The main argument for the rabbit-rodent connection is their similar sets of gnawing teeth (though rabbits have an extra pair of incisors). The main argument against it is that distinct rabbit and rodent fossils go back about 60 million years.
When two different groups of animals have descended from a common ancestor, the earlier fossil versions generally look more alike than the later ones. But with rabbits and rodents, the reverse is true: earlier versions look less alike than the later ones. This implies that the dental similarities we see now arose by “convergence,” by adaptation to similar diets rather than by direct genetic inheritance.
During recent decades, molecular comparisons have helped to sort out relationships among animals whose morphology is confusing. Human anatomy, for instance, doesn’t tell us which primate is our closest evolutionary relative, and every known ape and monkey had been suggested by one expert or another. In the past few years, DNA comparisons have shown decisively that we’re most closely related to chimpanzees.
DNA molecules make up the genes, the hereditary material, and protein molecules are programmed by the genes. All the anatomical and developmental features of an animal are determined by these molecules, which do not “converge” the way tooth shape and overall appearance may do. On the contrary, all the available evidence confirms that the longer two species have been diverging from a common ancestor, the more different their DNA and protein molecules are. The more closely related two species are, the more nearly identical their molecules.
In January, Dan Graur of Tel Aviv University, and Laurent Duret and Manolo Gouy of Claude Bernard University in Lyon, wrote an article in Nature, “Phylogenetic position of the order Lagomorpha (rabbits, hares, and allies).” They analyzed 88 different protein sequences and compared rabbits with virtually all other groups of mammals, including rodents, ungulates, whales, carnivores, hyraxes, bats, and primates. Using sophisticated computer programs, they ran thousands of family trees to determine which ones mathematically were most likely to be correct.
The first thing they found was that the rabbit-rodent connection doesn’t hold up. This family tree came out with a very low statistical probability. They concluded that the popular Glires cohort, putting lagomorphs and rodents together in one Easter basket, simply doesn’t work. Look-alike teeth don’t necessarily signal a true affinity. Surprisingly, the computer places whales, cattle, cats, and dogs, whose teeth are wildly different from those of gnawers, closer to rabbits than the rodents are.
Well then, which mammalian order gets the lucky rabbit foot? I never would have guessed, and neither did the hundreds of other eggspurts who have contemplated the Lagomorpha–and written learned dissertations to immortalize their mistakes.
The answer is… by heavy odds, rabbits come out most closely related to the primates!