A Left-Brain/Right-Brain Conundrum Revisited

A prominent British psychiatrist recently revived old arguments about the origins of language and the evolution of humans. Tim Crow at Warneford Hospital in Oxford says that reports on ape brain asymmetry are distorted by observer bias.1 Those criticized point to "plenty of evidence" that general functions and skills have gravitated to one side of the brain or the other in animals from chicks to chimps.Crow argues that researchers are finding evidence of language precursors in apes because they want to believe in a graduated theory of evolution, rather than the leap proposed by Thomas Huxley, Stephen J. Gould, and others. Crow points to studies that have reanalyzed data and found no support for initial conclusions of asymmetry.2 He also asserts his support for the model proposed by neuropsychologist Marian Annett in 2002,3 in which she suggests that a single gene gave rise to language in the brain's left hemisphere, and brought a shift towards right-handedness.In 1877, Paul Broca argued that brain asymmetry distinguishes humans from other animals and gives humans the capacity for language. Then scientists started finding evidence of asymmetry in other vertebrates. "Many of the lateralized functions of the human are the same as those in animals," says Lesley Rogers of the University of New England in Australia, who with Richard Andrew oauthored the 2002 book Comparative Vertebrate ateralization.4 "Language has a left-hemisphere location in most humans. It might rely on the evolution of some nuance of laterality, but the point is, it was superimposed on other lateralities that were already there."Rogers and Andrew offer examples, such as the left-footedness of parrots, the right-hand preference of toads, and the reliance of chicks on the right hemisphere for spatial cognition. Songbirds show strong lateralization for song production. But when it comes to the great apes, Rogers admits, the evidence for handedness is more controversial. Chimps at the Gombe National Park in Tanzania, for instance, showed no evidence of right- or left-hand preference at a population level according to a 1996 study.5Even in humans, says Richard Palmer at the University of Alberta, Canada, nobody really knows why handedness exists or whether it has a genetic basis. "The amount that we know with confidence about human handedness is so pitiful it's almost shocking," he says. Indeed, no one has ever demonstrated a causal link between handedness and language.Crow says he doubts that evidence from birds, rodents, and fish consistently points to population-level lateralization of certain functions in a given direction. Even if it did, he argues, "I need to see that the relevant anatomical and physiological lateralizations and their genetic correlates were transmitted through the primate lineage." Without robust evidence for handedness and lateralization in nonhuman primates, he says, a graduated theory of evolution isn't supported.On the other hand, if it were conclusively shown that a major transition from apes to humans had occurred, scientists could look for the genetic change that gave rise to it, and ask what else it brought about. Crow has identified a pair of genes, Protocadherin X and Protocadherin Y, which are expressed in the human brain and are, he says, candidates for the characteristics such as language that set humans apart.If Crow is correct about the evolutionary leap, researchers may need to reconsider the nature of the ape mind. Daniel Povinelli, a cognitive scientist at the University of Louisiana, Lafayette, has an intermediate view of the chimp mind: neither a watered-down version of the human mind, nor a thing apart. Chimps and humans share massive amounts of neural machinery, he says, but the human brain underwent an accelerated period of evolution that left it far more sophisticated.Povinelli says he's pessimistic about programs that aim to teach language to chimps and other apes. "I don't think we're pulling language out of these animals," he says. "I think we're showing that chimps can use their own language of understanding the world to cope with these problems that we're giving them. But I don't think chimps are moving in the direction of evolving language."Laura Spinney laura.spinney@btopenworld.com is a freelance writer in London.

A Left-Brain/Right-Brain Conundrum Revisited

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