Evolution and the lowly louse

mammals. Most lice are specialists,feeding on a single species to whose fur or feathers their claws are adapted. The adaptation is so precise that when a louse’s host species evolves into a new one,the louse will diversify into different species,too. The human head louse,for instance,evolved from the chimpanzee louse when the ancestors of humans and chimps split apart some 5 million years ago. Species of human lice thus mirror the splits in the tree of ape and human evolution. Two fossil lice discovered in the last few years—one of them 44 million and the other 100 million years old—provided the necessary anchors for Smith’s tree. The assembled family tree shows that lice started to radiate into new species well before the end of the Cretaceous period,Smith and his colleagues report in the current issue of Biology Letters . The finding implies that their hosts,both mammals and birds,had begun to flourish before the reign of the dinosaurs was over. The new tree bears on a longstanding dispute about the rise of birds and mammals. One view is that both groups proliferated early in the Cretaceous period,which began 145 million years ago,and many lineages survived the large asteroid 65 million years ago,that brought the dinosaurs to a sudden end. The opposing view is that mammals and birds did not become successful and radiate into many different species until after the demise of the dinosaurs.

The issue is contentious because the fossil record suggests that placental mammals did not expand,until after 65 million years ago. A plausible reason is that all the dinosaurs had been killed off,except the line that evolved into birds,and the placental mammals speciated into the ecological niches that had been left vacant. But biologists reconstructing the mammalian tree of evolution from the DNA of living species have said their molecular clock data suggest a much earlier speciation,perhaps prompted by the breakup of the ancient supercontinent of Gondwana around 120 million years ago. Michael Novacek,an expert in mammalian paleontology at the American Museum of Natural History in New York,said the louse tree was very interesting and showed that lice were diversifying during the Cretaceous. But the fossil record of placental mammals is reliable and does not record a speciation until later. “So the contradiction between the fossils and molecular clocks remains.” In his view,the hosts on which lice were speciating during the Cretaceous could have been a different branch of the mammalian family tree,all of whose species are extinct.NICHOLAS WADE