Why Do We Have Dominant Hands?

Camille Lee-Own

The evolution of handedness, where one hand is more competent at performing tasks, has been puzzling scientists for centuries. Recent research has shown that ancient human ancestors (hominids) may have been right-handed.

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Image Credit: Wikimedia

The direction of cut marks on Homo habilis teeth indicate they may have held meat with their left hand, and used their dominant right hand to saw chunks off. Our closest primate relatives have a 50:50 ratio of handedness whereas 90% of humans are right-handed.

Whilst this dental evidence is only from one specimen it suggests hand dominance arose early in human evolution. But why did it evolve, and why are ‘lefties’ so uncommon?

The organisation of the brain could have something to do with it. Our brains are formed of two hemispheres, a left and a right. Different abilities are specialised in specific areas of the hemispheres. In most people, the right hemisphere controls emotional and visual processing and the left hemisphere controls language, including fine motor skills such as moving your tongue.

Each hemisphere also controls the movement of the opposite side of the body; the left hemisphere controls right-handedness. It has been suggested that having two areas controlling movement in the same hemisphere increases efficiency, potentially explaining the high numbers of left-brained, right-handed people in the population.

This led scientists to assume left-handed people were the opposite, with the right hemisphere controlling language. However, studies have shown this isn’t the case: 70% of left-handers also process language in the left side of the brain.

However, there is evolutionary evidence to back up brain organisation’s effect on hand dominance. As previously mentioned, different areas of the brain can control different tasks at the same time. These separations may have led our ancestors to use the left side of the brain for routine tasks and the right to detect unusual changes in their surroundings.

In animals, predators like eagles are more likely to attack prey seen by their right eye.  As our ancestors began to walk upright, freeing their hands, the brain’s division of labour may have been echoed by hand dominance. On the other hand, this doesn’t explain the prevalence of right-handedness, and why ‘lefties’ are so rare.

Genetics might also play a huge role in handedness. Researchers have so far been unable to pinpoint a specific gene for dominance, suggesting multiple genes are at work. This could explain why left hand dominance still exists. If only one gene caused handedness, it would be far easier for random mutations to remove it from our DNA completely. It’s not as easy to eliminate multiple genes.

One idea suggests that genes may not even code specifically for different hands. Instead, a specific sequence of genes (known as RS+) is biased towards increasing the development of the left hemisphere, and therefore right-handedness.

The other form of the sequence (RS-) is neutral. Rather than being a gene sequence for the right hemisphere and left-handedness, RS- doesn’t specify dominant development of either side of the brain. The scientists behind the theory have suggested that RS- evolved to prevent brains from becoming lopsided due to overdevelopment in just one hemisphere.

So, lefties do not have a specific gene sequence for left-handedness, merely the lack of one for right-handedness. The absence of dominance could also help explain why some people are ambidextrous (able to use both hands equally well).

Despite theories that being left-handed is only useful to prevent lopsided brains, there are many positives to being in the 10%. In many one-on-one sports, such as tennis or fencing, left-handers are at an advantage, as right-handed people are less practiced at competing against them. Rafael Nadal is famously ambidextrous, but plays tennis with his left hand to take advantage of the higher proportion of right-handers.

Lefties are also supposedly more adaptable to unexpected situations. Their brains are more plastic, showing unpredictable activity and are able to reconfigure themselves. This could help left-handers recover from brain injuries faster.

The true origin of left-handedness is likely a mixture of genetic, evolutionary and environmental causes. But if it means you’re more likely to win sword battles, it can’t be all bad.

#CamilleLeeOwn #Handedness #Humanevolution

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