As Dinosaurs Waned and Mammals Rose, the Lowly Louse Kept Pace

The New York Times: As Dinosaurs Waned and Mammals Rose, the Lowly Louse Kept Pace

Biologists have found a new way to peer back 130 million years in time, illuminating the catastrophic period in which the dinosaurs perished and birds and mammals arose.

The new approach rests on reconstructing the family tree of lice. Vincent S. Smith, a louse taxonomist at the Natural History Museum in London, has found that the tree stretches so far back in time that the host of the first louse would have been a dinosaur, probably one of the theropod dinosaurs that were the ancestors of birds.

Dr. Smith and his colleagues reconstructed the louse family tree by analyzing DNA from present-day louse species that parasitize birds and 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 five million years ago. The human pubic louse, on the other hand, is related to the gorilla louse, from which it parted company some 13 million years ago. Species of human lice thus mirror the splits in the tree of ape and human evolution.

In the same way, species of lice on living animals and birds reflect the splits in animal and bird ancestry back to the time that lice first arose. Family trees based on DNA can be given precise dates at all their branch points if a few datable fossils of the right age are available. But this is difficult to do with lice, for which almost no fossils are known. Dr. Smith was fortunate that 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 his tree.

The assembled family tree shows that lice started to radiate into new species well before the end of the Cretaceous period, Dr. Smith and his colleagues report in the current issue of Biology Letters. The finding implies that their hosts, both mammals and birds, had also begun to flourish and speciate before the reign of the dinosaurs was over.

The new tree bears on a longstanding dispute about the rise of birds and mammals. One school holds that both groups proliferated early in the Cretaceous period, which began 145 million years ago, and that many lineages survived the cataclysm that brought the Cretaceous and the dinosaurs to a sudden end: the strike of a large asteroid 65 million years ago. 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.

Unfortunately, the new finding will probably not settle the issue of whether birds and mammals speciated before or after the asteroid hit. “I would put this down as an entry in the debate,” Dr. Smith said. “The louse phylogeny adds one more piece of data to this puzzle. It says lice are old, predate the Cretaceous-Paleogene boundary, and must have been living on something.”

The issue is contentious because the fossil record suggests that placental mammals did not expand, in terms of the number of different species, 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 come to a different conclusion. 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. In this view, placental mammals evolved into distinct species before the asteroid hit of 65 million years ago, and many would have survived the mass die-out that polished off the dinosaurs.

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. “The fossil record continues to show that the origin of modern placental mammals postdates or is at the Cretaceous-Paleogene boundary,” he said. “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.

Dr. Smith said Dr. Novacek was correct in saying that there were nonplacental mammals around in the Cretaceous on which lice would doubtless have fed. But these nonplacental mammals, which included several lineages of marsupials, all became extinct, taking their parasites with them. These lice would not show up in his tree, Dr. Smith said, unless they had been able to transfer to the placental mammalian species, and most lice do not regularly switch hosts.

Dr. Smith believes that lice infested birds before mammals, in part because lice are so common on birds. Every bird family but one has lice, and there is a species of bird, the great tinamou, that harbors 18 different species of lice, perhaps because it has many different kinds of feathers, each offering a special niche. Given the ancient root of the louse family tree, it is likely that the first louse infested the feathered dinosaurs that were the birds’ ancestors, he concludes.