Researchers at Yerkes National Primate Research Center (Atlanta, Georgia) have
shown that in a population of 70 chimpanzees, a substantial majority of the
animals showed a significant bias towards right-handed gestures when communicating.
As is well known, linguistic functions in humans are controlled by the left
cerebral hemisphere, and for a long time it has seemed possible that there is
some connection between this fact and the predominance of right handedness in
humans (resulting equally from left-hemisphere dominance). Anatomical differences
at the cellular level between left and right hemispheres have been demonstrated.
Many researchers also believe that spoken language had its origins in gestural
communication, so that evidence of right-handedness in pre-linguistic communication
by chimpanzees is significant.
As reported in the January 2010 issue of Elsevier's Cortex, a team of researchers,
supervised by Prof. William D. Hopkins of Agnes Scott College (Decatur, Georgia),
studied hand-use in 70 captive chimpanzees over a period of 10 months, recording
a variety of communicative gestures specific to chimpanzees. These included
'arm threat', 'extend arm' or 'hand-slap' gestures produced in different social
contexts, such as attention-getting interactions, shared excitation, threat,
aggression, greeting, reconciliation or invitations for grooming or for play.
The gestures were directed at the human observers, as well as toward other chimpanzees.
"The degree of predominance of the right hand for gestures is one of the
most pronounced we have ever found in chimpanzees in comparison to other non-communicative
manual actions. We already found such manual biases in this species for pointing
gestures exclusively directed to humans. These additional data clearly showed
that right-handedness for gestures is not specifically associated to interactions
with humans, but generalizes to intraspecific communication", notes Prof.
Hopkins.
The French members of the team, Dr. Adrien Meguerditchian and Prof. Jacques
Vauclair, from the Aix-Marseille University, say: "This finding provides
additional support to the idea that speech evolved initially from a gestural
communicative system in our ancestors. Moreover, gestural communication in apes
shares some key features with human language, such as intentionality, referential
properties and flexibility of learning and use".
Hemispheric lateralization of linguistic, and presumably pre-linguistic skills
is a fact, but there is no satisfactory answer to the question of why it should
have evolved. It doesn't seem likely that it was to solve a capacity problem,
although that remains a possible explanation. More convincing is the idea that
there is an advantage to handedness. William H. Calvin, in The Throwing
Madonna: Essays on the Brain, speculated that one-handed throwing could
have been the crucial advance that gave early humans their survival advantage
as against other apes, and that the requirement for intricate sequencing of
motor actions was best fulfilled in one hemisphere and was then taken advantage
of by language when it came along. But really this is just an elaboration of
the capacity argument, and more convincing (but only just) is the idea that
in gestural communication there is a group advantage if everyone gestures with
the same hand, to avoid the need to apply a mirror transformation to the gestures
you see when they are made by a left handed person, with some possible dangers
of misinterpretation.
Whatever the original reasons for handedness, meaning left-brain dominance
in certain functions, the fact that humans are mostly right-handed is of course
just a random result. Evolution had to pick either left or right, and metaphorically
it spun a coin, which happened to land right side up.
