| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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horizons Evolutionary Medicine Swimming Sloths Discovered Never underestimate the initiative of a sloth. That's one lesson to draw from a fossil discovery made in the coastal Peruvian desert. From an early Pliocene site called Sud- Sacaco, among the skeletal remains of seals, dolphins, penguins, and crocodiles, comes the first evidence of a swimming sloth. The animal is so unusual that it has been placed in a new genus and species, Thalassocnus natans. Paleontologists Christian de Muizon, director of the French Institute of Andean Studies in Lima, Peru, and Greg McDonald, of Hagerman Fossil Beds National Monument in Idaho, described the aquatic sloth, which grew as big as a grizzly bear, last May in the journal Nature. "I was totally surprised," says McDonald of the find. "We've been joking that our next trip will be to find the flying sloth." It's not the first time that a strange creature has been unearthed from this site southwest of Lima. Previously, de Muizon, a marine mammal specialist, found a small, tusked whale that resembled a narwhal, except that instead of a unicorn-like tusk projecting forward, the whale's two tusks pointed downward like those of a walrus. De Muizon has worked at Sub-Sacaco since 1976 and first recovered fossils of the sloth in 1977. It seemed strange that a terrestrial mammal would turn up on the shore of what was once a shallow bay and be more common there than some of the marine mammals. So in the early 1980s de Muizon began collaborating with McDonald, an expert on extinct ground sloths. The unique shape of the cheek bones gave away this animal as a sloth, but McDonald noticed some strange features. "The premaxilla is the thing that catches your attention immediately," he says. This bone at the front of the upper jaw is stretched forward and bent slightly downward. Other parts of the skeleton appeared both familiar and bizarre to McDonald. "You look at it and say, 'Yes it's obviously in this group [of ground sloths] but it doesn't fit the usual pattern.'" As McDonald picked out the anatomical peculiarities, de Muizon began to recognize a faint pattern similar to other marine mammals. For instance, the tibia, or shin bone, of ground sloths is always much shorter than the femur, but in Thalassocnus the tibia is almost as long as the femur. That suggests a shift-in-progress to aquatic life. Marine mammals have longer tibias than femurs, which enables their hind limbs to make powerful swim strokes. But the sloth's hind feet show no specific adaptations to aquatic life, such as longer digits. In the feet, McDonald says, "You still see the imprint of its terrestrial origin." The femur also changed compared to other ground sloths. The insertion points for the new sloth's gluteal muscles are closer to the hip joint, allowing a greater range of movement in the hind limb. The gluteal muscles of ground sloths helped them sit or stand while feeding from trees, but the apparently smaller gluteals and quadriceps in the new sloth suggest that it rarely adopted this position. Among other anatomical differences, the kneecap, which is expanded in ground sloths for extra muscle attachment space, is reduced in the aquatic sloth. Finally, the tail vertebrae of Thalassocnus have split bony projections on the sides as found in aquatic mammals and those with prehensile tails. This suggests that the sloth could move its tail up and down, perhaps to aid in propelling or supporting the body while it ate. The protruding premaxilla that first caught McDonald's eye has an expanded tip. This may have supported a strong, fleshy lip with which Thalassocnus grazed seagrass or kelp, much like manatees and their sirenian relatives. On land in this dry desert, there would have been little for an herbivorous sloth to eat. So judging from the skeletons themselves and the harsh environment, the paleontologists concluded that Thalassocnus had been partly--perhaps fully--aquatic and had used its modified hind limbs to push or pull itself through the water. A possible alternative is that these actually were ground sloths that had washed out to sea as carcasses, but their abundance and complete preservation make that explanation unlikely. Today, only five species of tree sloths exist, all specialized for spending their lives upside down. But an array of ground sloths, as small as dogs and as large as elephants, once inhabited the Americas and Caribbean islands. Four waves of sloth migrations northward from South America began about nine million years ago, before the Panamanian land bridge existed. Even though ground sloths must have island-hopped or swam across water before the bridge formed, no one had considered that a sloth might become adapted to life in the sea. McDonald says that Thalassocnus is a close relative of the small North American ground sloth Nothrotheriops, whose remains have been found in the La Brea Tar Pits and in caves throughout the Southwest, where even