The division of labor by the two cerebral hemispheres—once thought to be uniquely human—predates us by half a billion years. Speech, right-handedness, facial recognition and the processing of spatial relations can be traced to brain asymmetries in early vertebrates
In the human brain the left hemisphere controls language, the dexterity of the right hand, the ability to classify, and routine behavior in general. The right hemisphere specializes in reacting to emergencies, organizing items spatially, recognizing faces and processing emotions.
The left hemisphere of the human brain controls language, arguably our greatest mental attribute. It also controls the remarkable dexterity of the human right hand. The right hemisphere is dominant in the control of, among other things, our sense of how objects interrelate in space. Forty years ago the broad scientific consensus held that, in addition to language, right-handedness and the specialization of just one side of the brain for processing spatial relations occur in humans alone. Other animals, it was thought, have no hemispheric specializations of any kind.
- The authors have proposed that the specialization of the brain’s two hemispheres was already in place when vertebrates arose 500 million years ago.
- The left hemisphere originally seems to have focused in general on controlling well-established patterns of behavior; the right specialized in detecting and responding to unexpected stimuli.
- Both speech and right-handedness may have evolved from a specialization for the control of routine behavior.
- Face recognition and the processing of spatial relations may trace their heritage to a need to sense predators quickly.
Those beliefs fit well with the view that people have a special evolutionary status. Biologists and behavioral scientists generally agreed that right-handedness evolved in our hominid ancestors as they learned to build and use tools, about 2.5 million years ago. Right-handedness was also thought to underlie speech. Perhaps, as the story went, the left hemisphere simply added sign language to its repertoire of skilled manual actions and then converted it to speech. Or perhaps the left brain’s capacity for controlling manual action extended to controlling the vocal apparatus for speech. In either case, speech and language evolved from a relatively recent manual talent for toolmaking. The right hemisphere, meanwhile, was thought to have evolved by default into a center for processing spatial relations, after the left hemisphere became specialized for handedness.
In the past few decades, however, studies of many other animals have shown that their two brain hemispheres also have distinctive roles. Despite those findings, prevailing wisdom continues to hold that people are different. Many investigators still think the recently discovered specializations of the two brain hemispheres in nonhumans are unrelated to the human ones; the hemispheric specializations of humans began with humans.
Here we present evidence for a radically different hypothesis that is gaining support, particularly among biologists. The specialization of each hemisphere in the human brain, we argue, was already present in its basic form when vertebrates emerged about 500 million years ago. We suggest that the more recent specializations of the brain hemispheres, including those of humans, evolved from the original ones by the Darwinian process of descent with modification. (In that process, capabilities relevant to ancient traits are changed or co-opted in the service of other developing traits.) Our hypothesis holds that the left hemisphere of the vertebrate brain was originally specialized for the control of well-established patterns of behavior under ordinary and familiar circumstances. In contrast, the right hemisphere, the primary seat of emotional arousal, was at first specialized for detecting and responding to unexpected stimuli in the environment.
In early vertebrates such a division of labor probably got its start when one or the other hemisphere developed a tendency to take control in particular circumstances. From that simple beginning, we propose, the right hemisphere took primary control in potentially dangerous circumstances that called for a rapid reaction from the animal—detecting a predator nearby, for instance. Otherwise, control passed to the left hemisphere. In other words, the left hemisphere became the seat of self-motivated behavior, sometimes called top-down control. (We stress that self-motivated behavior need not be innate; in fact, it is often learned.) The right hemisphere became the seat of environmentally motivated behavior, or bottom-up control. The processing that directs more specialized behaviors—language, toolmaking, spatial interrelations, facial recognition, and the like—evolved from those two basic controls.
The Left Hemisphere
Most of the evidence that supports our hypothesis does not come from direct observation of the brain but rather from observations of behavior that favors one or the other side of the body. In the vertebrate nervous system the connections cross between body and brain—to a large degree, nerves to and from one side of the body are linked to the opposite-side hemisphere of the brain.
Evidence for the first part of our hypothesis—that the vertebrate left hemisphere specializes in controlling routine, internally directed behaviors—has been building for some time. One routine behavior with a rightward bias across many vertebrates is feeding. Fishes, reptiles and toads, for instance, tend to strike at prey on their right side under the guidance of their right eye and left hemisphere. In a variety of bird species—chickens, pigeons, quails and stilts—the right eye is the primary guide for various kinds of food pecking and prey capture. In one instance, such a lateralized feeding preference has apparently led to a lateralized bias in the animal’s external anatomy. The beak of the New Zealand wry-billed plover slopes to the right; that way, the plover’s right eye can guide the beak as the bird seeks food under small river stones.
As for mammals, the feeding behavior of humpback whales is a spectacular example of a lateral feeding preference. Phillip J. Clapham, now at the Alaska Fisheries Science Center in Seattle, and his colleagues discovered that 60 out of 75 whales had abrasions only on the right jaw; the other 15 whales had abrasions only on the left jaw. The findings were clear evidence that whales favor one side of the jaw for food gathering and that “right-jawedness” is by far the norm.
In short, in all vertebrate classes—fishes, reptiles, amphibians, birds and mammals—animals tend to retain what was probably an ancestral bias toward the use of the right side in the routine activity of feeding.
Origins of Right-Handedness
What do these findings say about the alleged uniqueness of human right-handedness? Evidence for a right-side bias in birds and whales is intriguing, but it hardly makes a convincing argument against the old belief that right-handedness in humans had no evolutionary precursors. Yet more than a dozen recent studies have now demonstrated a right-handed bias among other primates, our closest evolutionary relatives—clearly suggesting that human right-handedness descended from that of earlier primates. The right-hand preference shows itself in monkeys (baboons, Cebus monkeys and rhesus macaques) as well as in apes, particularly in chimpanzees.
Many of the studies of apes have been done by William D. Hopkins of the Yerkes National Primate Research Center in Atlanta and his colleagues. Hopkins’s group observed right-hand preferences particularly in tasks that involved either coordinating both hands or reaching for food too high to grab without standing upright. For example, experimenters placed honey (a favorite food) inside a short length of plastic pipe and gave the pipe to one of the apes. To get the honey, the ape had to pick up the pipe in one hand and scrape out the honey with one finger of the opposite hand. By a ratio of 2 to 1, the apes preferred to scrape honey out with a finger of the right hand. Similarly, in the reaching experiments, the apes usually grabbed the food they wanted with the right hand.
The Yerkes findings also suggest to us that as early primates evolved to undertake harder and more elaborate tasks for finding food, their handedness preferences became stronger, too. The reason, we suspect, is that performing ever more complex tasks made it increasingly necessary for the control signals from the brain to pass as directly as possible to the more skilled hand. Since the most direct route from the left hemisphere—the hemisphere specialized for routine tasks—to the body follows the body-crossing pathways of the peripheral nerves, the right hand increasingly became the preferred hand among nonhuman primates for performing elaborate, albeit routine, tasks.
Communication and the Left Brain
The evolutionary descent of human right-handed dexterity via the modification of ancient feeding behavior in ancestral higher primates now seems very likely. But could feeding behavior also have given rise to the left-brain specialization for language? Actually we do not mean to suggest that this development was direct. Rather we argue that the “language brain” emerged from an intermediate and somewhat less primitive specialization of the left hemisphere—namely, its specialization for routine communication, both vocal and nonvocal. But contrary to long-held beliefs among students of human prehistory, neither of those communicative capabilities first arose with humans; they, too, are descended from hemispheric specializations that first appeared in animals that lived long before our species emerged.
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