There is fresh evidence that, ethnically speaking, you really can't judge a book by its cover: A new study has found that skin colour is a poor indication of ancestry.

Men and women who look like black Africans can carry genetic traits similar to those of white Europeans, according to a multipronged study carried out on Brazilian populations. Some of the lighter-skinned Brazilians showed more genetic evidence of African heritage than their darker-skinned counterparts.

The research, published today in the Proceedings of the National Academy of Sciences, adds to the growing body of science that says race is merely a social invention and, biologically, does not exist.

"They're saying you can't go out on the street and say this person looks African or something else because really what they're saying is you can't tell," said Tom Hudson, director of the Montreal Genome Centre at McGill University Health Centre.

"This appears to be a well-done study showing that between race, skin colour and DNA markers there is no clear-cut connection."

Scientists from the Institute of Biological Sciences in Brazil set out to investigate whether Mother Nature is in fact colourblind, since race -- as defined by skin colour and other physical features -- continues to shape the country's social prejudices, just as it has always fuelled conflicts and discrimination the world over.

"The physical traits of an individual -- especially skin pigmentation, hair colour, hair texture, and the shape of the lips and nose -- are constantly used for racial categorization and thus play an extremely influential role in human social relationships," the authors write. Yet, they point out, "It is possible for two siblings differing in colour to belong to completely diverse racial categories" in Brazil.

Earlier research has already made it clear that there are far more genetic similarities than differences between ethnicities. Dr. Hudson, who is leading Canada's efforts to map disease genes in African, Caucasian and Asian populations, said it is difficult to say whether the Brazilian research applies widely to other populations.

Still, the researchers seem confident that even scientists should consider their findings' wider implications: "Our study makes clear the hazards of equating colour or race with geographical ancestry and using interchangably terms such as white, Caucasian, and European on one hand, and black, Negro or African on the other, as is often done in scientific and medical literature."

Included in the study were 173 people from a rural village in southeast Brazil who agreed to be examined clinically and classified by their physical features. Twenty-nine were listed as white, 30 as black and 114 as intermediate.

Two hundred Brazilian males from different metropolitan centres who described themselves as white also took part. As well, the study included representatives of the country's founding populations: 10 native Amerindians from Amazonian tribes, 20 men from northern Portugal and 20 individuals from the west coast of Africa, the point from which black slaves were once shipped to Brazil.

The researchers then correlated the subjects' physical characteristics with a group of 10 genetic markers known to be common to people of African ancestry.