our hand preference and language could have developed in the same side of the brain.
Until earlier this week there was very little evidence that supported this possible relationship.
A new study from the UK identified a genetic variation that was found in people with dyslexia and were strongly right-handed. This variation was found in a gene called PCSK6, which has a known role in embryonic development. PCSK6 in involved in a process that makes an embryo asymmetrical i.e. gives an embryo a left and right-half, which is part of the basis of forming a body plan for limbs and organs. Deleting this gene in mice causes left-right symmetry defects, where internal organs are not in the correct locations (heterotaxia).
When other genes in this left-right asymmetry pathway are deleted in mice other internal organ placement problems occur. The research team identified a set of these genes to test if handedness in humans was associated with known asymmetry genes.
In a cohort of over 2000 people without dyslexia another PCSK6 variant was found to be associated with handedness.
When the PCSK6 gene was taken out of the analysis (as it had such a strong effect it could mask other associations) several other genes in the asymmetry pathway were revealed as been associated with handedness.
The results from this study suggest that handedness is at least partly caused by the same mechanisms that determine asymmetry in the body. The exciting point of this information, is there is now evidence that asymmetry in the brain is probably created, in part, by the same pathway that determines asymmetry in the body. This links back to language, as language is also asymmetrically distributed in the brain. At this stage it is a bit of stretch to directly link the two, but it s a step forward.
‘As with all aspects of human behaviour, nature and nurture go hand-in-hand. The development of handedness derives from a mixture of genes, environment, and cultural pressure to conform to right-handedness.’- William Brandler, MRC Functional Genomics Unit, Oxford University
The asymmetry genes do not tell the whole story – there is still plenty of mystery in this space!
Media Release: Oxford University
LOL at lefties: Cracked