A new study of insect DNA has shown that lice evolved more than 65 million years ago, at a time when dinosaurs still ruled the Earth.
The research also suggests that mammals and birds began to diversify into the vast range of different species seen today far earlier in prehistory than was previously thought.
Dr Kevin Johnson, of the University of Illinois, said: 'Our analysis suggests that both bird and mammal lice began to diversify before the mass extinction of dinosaurs.
'And given how widespread lice are on birds, in particular, and also to some extent on mammals, they probably existed on a wide variety of hosts in the past, possibly including dinosaurs.'
The researchers created an evolutionary family tree for lice using DNA from 69 different lineages.
Because changes in DNA accumulate over the millennia, the changes can also be used to create a time-line of the evolution of a related group of animals.
|A 44million-year-old louse fossil (left) and a modern version of the parasite |
that infests aquatic birds. Scientists believe lice may have fed off the blood
of feathered dinosaurs
'The record of our past is written in these parasites, and by reconstructing their evolutionary history we can use lice as markers to investigate the evolutionary history of their hosts.'
It was once thought that there were relatively few different species of birds and mammals while the dinosaurs were alive.
It was only after the giant reptiles died out 65 million years ago that birds and mammals went through a period of rapid diversification - filling the niches in the seas, air and land left by the dinosaurs.
The new study suggests birds and mammals had begun to diversify long before the dinosaurs went extinct.
'Ducks do different things from owls, which do different things from parrots, for example,' said Dr Johnson.
'I was thought that after the dinosaurs went extinct that's when these birds or mammals diversified into these different niches.
'But based on the evidence from lice, the radiation of birds and mammals was already under way before the dinosaurs went extinct.'
Many scientists believe that birds are the descendants of feathered dinosaurs.
Dr Johnson added: 'So maybe birds just inherited their lice from dinosaurs.'