The Evolution Of Handedness

The nature of human handedness is unique among mammalian species. We can set up situations, analogous to handedness tests in humans, in which animals must manipulate something with only one paw. This might be a task in which a cat must reach down a relatively narrow tube to pull out a treat or in which a monkey must insert a stick into a small hole to get something to eat. When we do this, we find that most cats, rats, and monkeys are right- or left-pawed. Although individual animals show behaviors analogous to handedness, there is one major difference between these animals and humans. Whereas 9 of 10 humans are right-handed, in other species the proportion of right- and left-sided individuals is approximately 50%. In other words, there is not right-sided bias to the animal population.

It is possible to estimate the handedness of humans over history to determine if we were always right-handed. Figure 1 shows the results of a study that examined paintings and drawings, reasoning that if artists were drawing from life, then they would draw the tools and weapons held by their models in the hand that they saw the person using. Analysis of more than 50 centuries of such drawings found that the proportion of right-handedness remained at approximately 90% from the Paleolithic Era (the Old Stone Age) until

Figure 1 Analysis of handedness as shown in artworks over a period of approximately 50 centuries.

the present. The date when right-handedness became dominant in our species can be pushed further back in time. Paleontologists have examined the wear patterns on stone tools and the grinding marks on devices used to grind grains. These tools and implements date back between 8000 and 35,000 bc and involve the early humanoid Homo habilis, one of the earliest tool makers. The wear patterns on these artifacts confirm that even at that early date, there appeared to be a consistent predominance of right-handers. Perhaps the most astounding evidence derives from more than 1.5 million years ago, involving one of our very early hominid ancestors, australopithecus. Although the australopithecines were not tool makers, they were tool users and would pick up an appropriately sized and shaped rock or stick and use it for a weapon. Examination of the skulls of baboons hunted by this early precursor to humans shows that the vast majority of these hominids were already right-handed.

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