As long as 1.5 million years ago, according to archaeologist Nicholas Toth, more than half of early stone tools were chiselled with the right hand - one of the earliest documentable manifestation of handedness - although the preponderance of early Stone Age right-handers was not dramatic (somewhere in the region of 56%, according to Toth's study). A 1.6 million year old Homo ergaster skeleton found in Kenya by Richard Leakey and his team shows some evidence of right-handedness in the length of the ulna bones and the depth with which the deltoid muscles attach to the clavicle, although little can be justifiably concluded from the evidence of just one individual. Other evidence from the smashed skulls of baboons suggest that Australopithecus africanus, a distant ancestor from 2-3 million years ago, appears to have mainly clubbed his prey to death in a right-handed manner.
Studies of the skeletons and teeth of Homo neanderthalensis and Homo heidelbergensis generally indicate more robust right arms and shoulders (consistent with right-handed spear-throwing) and tooth wear consistent with food held in the right hand, suggesting a trend toward right-handedness as long ago as perhaps 500,000 years. A rather eccentric theory propounded by Stan Gooch suggests that Homo neanderthalensis was a left-handed race, but Gooch offers no real scientific evidence, and the idea has been comprehensively disproved.
As humans evolved and their brains grew larger, the brain became lateralized and one side became dominant, although quite why it was the left hemisphere that became dominant is still open to conjecture. It is thought that handedness and the specialization of the brainís hemispheres was an evolutionary step towards greater efficiency somehow, in much the same way as the division of labour in a factory makes production more efficient due to specialized and separate job functions. The development in human evolution that many scholars suggest began the whole process of brain hemisphere specialization and handedness in humans was the early use of tools, specifically throwing rocks at prey and predators. Indeed, according to some, the development of that quintessential human trait, speech, may even be considered an offshoot of the intricate tangle of interconnected neurons that originally developed in one hemisphere of the brain to aid the gross and fine motor skills of rock-throwing. There is more discussion of the evolutionary aspects and implications of handedness in the section on Evolutionary Theories of Handedness.
The question then arises, though, as to what evolutionary benefit left-handedness in particular confers. Why are we not all right-handed? It has been argued by some that the more distributed and right-hemisphere dominant brain arrangement of left-handers may help them remember landmarks and images that involve direction (visual-spatial functions normally carried out in the right-hemisphere of the brain), and so left-handed people may be better at navigating their way through uncharted terrain, a useful skill in prehistoric times. There is, however, a distinct group advantage in a common and consistent handedness, in terms of efficiency, speed and reduced accidents (imagine, for example, one out of a team of scythers scything on the other side to the rest). As human societies became more fixed and agrarian, therefore, they began to benefit from the consistency of right-handed conventions, creations and tools, while the left-handersí advantages in spatial integration and navigation became less important in settled agricultural communities.
Given that handedness is, at least to some extent, a heritable trait (see the section on Genetic Theories of Handedness), this suggests that there must be at least some evolutionary advantage to left-handedness, in some situations at any rate. Some consider the putative combat advantage of left-handedness, the so-called Fighting Hypothesis, although this is by no means undisputed. Michael Corballis, who supports the genetic theory of handedness, argues that handedness emerged due to a mutation at some point in the evolution of Homo sapiens in Africa some 150,000 to 200,000 years ago, and spread quite quickly thereafter through the population. He also uses this theory to explain the apparent stability in the relative proportions of left-handers and right-handers over long periods of time (see below).
As the Stone Age progressed, the proportion of right- to left-handers appears to have increased, suggesting even more strongly an evolutionary advantage in strong right-handedness. Studies by Stanley Coren and Clare Porac, and by Charlotte Faurie and Michel Raymond, of the prehistoric hand tracings in caves in France and Spain, indicate that, around 30,000 years ago, at least 75% were probably right-handed. Other studies of tool use during this period back this up.
Interestingly, though, the indications are that, since the Upper Paleolithic Age (around 10,000 years ago), there has been very little change in the proportion of left-handers, which continues at around 10% (with some cultural variations). An analysis by Stanley Coren and Clare Porac of left-handedness in works of art (paintings, drawing and sculptures in which a person uses one hand or the other to perform some skilled action, from throwing a spear to firing an arrow to needlework), shows a remarkably consistent record of around 10% left-handedness going back over 5,000 years.
Either way, it seems that right-handedness became prevalent across all human cultures. Many modern-day indigenous cultures, from the Niger River to the veldt of South Africa to the islands of the Pacific, who still cling to very ancient traditions, go so far as to forbid outright the use of the left hand, with some cultures even binding the left arm to the body or ritually scalding it in early childhood. However, some researchers, notably Fred Previc, have claimed that the percentage of right-handedness is actually smaller in more primitive cultures.
Other studies, such as that of John Dawson in the 1970s, have also found that nomadic peoples, like the Inuit and the Australian Aborigines, showed higher than average rates of left-handedness, although the differential, if any, is not great. Claims of the existence of wholly (or even predominantly) left-handed tribes, as have been made from time to time in the literature, have all turned out to be spurious or based on unreliable travellersí tales.