I do not write about biology-related issues here much: official participation ended with a B grade at ‘O’ level in 1982, but a New Scientist article on the evolutionary history of the (various) human lice (which does not yet appear to be online) is just too fascinating to ignore.
Primate lice are different from most species of wingless insects of the order Phthiraptera in that they suck blood: most lice just live on dead skin and similar detritus. Not all primates are infected either – Orang Utangs and Gibbons do not suffer. But the human head louse shares a common ancestor with the louse of the Chimp – just as we and Chimps share common ancestors.
But it turns out there is more than one species of human head louse and it is likely that the rarer forms – found in two groups, the first in the Americas and Asia and the second only in Nepal and Ethiopia – are descended from the lice of other (now extinct) hominids. The most common form of head louse can be dated back to about 6 million years ago, but the less common forms appear to have only established themselves with homo sapiens about 0.5 million years ago.
Then there are the pubic lice – commonly known as crabs – which, as the name suggests, live on pubic hair. These are not descended from head lice but actually, it appears, from the lice of the gorilla and crossed to humans about 3 million years ago. This leaves open the prospect that human had sex with gorillas or (perhaps more likely as it still happens today) ate gorilla meat.
Head and pubic lice are a public health menace but in general pose no serious threat. Not so the clothes louse. Typhus – the disease carried by these – killed millions in Europe in the 20th century (particularly in times of war) and is still killing around tens of thousands of people across the world every year. But it would appear the clothes louse is merely a mutated form of the head louse.
In experiments head lice transferred to clothes die in massive numbers but a few have a genetic disposition to survive and will then reproduce in massive numbers. It is this overwhelming number that may make them deadly, rather than any other particular characteristic. The genetics suggest that humans began to wear clothes (as we became less hairy and gained new skills and tools) perhaps 170,000 years ago.
My scalp feels quite itchy now. So I’ll stop.