Ask Mr. Brain

PUBLISHED : Friday, 20 August, 1999, 12:00am
UPDATED : Friday, 20 August, 1999, 12:00am

Why are there white, black and yellow people? Does the colour of the skin serve any special purpose? WANDA TSANG St Rose of Lima's School In common with many species, there is a wide range of variations in hair and skin colour, size and body shape among humans. The most obvious is the range of skin colours. Despite many years of research into skin colour and many hypotheses, no one has yet come up with a totally convincing reason as to why humans have different skin colouring and why they are spread across the globe in the way they are.

One of the oldest - and most popular - hypotheses held that skin colour was due to natural selection in relation to how sunny the place was. At first glance this appears to make sense, as darker skinned peoples tend to be found in sunnier climates and lighter-skinned in cooler and cloudier climates.

The reason, so one hypothesis goes, is that dark skin protects against sunburn and skin cancer in sunny areas. Along similar lines, another hypothesis claims that lighter skins are needed in less sunny areas to ensure the body is able to manufacture sufficient vitamin D. However, as Jared Diamond points out in The Third Chimpanzee, the match between dark skin and a sunny climate is far from perfect.

He cites the example of Tasmanian Aborigines, who are very black even though their land is located on the southern hemisphere's latitude equivalent of New York and southern Europe. Similarly, the people of tropical Southeast Asia - including Hong Kong - generally have much lighter skins than Tasmanian Aborigines, even though their climate is far sunnier and hotter.

In the Solomon Islands in the Pacific Ocean, the people vary over a short distance from very black to much lighter depending on which island they are from. Among American Indians, the opposite is true - there are none with black skins, even in the sunniest areas of Central America.

Genetic research has revealed some findings which have surprised many people. While it seems obvious to assume people with similar skin colour are likely to be closely related, this is not true.

In their book The History and Geography of Human Genes, Luca Cavalli-Sforza, Paolo Menozzi and Alberto Piazza reveal that Australian Aborigines are genetically most closely related to Southeast Asians, not to Africans.

So, unfortunately no one has yet come up with a watertight theory as to why humans are of different colours, especially given the wide geographic and climatic range over which different skin colours are found. While natural selection may have played some role, its part is far from obvious and it is not likely to have been the only deciding factor.

Why does a fly have so many eyes? Flies have two eyes, but they are compound eyes. Most adult insects have compound eyes, each of which can have as many as several thousand individual units. Each unit is called an ommatidium and has a group of cells capped by a lens. The picture created by the fly's eyes is like a mosaic.