| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Counterpoints in science The Bunny Club Everybody knows what a rabbit is--a distinctive furry mammal with long ears, a twitchy nose, a short tail, and extensible hind legs admirably adapted for jumping. To evolutionary biologists, however, the rabbit is a puzzle, its relationship to other mammals a subject of endless debate. In 1957, Albert E. Wood reviewed the many conflicting theories in an article titled, "What, if anything, is a rabbit?" The mammalian order Lagomorpha includes rabbits, hares, and pikas. Pikas are small, tailless, and well adapted to cold climates. The Old Testament classified rabbits as ruminants, relatives of cattle and camels. In the vulgate Latin translation of the Bible, rabbits were equated to hyraxes. Their resemblance to these small squeaking African mammals is so strong that hyraxes are sometimes known as "rock rabbits." In fact, though, much anatomical and biochemical evidence shows that hyraxes are more closely related to elephants and sea cows than to lagomorphs. Linnaeus, the Swedish botanist who created the biological classification system we use today, thought that lagomorphs were rodents, like rats, beavers, and squirrels. Various versions of this idea have remained popular up to the present day. George Gaylord Simpson, the most celebrated mammalian systematist of this century, lumped lagomorphs and rodents together in a supergroup called Glires, which he admitted was "permitted by our ignorance rather than sustained by our knowledge." 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. Rabbits, of course, are famous for their reproductive prowess, producing as many as seven large litters per year. Male rabbits are unique among placental mammals in having their scrotum and testicles in front of the penis rather than behind it. Taxonomists love to use special features like this one as the basis for their deductions. Marsupials such as kangaroos also have this scrotum-forward arrangement of their genital equipment. Kangaroos do rather resemble giant bunnies, with their antenna ears, chisel teeth, and leaping locomotion. But most systematists would agree that all placental mammals, including rabbits, are more closely related to each other than any of them is to marsupials. Regardless of whether their cojones are fore or aft, all marsupials lack placentas, a much more fundamental distinction. Marsupials have been around for at least 130 million years, while the 16 or so orders of placental mammals arose later and started to diversify about 65 million years ago, after the demise of the dinosaurs. This game of phylogenetic jack-straws, with its jumbled heaps of anatomical traits, have not helped us much to understand what, if anything, a rabbit is. 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! Bunnies are us! The only other mammalian order that competes in molecular kinship with the primates is Scandentia, the tree shrews, which are generally considered to be primitive relatives of the primates. Nobody, except possibly Hugh Hefner, has previously made out any convincing physical resemblance between ground-living, leaf-nibbling, hopping, wall-eyed rabbits; and tree-climbing, big-brained, omnivorous primates with stereoscopic vision. Nevertheless, the message of the molecules is that way back in the Paleocene epoch 65 million years ago, after something, probably a big asteroid, wiped out the dinosaurs, some strange critter emerged from the wreckage and became the common ancestor of pottos and pikas. We don't yet know what that forebear (or forehare) looked like, but paleontologists will be taking another look at the fossil record to see what chimerical candidates may be lurking there. Perhaps folk mythology, children's stories, and cartoons show an insight that scientists lack when they attribute human emotions and intelligence to characters like Peter Cottontail, Br'er Rabbit, Bugs Bunny, and the White Rabbit and March Hare of Alice in Wonderland. Even if real bunnies can't talk, their molecules have a lot to tell us about their mysterious origins, and about ours.
Jerold M. Lowenstein is professor of medicine at the University of California, San Francisco. jlowen@itsa.ucsf.edu |
Summer 1996
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