What the left hand's doing
Socialist Organiser1982 1982-07-15Read it in the scan (PDF), scan page 11 →
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and right-handed individuals but in left- and rightpawed rats, differing levels of the chemical dopamine (which carries impulses across the gaps between the endings of certain nerves) are found on the right and left sides of the brain.
Whether this is an effect or a cause of handedness is not known, but it could be that general messages from the brain ordering movement of a limb are more easily transmitted on one side of the brain than the other.
In all other species with handedness (or pawedness) including our closest relatives, the chimps, the split is 50-50 right-left. Handedness is not inherited in other species, so it seems that it is caused by some random event (perhaps an accidental imbalance in dopamine production) which has no evolutionary significance.
In humans, there is a weak link between handedness of parents and children, but this could be due to environmental influences. One attempt to establish such an influence was the assertion in 1975 by a. Canadian psychologist that left-handedness was a deviation from a "natural" state caused by minor brain damage during complicated and traumatic births - an updated version of the 'left-handed equals daft or clumsy" view. This theory was later shown to have no support in fact.
So the environmental influence may be no more than the example set by parents teaching their children certain skills. This might be enough to change a weakly. left-handed child into a right-hander or vice-versa.
Uniquely amongst species with handedness, humans exhibit a stable split of 90-10. So it would seem that there is some evolutionary advantage to right-handedness amongst humans. But there must also be some advantage to having around 10% of lefthanders. What these advantages might be, though, is unknown.
Prejudice against lefthanders may no longer be important, but they still find that modern society takes them little into account. Right-handed designers don't cater for left-handers, who often have to undergo strange contortions to use a pair of scissors or a potato peeler!
Information: New Scientist.
Socialist Organiser Science Group, contact Les Hearne, c/o Socialist Organiser, 28 Middle Lane, London N8 SPL.
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July 1982