The puzzle of left-handedness seems to be a modest growth area in current research. One of the latest proposals links left-handedness both with an overproduction of the male hormone and with deficiencies in the body’s automatic defense mechanisms, although it is not clear whether the overproduction of hormone causes the defense defect, or is simply one symptom of a general imbalance. Could this explain why 13 per cent of male children, but only 10 per cent of females, are left-handed?
Today, very roughly 10 per cent of the population is left-handed. The odd thing is that this percentage seems to have remained constant throughout history, and indeed throughout human prehistory. From written records, and even painting, researchers can get a guide to the incidence of left-handedness among our recent ancestors.
Looking further back, some experts claim to be able to tell from the shaping of flint(打火石) tools whether our Stone Age forebears were left-or right-handed while the habit of cave painters of leaving hand prints (as “signatures”)on walls at least tells us which hand an individual used for marking walls. The evidence suggests that 10 per cent of the population was left-handed.
This is a common evolutionary pattern where there are variations that each provides some advantage for the individual that carries them, but not an overwhelming advantage, so that all the variations exist somewhere in the population.
A balance is struck in which the proportions of the population in which each version is expressed stay roughly the same form generation to generation - as in the case of blood groups, for example. There must be some advantage, in Darwinian terms, in being a left-hander among a population of predominantly right-handed people. But this is very different from the pattern seen in most animals.
Our near relatives, the chimpanzees, also show individual preference for one hand or the other, but half the chimp population is left-handed. Even rats show the same split - 50 per cent left-pawed and 50 per cent right-pawed. Man may not quite be unique among mammals; there have been claims that polar bears are more likely to be left-handed than right, although understandably few experiments have been done with polar bears under laboratory conditions. But by comparison with our cousin apes, it looks as if research into left-handedness in people may be asking the wrong question.
The real puzzle is not why 10 per cent of people are left-handed, but why 90 per cent are right-handed. What has happened to the human species, in evolutionary terms, to tilt the balance away from left-handers in favor of right-handers? Why is man the right-handed ape? Nobody has a satisfactory answer. But it is one of the curiosities of scientific research that studies of something seemingly so mundane as left-handedness in people could actually shed new light on human evolution.
1.The article suggests that left-handedness might be due to（ ）
2.It appears from the article that （ ）
3.What do we learn about polar bears?
4.What advantage is shown in being a left-hander?
5.From the passage, we can guess the word "mundane" in the last paragraph means（ ）.
