Without a brain, there is not much we would be able to do. Likewise, the architecture of our brain makes us what we are. And any meddling with it is likely to have an effect on our behaviour, one way or another. A form of behaviour particular to humans and which can be distinguished from other animals – even its closest relatives – is our handedness. We have been using our hands for millions of years. So have our nearest cousins you may argue. Yes. But humans are singular in that they have a distinct preference for right-handedness – a fact which must echo a singular brain structure. Indeed, the human brain is divided into two hemispheres: the left hemisphere and the right hemisphere. And we now know of one protein which plays a role in sculpting them: leucine-rich repeat transmembrane neuronal protein or LRRTM1.
So the human brain is divided into two – a left and a right side. The right side deals with our emotions and our imagination while the left one deals with talking and logical processes. But what does this have to do with handedness? Many years ago, our ancestors probably got up onto their legs to free their hands for collecting food and seized the opportunity to use them to communicate. In time, they realised that they could free their hands even more by learning how to use their voices to make conversation. This is the basis on which speech evolved. Speech turned out to be very profitable and natural selection made sure it never disappeared. In doing so though, it moulded the brain into two distinct hemispheres. And nudged humans towards right-handedness as opposed to left-handedness. A phenomenon which has been baptized the ‘right-shift factor’.
Consequently, right-handedness in humans is purely due to circumstances. What of left-handers? Human brain structure is asymmetric. In left-handers, the brain symmetry is flipped around, or reduced. LRRTM1 is involved in brain development and possibly neuronal connectivity. It is thought that left-handedness may be due to LRRTM1 dysfunction. However, chimpanzee LRRTM1 is identical to human LRRTM1 so there must be more than just one protein which makes us left-handed, or right-handed for that matter. Like all proteins, LRRTM1 does not act on its own and the environment is also very important.
With a role in brain development and possible neuronal connectivity, it is hardly surprising that LRRTM1 has been linked to neuronal diseases. Likewise, scientists wondered whether left-handedness was not indicative of a possible predisposition to neuropsychiatric disorders. A study carried out on schizophrenic individuals showed that many were left-handed. This does not make all left-handers prone to schizophrenia of course – such a result must be taken with caution. However, it does suggest that there could be a genetic predisposition to certain types of psychiatric disorders. And if LRRTM1 can be directly linked to such conditions, then it could prove to be very useful in the design of novel therapies to lessen such disorders.