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2014-12-17 10:58:43
Jason G Goldman
Right-handed people are dominant worldwide but why? Jason G Goldman
investigates.
We humans don t typically agree on all that much, but there is at least one
thing that an impressive amount of us accept: which hand is easiest to control.
If you use one hand for writing, you probably use the same one for eating as
well, and most of us around 85% of our species prefer our right hands. In
fact, "there has never been any report of a human population in which
left-handed individuals predominate", according to archaeologist Natalie Uomini
at the University of Liverpool in the UK.
Lateralisation of limb use that is, a bias towards one side or the other
usually begins in the brain. We know that some tasks are largely controlled by
brain activity in the left hemisphere, while the right hemisphere governs other
tasks. Confusingly, there is some crossing of nerves between the body and the
brain, which means it s actually the left side of the brain that has more
control over the right side of the body and vice versa. In other words, the
brain s left hemisphere helps control the operation of the right hand, eye, leg
and so on.
Some argue that this division of neurological labour has been a feature of
animals for half a billion years. Perhaps it evolved because it is more
efficient to allow the two hemispheres to carry out different computations at
the same time. The left side of the brain, for instance, might have evolved to
carry out routine operations things like foraging for food while the right
side was kept free to detect and react rapidly to unexpected challenges in the
environment an approaching predator, for instance. This can be seen in
various fish, toads and birds, which are all more likely to attack prey seen in
the right eye.
So it is possible (though hard to prove) that as our hominin ancestors began
walking on two legs rather than four, freeing up their hands to perform new
tasks like making tools, they were predisposed to begin using those hands
differently. Or, as cognitive scientist Stephanie Braccini and colleagues put
it in a Journal of Human Evolution study, "a strengthening of individual
asymmetry [may have] started as soon as early hominins assumed a habitual
upright posture during tool use or foraging".
In support of the idea, Braccini and her colleagues looked at handedness in
chimpanzees, and found that when the apes stand on all fours, they displayed no
real hand preferences. It was only when forced to assume an upright stance that
a lateral preference emerged although individual chimps in the study were
equally likely to be left-handed as right-handed.
Evidently, then, something else was needed to push early humans from a lateral
preference in general to the extremely high levels of right-handedness we see
today.
We know roughly when that change occurred from experiments in which researchers
made their own versions of ancient stone tools using either their left or right
hands to chip or knap the tool into shape, before comparing them with the
tools made by early hominins. Doing so suggests there is only limited evidence
that hominin toolmakers working more than 2 million years ago were primarily
right-handed.
However, stone tools that were made some 1.5 million years ago in Koobi Fora,
Kenya, by two ancient human species Homo habilis and Homo erectus do show
some evidence of species-wide right-handedness. And by the time a species
called Homo heidelbergensis had appeared, perhaps around 600,000 years ago,
there was a clear right-handed preference in prehistoric societies. Wear on the
preserved teeth of Homo heidelbergensis, for instance, suggest that food was
usually brought to the mouth with the right hand.
This tells us when that shift occurred, but not why. Some have argued that it
all comes down to language. Just as most people are right-handed a trait,
remember, controlled by the left side of the brain so do most people do the
bulk of their linguistic processing in their brain s left hemisphere. Indeed,
this left-brained specialisation for language is even more common than
right-handedness which might suggest that as the left hemisphere evolved for
language, the preference for the right hand may have intensified simply as a
side effect. This is called the Homo loquens hypothesis: lateralisation in
general was driven by the evolution of an upright, bipedal stance, while the
rightward preference was driven, some time later, by the evolution of language.
Right-handedness, then, may simply be an accidental by-product of the way most
of our brains are wired up. But proving the hypothesis is difficult, or even
impossible, since it would ideally involve running neurological tests on our
long-dead ancestors. The truth is we'll probably never quite know what the
sequence of events was that led our species to lean so overwhelmingly on the
right sides of our bodies and the left sides of our brains.
As for the left-handers out there? Take heart! According to a 1977 paper in the
journal Psychological Bulletin, "there is remarkably little evidence for any
association of left-handedness with deficit, as has often been suggested". In
fact, some research shows that left-handed folks might even have an easier time
recovering from brain damage. And their left hand seems to have the advantage
of surprise in a fight, which means they can be better at combat sports. All of
which suggests there are advantages to breaking from the norm.