National Geographic : 2003 Apr
Mammals suddenly found themselves inaworld without large carnivores. Restraints were off. but also a strange assortment of hyraxes. Today hyraxes resemble guinea pigs. But 35 million years ago hyraxes took many forms. Some were the size of rhinoceroses; others had long legs like gazelles. Most mammals on the African ark began to disappear around 20 million years ago, after Africa came into contact with the rest of the world again. But Africa wasn't the only ark. An ancient seaway split South America from Eurasia and North America for millions of years, and South America became home to what geneticist Springer calls xenarthrans, another group of placental mammals. South America's fossil record during its isolation is far better than Africa's, and includes such xenarthrans as sloths, armadillos, and anteaters. Springer's data, in other words, indicate that the most recent common ancestor of placental mammals is Gondwanan. Contrary to more than a century of northern chauvinism, the northern continents have the youngest placental mam mals. One group, the laurasiatheres, includes seals, cows, horses, whales, and hedgehogs. The other group, euarchontoglires, includes rodents, tree shrews, monkeys, and humans. These genetic findings reveal more than simply which came first. They also redefine relationships among placental mammals. For one, anatomists have always assumed that bats were in the same superorder as tree shrews, fly ing lemurs, and primates. But genetic data place bats with pigs, cows, cats, horses, and whales. The data further show that these superorders of living mammals started to diversify much earlier than the fossil record suggests. What gets fossilized is a record of an animal's shape. But geneticists contend that genes in an organ ism's mitochondria, the parts of a cell that are used to trace and date lineages, can be evolving rapidly without changing what would be left behind in the fossil record. 28 NATIONAL GEOGRAPHIC * APRIL 2003 "An animal's shape may be heavily affected by its environment," says Uljfur Arnason, a geneticist at Sweden's University of Lund. "Crocodiles haven't changed much physically in 250 million years, yet they have a high rate of change in their mitochondrial DNA. Birds have a slow rate, yet they can evolve physically very rapidly." However surprising the claims of geneticists seem at first, paleontologists and DNA researchers are finding that their theories can be complementary. Some stunning new fossils have confirmed a previously controver sial DNA finding about whales. Most pale ontologists long believed that whales and dolphins-or cetaceans-descended from an extinct line of carnivorous mammals that for unknown reasons became aquatic between 50 and 45 million years ago. At the time of these fossils' discovery, molec ular biologists were maintaining that new DNA work indicated the cetaceans were actually aligned closely with artiodactyls, an order that includes even-toed ungulates such as pigs, camels, deer, and hippopotamuses. Paleontologists first dismissed this unlikely connection because nothing in the fossil record supported it. Then in September 2001 two teams of fossil hunters published finds that backed up the claims of the biologists. A group led by Hans Thewissen of Northeastern Ohio Universities College of Medicine found two species of the earliest known whales in 50-million-year-old deposits in Pakistan. Both had ear bones unique to whales, but the legs and anklebones of artio dactyls. "The first whales, it turns out, were ful ly terrestrial and good runners," Thewissen says. Almost simultaneously, a group from the University of Michigan led by Philip Gingerich announced similar fossils from Pakistan that Fruits of Their Labor It was the perfect partnership. Megabeasts like the giant sloth (right) may have dis persed big-seeded plants back in the late Pleistocene, some 15,000 years ago. A seed hidden in slippery fruit is easily swallowed whole and later passed, nicely fertilized. These big-seed sowers are now extinct, so humans do the job with trowel or plow.