mummified sloths and dung have been preserved. Nothrotheriops entered North America during the last round of sloth dispersal about 1.9 million years ago and only went extinct around 11,000 years ago. Fossils of the Peruvian sloth occur throughout the geologic sequence, which ranges in age from almost ten million years to just under four million years. When Thalassocnus became extinct remains a mystery. Had it survived, McDonald thinks that this sloth would have developed more aquatic adaptations, but it died out before becoming an agile or strong swimmer. "I think we're looking at an animal that was doing the best it could to be aquatic considering that it started out as a terrestrial sloth," he concludes. "I can't visualize a sloth with that sleek movement through the water like an otter. It was still being a sloth, but in a wet environment." Evolutionary Medicine Feeling feverish? Wait a minute before you run to the medicine cabinet. Popping pills may prolong or worsen your illness. Whether or not you decide to feed a fever, you may be better off sweating it out. So says the emerging field of Darwinian medicine, which aims to understand the deeper causes of disease and why our species suffers from certain sicknesses. "'Take two aspirin and call me in the morning' is probably, in most cases, bad advice," asserts Paul Turke, an anthropologist and medical researcher. He's also a fourth- year medical student at Michigan State University who plans to apply his knowledge of evolutionary theory to clinical research and practice. But Turke is an exception. There may be only a score of active Darwinians out there in the medical research world, according to psychiatrist Randolph Nesse of the University of Michigan Medical School. Nesse is a leading proponent of the new field and co-author with evolutionary biologist George Williams of Why We Get Sick (Times Books, 1994), which lays out the case for applying the central theory of biology to medicine. Nesse wants doctors and patients to ponder possible evolutionary causes for physical and psychological illness. "All we've done is say the vulnerability to disease should be explained in the same way as every other trait." So the book offers explanations, culled from the work of several researchers, for everything from aging, allergy, cancer, and gout to myopia, premature ejaculation, and strep throat. Want to know why kids tend to hate vegetables? There's a potential answer buried in our evolutionary past: our Stone Age ancestors may have wisely avoided plants that naturally contained toxic chemicals. Children would be especially susceptible. And despite millennia of cultivation, toxins persist in many plants and other foods. Toxic chemicals in foods, metals, and other parts of our environment may also be the cause of allergies, another new hypothesis developed by Darwinian medicine. Most illness, argue the Darwinians, can be placed into one of these categories: defenses (such as a cough or allergy), infections (colds, malaria), new environments (heart disease, some cancers), genetic quirks (myopia, sickle cell anemia), design compromises (lower back pain, osteoporosis), and historical legacy (appendicitis, detached retina). The novelty of our civilized, urban environments and lifestyle--radically different from our hunter-gatherer past--contributes to lots of medical problems, from orthodontia to breast cancer. Write Nesse and Williams, "Natural selection has not had time to revise our bodies for coping with fatty diets, automobiles, drugs, artificial lights and central heating. >From this mismatch between our design and our environment arises much, perhaps most, preventable modern disease." Now about that fever. Because a diverse range of distantly related animals can self-induce fever, it must have an ancient origin and probably provides some evolutionary edge; if not, natural selection would have discarded it long ago. Evidence suggests that the higher body temperature brought by fever is actually an evolved defense that boosts our immune system to combat infection by bacteria. Studies of lizards showed that preventing infected iguanas from raising their body temperature to fever levels increased the odds of death. Giving infected iguanas an aspirin-like pain reliever, which depresses body temperature, also increased mortality. Other animal studies found similar results. But whether aspirin poses any threat to feverish humans, or whether taking it prolongs our suffering, has yet to be tested. In a review of Nesse and William's book for Nature last July, anthropologist Melvin Konner wrote, "It is high time Hippocrates gave Darwin his due." But Darwinian doctors haven't made much headway against the inertia of mainstream medicine. Even so, their ideas deserve serious thought and should stimulate some fascinating debates. "We're at the idea stage," says Turke. "Medicine needs ideas now as much as it needs methods."
Blake Edgar is Associate Editor of California Wild. |
Fall 1995
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