Humans only started wearing clothes as little as 40,000 years ago, according to a new genetic study which has calculated when the human body louse evolved - a creature which needs clothes to lay its eggs on.

According to the research, by Professor Mark Stoneking and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany and published in the latest issue of the journal Current Biology, humans might have first worn clothes around 42,000 to 72,000 years ago.

Anthropologists have long wondered when clothes began to appear. Since fur and fabrics do not fossilise, no evidence has been left, apart from some fabrics more than a few thousand years old.

The new approach focused on the subtle genetic differences between the head louse (Pediculus humanus capitis) and the body louse (P. humanus corporis or P. h. humanus). These human ectoparasites differ mainly in their habitat on the host: head lice live in the hair and scalp, while body lice feed on hairless parts of the body but lay their eggs only in clothes.

"This ecological differentiation probably arose when humans adopted frequent use of clothing," write the researchers. Thus, an indirect measure of when our ancestors first wore clothes would have emerged by figuring out when body lice first appeared, the researchers concluded.

Stoneking's team used a molecular clock approach - a dating method based on the rate that specific types of mutations accumulate in DNA.

"Sequences were obtained from two mtDNA [mitochondrial DNA] and two nuclear DNA segments from a global sample of 40 head and body lice. ... We also included a chimpanzee louse (Pediculus schaeffi), assuming that human and chimpanzee lice co-speciated with their hosts," the researchers reported.

"The divergence time of 5.5 million years between humans and chimpanzees also corresponds to the P. humanus-P. schaeffi divergence, and hence was used as a calibration point for molecular clocks."

DNA analysis of the 40 human head lice and body lice sent from around the world revealed the modern genetic variation in the parasites.

Assuming that mutations occur at a given rate, Stoneking's team came to the estimate that "body lice originated not more than about 72,000 to 42,000 years ago." The date fits with fossil and archaeological evidence: the only tools that can be definitely associated with clothing, such as needles, are about 40,000 years old.

The genetic results also indicate greater diversity in African than non-African lice, suggesting an African origin of human lice which matches human origins.

"It is a clever study and their results makes sense to me," evolutionary biologist Blair Hedges of Pennsylvania State University, told Discovery News.

"Molecular clocks are not perfect chronometers - they need a lot of sequence data for precision. In this case, the estimate of 72,000 years has a large statistical error associated with it. With more genes, that error can be reduced considerably," Hedges said. "Nonetheless, the ballpark time - 50[,000] to 100,000 years ago - makes a lot of sense because that is when modern humans were leaving Africa for the cooler northern latitudes and would have needed clothing."

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