National Geographic : 2003 Apr
had the same dual traits. The evolutionary tran sition among major groups of mammals is rarely illustrated so clearly. And no other dis coveries have linked fossils to DNA findings with such precision. ntil 65 million years ago dinosaurs dominated the land. The oceans swarmed with huge sharks and voracious marine reptiles. The dinosaurs and other large predators occupied the richest and most obvious evolutionary niches, keeping mammals at the margins. Then an event occurred whose scale is still hard to comprehend. An object six miles across crashed near the present-day Yucatan Penin sula, punching out a crater 110 miles across. That impact may have been one of many over the next several hundred thousand years, each adding to the destruction. But the damage done by the Yucatin impact alone is impressive: Tsu namis 500 feet high battered North America. The temperature reached 500 degrees in parts of the world. "Everything big bit it," says Kirk Johnson of the Denver Museum of Nature & Science. "The key to survival was to be small." Mammals fit that profile. They suddenly found themselves in a world without large carnivores. Restraints were off. Within 270,000 years they were diver sifying and growing bigger. Still, the majority of mammals didn't get much larger than a pig until the Eocene epoch, which began about 55 million years ago. Then a rapid increase in global temperature encour aged the spread of forests around the world even near both Poles. This abundance of rich vegetation opened yet more ecological niches for mammals to exploit. Mammal diversity soared. One of the newcomers in the fossil rec ord was our own order, the primates. The earliest primates belonged to the lemur branch. Today lemurs are confined to the island of Madagascar, where one species made it from Africa perhaps 50 million years ago, probably on rafts of storm-tossed debris. A few million years later, more advanced primates appear in the fossil record of eastern Asia. These higher primates are anthropoids monkeys, apes, and humans. Chris Beard, a specialist in primate origins at the Carnegie Museum of Natural History, has unearthed 30 NATIONAL GEOGRAPHIC * APRIL 2003 in China what may be the earliest known example, called Eosimias. These creatures evolved in the mid-Eocene as the world was cooling and concentrated in the midlatitudes where forests remained lush. Beard says they "must have been frenetic little animals. Kind of caffeinated. They prob ably ate all the time. When you are that small, you have to. They probably lived in troops and maybe never left the tree they were born in." Despite its primitive anatomy, Eosimias had already adopted the monkeylike habit of walk ing along the tops of branches rather than leap ing from tree to tree like earlier primates. About 34 million years ago smarter, bigger, and more aggressive monkeys evolved. Fossils from the Faiyfm Depression, where Elwyn Simons of Duke University has led a dig since 1961, reveal how anthropoids were changing. Catopithecus,one of many anthropoids his team has uncovered, has a skull the size of a small monkey's, a relatively flat face, and a bony enclosure at the rear of its eye sockets. It is the first anthropoid to show the same arrange ment of teeth humans have-two incisors, one canine, two premolars, and three molars leading Simons to argue, "This is the first chap ter of human history." At the start of the long Miocene epoch-23.5 million to 5.3 million years ago-yet another major climate change occurred. The world was warming again and more seasonal climate pat terns may have emerged. At higher latitudes, forests gradually gave way in many places to grassland meadows and savannas. Because grass is abrasive, some mammals developed new den tition. Horses, for instance, emerged as little leaf-eaters in the forests but later developed molars that are much better adapted to eating grass. Horses' crowns extend into the jawbones. As the crown gets ground down, new crown will emerge from the jaw to replace it. Early in the Miocene, Africa's long isolation ended when it and Arabia came back into con tact with Eurasia. That's when the ancestors of many mammals we think of as native to Africa arrived there. First came the ancestors of ante lope, cats, giraffes, and rhinos. Later, around ten million years ago, North American mammals camels, horses, and dogs-began to arrive. Almost every animal that roams the Serengeti today is a relative newcomer to the continent.