DEXTEROUS EARLY HOMINIDS?

In modern human populations, approximately 90 percent of people are dominantly right-handed (dextral) and about 10 percent are dominantly left-handed (sinistral). This is a very unusual pattern, and it is unique to humans. In the rest of the animal world, including nonhuman primates, the breakdown of handedness (or pawedness) tends to be about 50 percent left-handed and 50 percent right-handed. (Among some primates there may be slight populational asymmetries in handedness for certain activities such as hanging from and feeding from trees, but these do not come close to the 9:1 ratio seen in modern humans.) It would seem probable that this preference for right-handedness in modern humans has a strong genetic component, and its absence among the apes would suggest that this trait became established in the human lineage after the split with the African apes.

Why should the human lineage have become so dominantly right-handed? No one knows for sure. It has been suggested that our extreme asymmetry in handedness may be correlated with an increased specialization (lateralization) in the hominid brain during the course of human evolution. Although there is increasing evidence to suggest that many animals appear to exhibit some lateralization in brain specialization, it seems to be on a much smaller scale than in human beings. In Homo sapiens the left hemisphere (which controls the dominant right hand) has become more specialized for such activities as time sequencing and language, and the right hemisphere has become more involved with spatial perception. With the development of manual skill during the rise of stone technology, perhaps natural selection favored populations that tended to use the same dominant hand to facilitate learning by observation (imagine a right-handed person trying to learn to play guitar by watching a left-handed person)....So far, every site we've examined from the Early Stone Age, including those at Koobi Fora dated from about 1.9 to 1.5 million years ago, shows exactly the same pattern. Thus it appears that by the time of early tool making in the archaeological record, these ancestral hominid populations may have already become preferentially right-handed...right-handedness seems to be an ancient trait in humans.