Sometimes little critters can hold the answers to big questions. In a new study, researchers claim that parasitic lice can tell us much about the course of bird and mammal evolution, including whether the ancestors of these animals made it through a mass extinction that wiped out most of the dinosaurs.
Researchers have long debated whether the forerunners of today's birds and mammals survived the Cretaceous-Tertiary (K-T) extinction, which was probably caused by an asteroid hitting Earth 65 million years ago, ending the Cretaceous period. Most DNA studies say yes, but many paleontologists think that the majority of today's lineages of birds and mammals emerged only after the Cretaceous period was over.
In an attempt to settle the debate, an international team led by Vincent Smith, a taxonomist and expert on lice evolution at the Natural History Museum in London, turned to the tiny louse—a parasitic insect that infests birds and mammals and is thought to have evolved alongside them—for new insights. The researchers used the recent discovery of two fossil lice, one 44 million years old and the other about 100 million years old, to calibrate a molecular clock for louse evolution. Then they analyzed partial DNA sequences from 69 existing louse species to trace the pest's evolutionary origins. The team reports online this week in Biology Letters that parasitic lice arose at least 115 million years ago, long before the K-T event.
This means, the researchers conclude, that the lice's hosts, presumably the ancestors of today's birds and mammals, also existed at that time. Moreover, because some bird lice seem to have deeper evolutionary roots than mammal lice do, the team suggests that birds—whose feathers make a good roosting place for lice—became infested first and then passed the pests on to mammals. And the first victims, the team concludes, may well have been the feathered dinosaurs that gave rise to today's birds.
But researchers who have been involved in these evolutionary debates are not convinced that louse evolution closely tracked the evolution of their hosts. Evolutionary biologist Olaf Bininda-Emonds of the Carl von Ossietzky University of Oldenburg in Germany says that the authors have used a "novel approach to answer the question of when the birds and mammals began diversifying." Yet he cautions that the "conclusions of the paper are overstated" because the study does not prove that prehistoric lice were actually infesting the direct ancestors of today's bird and mammal species rather than other animals that may have led to evolutionary dead ends.