National Geographic : 2003 Apr
Evidence that mammals evolved inthe Southern Hemisphere came inthe form of jawbones and teeth. perhaps turning the Sherwin-Williams world upside down. On another front, geneticists comparing the genes of living mammals have found that cer tain groups thought to be very distant cous ins-hippos and whales, say-are in fact next of kin. They have also found evidence that mammals began to diversify into today's 18 liv ing orders much earlier than the fossil record shows. Fossils suggest that most modern groups appeared around 60 million years ago, after the dinosaurs were gone. Molecular data suggest they actually began diversifying about 100 mil lion years ago. "It's been a complete upheaval," says Mark Springer, an evolutionary geneticist at the Uni versity of California, Riverside. "We've come up with a very different family tree for mammals." Many paleontologists angrily reject the DNA findings, arguing there must be something wrong with the molecular clocks the geneti cists use to date their findings. The geneticists counter that paleontologists just haven't found the right fossils yet. cientists who trust the fossils and those who look to the genes agree on at least one thing: Mammals were starting to come into their own around the time of the morganucodontids. Their tiny jawbones about an inch long-show just how differ ent the mammalian form was from the giant reptile world. Their jawbones were beginning to fuse into one piece. "This is very different from reptile jaws, which are made up of several bones," says pale ontologist Rich Cifelli of the Sam Noble Okla homa Museum of Natural History. "Modern mammals' bones migrated backward to become the small bones of the middle ear. That's why mammals hear so much better than reptiles." The separation of the jaw and the ear bones 16 NATIONAL GEOGRAPHIC * APRIL 2003 allowed the skulls of later mammals to expand sideways and backward-enabling mammals to develop bigger brains. The teeth of the mor ganucodontids were another important inno vation that later mammals would improve upon. The upper and lower molars of morgan ucodontid jawbones interlocked, letting them slice their food into pieces. That released more calories and nutrients. "Reptiles don't cut up their food," says Cifelli. "They grab and gulp. But these little guys were so active they had to get every calorie they could out of what they ate. The more they could process their food in their mouths, the more energy it gave them." The jawbone also indicates that the mor ganucodontids had another important mam malian trait-they drank mother's milk. Researchers can infer that morganucodontids nurtured their young with mammary glands that produced milk because, like all mammals today, this animal had only one set of per manent teeth-as its jawbones show. This is in contrast to the growth pattern of reptiles, non milk-drinkers, which must continually replace teeth their entire lives. Scientists believe mammary glands began as sweat glands at the bases of hairs. Both sweat glands and mammary glands produce water, salts, and proteins, all of which a new born needs to survive. The duck-billed platypus of Australia gives us a glimpse of how those primitive mam mary glands worked. The platypus and the spiny anteater are the only surviving examples of a mammalian subgroup called monotremes. "The platypus female doesn't have nipples," explains Peter Temple-Smith, a platypus spe cialist at the Melbourne Zoo. "Rather, there is a region where milk ducts come together and Multi-tasking Molar It slices, it grinds, it pulverizes. The revolu tionary tribosphenic molar (right, from an opossum) may have evolved independently in the Northern and Southern Hemispheres 167 million years ago. It led to improved digestion and nutrient absorption in early mammals. Diets expanded, opening new niches for mammals among the dinosaurs.