Ever since Linnaeus sorted all known living things into families, genera, and species--in other words, ever since the eighteenth century--rabbits have been classified as rodents. The classification seemed only natural. Like mice, rats, guinea pigs, and other rodents, rabbits have large, gnawing incisors. But early last year a zoologist found genetic evidence that rabbits may in fact be more closely related to primates, including humans, than to rodents.
Dan Graur of Tel Aviv University bases his surprising claim on a study of genetic mutations, which produce changes in the amino acid sequence of the protein a gene codes for, and which are assumed to accumulate at a fairly steady rate. Thus the more amino acid differences you find in the same protein taken from two different animal species, the more time must have elapsed since the two diverged from a common ancestor.
Graur compared amino acid sequences in 91 proteins from humans, monkeys, rats, rabbits, and other mammals. He found that, on average, the proteins from rabbits differed from the same proteins in rats by almost 13 percent, while proteins from rabbits and primates differed by less than 10 percent. Although it’s difficult to assign an exact timescale to the relationships among the groups of mammals, Graur’s results imply that rabbits and primates separated only after their common lineage had diverged from that of rats and mice. Thus rodents, Graur thinks, may not be a natural grouping at all, Linnaeus notwithstanding. Most people have assumed that all rodents evolved relatively recently, along with the other mammals, says Graur. I think that the order is going to be fragmented into many different lineages.