The Puzzle of Left-Handedness

by Rik Smits

  This sounds more pleasant than it would have been in reality. An individual with sufficient determination can get the better of a mouse on open ground, but a rabbit is just as nimble, larger and much faster. Like most other animals of the savannah, rabbits are more than a match for humans or hominids equipped with nothing but their bare bands. We are too slow and lumbering, too vulnerable to attacks by large predators. So these early humans cannot have had a particularly meaty diet; they must have been vegetarians most of the time. This in itself is something of a problem, since little energy can be derived from plant material, especially if eaten raw. Being a vegetarian on the ancient savannah meant working long and hard for relatively few calories. Man’s prospects did not look good.

  Nevertheless, this comparatively slow hunter-gatherer proved himself unprecedentedly energetic. Some two million years ago Homo habilis spread from Central Africa across the entire African continent with phenomenal speed and within less than half a million years his successor, Homo erectus, inhabited large parts of the globe, fertile or barren, rainy or dry. Eventually he could be found in practically all regions that are warm enough for humans to survive without clothing or heated dwellings. He even settled in areas where it was virtually impossible for him to get by on roots, tubers, nuts and seeds, since they are not productive enough in those environments or simply do not grow in sufficient quantities.

  So man must have survived on the flesh of birds and small animals. Meat is a far more compact and digestible form of energy than plant matter, and it’s available everywhere. In all environments, whether the frozen wastes near the poles, the tundra or arid sandy deserts with extreme variations in temperature, it’s possible to find edible species of animal that are adapted to the prevailing conditions. The largely vegetarian gatherer who from time to time catches a hare must therefore, Calvin claims, have developed in this period into a competent hunter who gathered plant material to supplement a meaty diet.

  The surprising thing about all this is that the forerunners of modern man spread so far out across the globe long before hunting implements such as the spear or the bow and arrow were invented. How did our ancestors get hold of enough animals to feed themselves? Calvin suggests we need to look at various differences between our methods of hunting and those of today’s apes, which are still limited to their original tropical environments.

  One such difference is the impressive arsenal we eventually developed, but here we are looking at a time before weapons were invented. Second, it’s often claimed that what made the difference was language, perhaps the most spectacular cultural artefact that humans possess and apes lack, since it’s said to have brought about decisive improvements in cooperation between individual hunters. But this does not stand up to scrutiny either. During a hunt, language is not used to any significant extent, as we can see from the success of animals that hunt in packs, such as wolves. They are living proof that ancient techniques like surrounding, driving and wearing to exhaustion can be applied without the slightest hint of language. In fact when people go hunting their conversations usually have nothing to do with the hunt. They talk about the ordinary things in life: relationships, work and money. As soon as they start seriously stalking their prey, they keep their mouths firmly shut. The quarry does not understand what people are saying, but it hears them all the better for that.

  There’s a third difference between humans and apes, however, and Calvin believes that it’s of huge significance. People are capable of throwing accurately. Apes sometimes throw stones, but always more or less indiscriminately. Humans use one arm as a sling of sorts, and this enables them to hit a small target many metres away with considerable force. The capacity to throw in this way gave the otherwise rather unimpressive erectus a deadly prehensile claw with a range of at least twenty metres.

  The well-aimed throw has a considerable effect on hunting success, and in evolutionary terms it’s a skill that can spread through a species extremely quickly. The connection with highly nutritious food is so direct that it exerts enormous selective pressure. Anyone who can throw even a little better than the next individual has a far greater chance of survival and is therefore more likely to pass on his superior throwing ability to a new generation. If, that is, the throwing skill is rooted in something heritable.

  Partially reconstructed skull of Homo erectus, with a modern human skull behind it. Erectus had a relatively small cranium but was nevertheless a competent hunter.

  This does appear to be the case. Success in well-aimed throwing relies on a powerful capacity for control, which is lodged in the brain. A good throw requires the thrower to develop and implement a plan of action for dozens of muscles, based on his observation of the course and speed of the prey, the weight of the stone in the hand and a great deal more besides. Nature has little to show that can compare to man’s accuracy in throwing. Neurons, the electrical switches of which our brains are composed, are not particularly good at such tasks, being relatively slow and imprecise, but if a great many neurons share the same job speed and accuracy increase markedly. Though individually no more than approximations, average results from hundreds of neurons produce extremely accurate estimates, and by having several groups of neurons work simultaneously we avoid the need to spend so much time waiting for intermediate results. The trick is to bring a huge amount of brain capacity to bear. But to do that, of course, you need to have a large brain.

  Call it a historical coincidence, or even in the words of Stephen Jay Gould a ‘glorious evolutionary accident’, but that brain capacity was available. Roughly in parallel with the rapid spread of our early ancestors across the globe, an equally rapid, hugely impressive expansion in their brain mass took place, the first phase of growth that would culminate in the almost one and a half kilos of grey matter housed in the skull of the average modern human. Our new capacity to calculate could not have arrived at a better moment.

