An evolutionary comparison of human head and body lice has shed light on the history of clothing.
A team based in Germany has worked out that humans probably first began wearing clothes 72,000 years ago - give or take 40,000 years.
At what stage in human development did clothes become de rigueur
Their calculation is based on the fact that as species evolve, they become distinct by inhabiting different environments and gradually changing to suit them.
While head lice live solely on the human scalp, body lice prefer to inhabit those areas covered by clothing.
Dr Mark Stoneking's team, from the Max Planck Institute for Evolutionary Anthropology in Leipzig, worked out when the two organisms began to diverge and became distinct species.
They compared DNA sequences from both types of lice, arriving at their result by counting the number of DNA mutations.
Head lice stayed put when body lice moved south
"DNA mutations occur at a roughly constant rate over time, so if you know what that rate is, you can use the number of mutations between head and body lice to estimate when body lice arose," explained Dr Stoneking.
They also analysed the DNA from chimpanzee lice, and found that the human and chimp bugs became separate species around 5.5 million years ago - not far off the time their hosts' lineages are thought to have diverged.
Margin of error
The calculation that clothing appeared about 72,000 years ago points towards it being a relatively new invention, given that Homo sapiens has probably been around for less than 200,000 years.
"A large time window is inevitable with any molecular clock approach to dating, but even if you take the extremes of the range, the result still associates clothing specifically with modern humans," said Dr Stoneking.
Given that Homo sapiens are generally believed to have expanded out of Africa about 100,000 years ago, perhaps clothing was invented to cope with the cooler climes of their new habitats.
Dr Stoneking's next project is the DNA analysis of pubic lice, using the molecular clock to work out when humans lost the majority of their body hair.
His research is published in Current Biology.