  Well-aimed throwing is a typically one-sided affair. Its great speed and precision, and therefore the considerable firepower it provides, are achieved by the use of one or other arm as a sling. This means that the required computing power needs to be present in one side of the brain only. Nevertheless, the symmetrical structure of the physical human being meant that the other side of the brain increased in size to almost the same degree. If that capacity was not needed for throwing, it would be available for other skills that conferred evolutionary advantages of their own. Were the throwing capacity to be lodged in both halves of the brain, it would mean extra investment and the use of additional brain-power without much to show for it. After all, a person who can throw equally well with either hand is barely any more effective than someone who can throw accurately only with one or the other. The two-handed thrower may even be worse off, because of the need to choose which hand to use at a point when every millisecond counts.

  All this, Calvin argued, sowed the seeds of an increasing dissimilarity in functions between the two cerebral hemispheres, possibly even for the developmental boom in the human brain that eventually produced consciousness, language and all the other characteristics that make us what we are today. At the same time, inevitably, we developed a hand preference.

  Of course Calvin’s line of reasoning did not emerge out of thin air. He was inspired above all by what we know about the differences in function between the two halves of the brain. It does indeed seem as if, generally speaking, the left cerebral hemisphere is devoted to the kind of tasks in which the precise sequencing of acts and calculations is of importance. This fits in with the fact that humans are predominantly right-handed, since the left half of the brain controls the right half of the body. But now we are starting to get ahead of the argument. Calvin’s reasoning offers a fair explanation as to why only one half of the brain specializes in this kind of task, and therefore why we have a hand preference, but his story has nothing to say about which of the two halves would be best. It therefore leaves us with the mystery as to why nine out of ten people have the same hand preferenc
e. In other words: what is it that makes the left side of the brain the seat of the throwing mechanism in almost all cases and not the right, or one side in half of us and the other side in the rest?

  In his search for an explanation, Calvin joins those who turn to the carrying habits of mothers with children and the position of the heart. As we have seen, most mothers carry their children with the left arm, probably because babies are happiest there and therefore quietest, since a mother’s heart is more clearly heard on the left side of her chest cavity than the right. Calvin assumes that erectus women were just as active during a hunt as their menfolk and that they carried their offspring with them while hunting, so women with quiet children had a clear advantage. Not only could they move more easily, there was less chance that their positions would be given away by the crying or crowing of their infants as they crept up on their prey. Those quieter children were generally carried on the left, leaving their mothers’ right hands free. So it was not the first stone-thrower but the first stone-throwing woman, Calvin’s ‘Throwing Madonna’, who took the decisive step on the road towards the modern human being.

  There seems to be little arguing with the first part of Calvin’s story. It’s both appealing and based on facts. Clearly the forerunners of modern man, once they had climbed down out of the trees, spread across all kinds of terrain without having any special tools at their disposal and started eating more meat. It’s undoubtedly the case that brain size increased spectacularly in the same period, and we can throw incomparably better than our closest relatives in the animal kingdom. Finally, it has gradually come to be accepted as a fact that in the vast majority of people living today the left half of the brain is responsible for activities that involve careful planning, considerable speed and correct sequencing. Until someone throws a spanner in the works, therefore, we can assume that his explanation for the existence of hand preference holds water.

  The same cannot be said of Calvin’s story about why most of us are right-handed. It relies upon assumptions about the social role of erectus women that are impossible to prove and against which various counter-arguments can be put. Firstly, the advantage to right-handed women who carry their infants in their left arms ceased to apply once children were no longer taken hunting by their parents. We don’t know when this happened. Secondly, we may well wonder whether hunting did not involve such risks to mother and child that women had the best chance of survival if they left it to men and the childless, relying on the solidarity of the group. Occasional examples in the animal kingdom suggest this is the way it worked.

  Calvin’s idea also fails to chime with the fairly well-substantiated theories put forward by Richard Wrangham, an American biological anthro pologist who sees in the sharp contrast between Homo habilis and Homo erectus the beginnings of cookery. He claims this was an activity that fell to women from the start, partly because they were less mobile as a result of their roles in procreation and childrearing. Based on ideas about precisely the period in which Calvin looks to huntresses as the motor for one-sided brain development, Wrangham places women at the kitchen stove for the rest of eternity.

  There is a fourth objection. If the behaviour of a child truly had such a powerful influence on the hunting success of its mother, then the effect on the survival chances of the child itself would be just as significant. In that case, why was the behaviour of the mother rather than that of the child subject to evolutionary pressure? If Calvin is right, you would surely think that quiet children of right-handed mothers had the best survival chances, and therefore tended to survive in greater numbers. It would therefore be perfectly sensible to suppose that modern babies ought to resemble their quietest ancestors and lie absolutely silently in their mothers’ arms. To the chagrin of many parents, this is not the case. Babes in arms still shriek, whine and wriggle to their hearts’ content.

  25

  Thinking About Brains

  It seems natural to assume that hand preference could have something to do with the difference between the two halves of the brain. Yet it was not until well into the nineteenth century that the connection was made.

  Hippocrates, the patriarch of doctors, suspected 2,500 years ago that brains were associated with thinking. That was anything but an obvious assumption. Aristotle, for instance, believed that the warm, beating heart was the centre of everything. He regarded the brain as no more than a system for cooling the blood. Yet even in the ancient world some real, concrete knowledge was available about how we control our limbs, for example. In the third century BC, Alexandrine scholars Herophilus and Erasistratus discovered the nervous system and were able to distinguish between efferent and afferent nerves. The former are the lines of communication that send instructions to the muscles, while the latter respond to external stimuli by transporting signals to the spinal cord and from there to the brain.

  Some four centuries later Galen, the Greek personal physician to Roman emperor Marcus Aurelius, performed experiments on animals in which he partially severed their spinal cords in various places. This revealed a great deal about the composition and function of the nervous system. He also knew that the brain had something to do with the mind, but he thought the mind was to be found in the fluid that fills the hollow spaces between the cerebral hemispheres. That idea would persist until the sixteenth century, when the brilliant Flemish physician Andreas Vesalius showed it could not possibly be true. There are those who claim that as early as the first century ad people knew that the left half of the brain controlled the muscles of the right half of the body and vice versa, but we cannot be certain of this.

  One of the drawings of the brain made by the great anatomist Andreas Vesalius (1514–1564). At the centre are the ventricles, filled with cerebrospinal fluid.

  These were impressive achievements, of course, but after Galen hardly anything happened in the field of brain research for almost 1,500 years. The dissection of animals, living or dead, in order to investigate their inner structures had virtually gone out of fashion even in Galen’s time and it would be almost unknown for centuries, making way for all kinds of speculative theories that in the end produced hardly anything useful. Still, it’s possible this was of little consequence for the growth of knowledge about how the brain works, since across all those centuries a range of dogmas, most of them emanating from the churches, blocked the free development of ideas about the connection between our bodies and our mental processes. The mind, the spirit and the soul were things of a higher order, mysteries that must at all costs remain mysterious. In any case people had few techniques, however primitive, that would have enabled them to study the brain, and there is nothing to be seen in the grey matter itself that could possibly divulge how it works. A damaged brain, like a diseased heart or failing kidneys, caused death. Why this was so inevitably remained a mystery.

  The idea that the brain has something to do with thought, emotions and the intangible thing we call the mind nevertheless remained alive down through the centuries. A strange grey lump of matter fills the head, while the face is the location of our most striking features and means of contact with the outside world. Throughout history, people have believed that a person’s character and intelligence could be read from his or her face. Aristotle, who felt his own appearance to be no better than ordinary, publicly declared that he had his training in philosophy to thank for the fact that his mind was less ordinary than his face would suggest. It’s an extremely deep-rooted notion, as evidenced by the success of comparable ideas presented by Cesare Lombroso in about 1900, to which we shall return.

  Even Vesalius’ skeletons wondered what was going on inside the brain.

  The turning point came in the eighteenth century, with the success of the Enlightenment. The most important achievement of that school of thought was to dispense with traditional, mainly religious convictions. According to the philosophical innovators of the time, the nature of things could be discovered through reasoning, aided by empirical science. In other words, people no longer wished
to speculate randomly, nor to adhere to an unquestioning faith in the old, respected authorities simply because they were old and respected. From this point on, the message was: look carefully and draw rational conclusions from what you see. Whether or not God and the pope were pleased by this approach no longer really mattered to serious scientists, who were now free to start examining those aspects of humanity that had to do with the mind.

  It wasn’t long before a great many people started studying the brain, among them the Viennese doctor and anatomist Franz Joseph Gall (1758–1828). He was the first to claim that our brains consist of a layer of grey matter, the cerebral cortex, over a core of a whiter substance. He was also a central figure in phrenology, a movement that had been gaining in influence ever since the mid-eighteenth century and from which many of the terms and concepts used in today’s psychology are derived, as well as a whole series of misconceptions and popular fallacies. Phrenology was the first real attempt to link human functions and behaviours with the brain. It was based on the notion that specific human characteristics were anchored in specific parts of the brain, and that you could see from the shape of the skull and its bumps and irregularities how highly developed, or not, certain functions were. Phreno logists were rather casual in the way they went about localizing characteristics. Gall, for example, ‘discovered’ that the centre for individuality and new ideas lay just above the nose, since, or so he claimed, that particular area was large in the case of Michelangelo but generally small among the Scots.