National Geographic : 2010 Jul
Wear patterns on the hominid teeth and analysis of isotopes in their enamel also suggested a gener- alized diet more tted to a woodland environment. If indeed the creature was bipedal---so far, the evi- dence was only indirect---then one of the hallowed tenets of human evolutionary science might be dead. e team would give the new hominid the name Ardipithecus ramidus. (Ardi means "ground" or " oor" in Afar, and ramid, "root.") In 1994 the team was eager to return. Nor- mally everyone's energy and nerves are spent on the rst day in the eld in the frenzied logistics of setting up camp. (White is an exacting " eld general," and woe to anyone who pitches his tent before the common area is set up or forgets to bring along a key set of maps.) But with a little light le at the end of that rst day, everyone rushed back to the outcrop. As the sun was setting, Yohannes Haile- Selassie found a hand bone not a stone's throw from where the teeth had been found the previ- ous year. e next day the team began to sieve the loose, sandy silt around the spot---more hand bones and foot bones turned up. en a sweep of the area produced a tibia. Eventually represent two arbitrary points in a single evolving lineage, with no clear dividing line between them. Below the Au. anamensis level, the view on hominid evolution in the Middle Awash goes temporarily black. e yellowish green clay we were walking through was laid down between 4.4 and 4.3 million years ago, when this part of the CAC was a lake much like Yardi. Nothing was preserved in the clay but sh. Below this sh layer, however, lay the ultimate prize. We trudged out onto a cobbled, sunbaked pan, featureless but for a rough semicircle of basalt rocks. e cairn marked the spot where, on De- cember 17, 1992, paleoanthropologist Gen Suwa of the University of Tokyo noticed an enigmatic molar peeking out of the ground. ere was just enough detail on it to reveal it was hominid. A couple of days later near the same spot, fos- sil hunter Alemayehu Asfaw found a piece of a child's jaw with a rst molar tooth. " at milk molar was like no other hominid baby tooth I'd ever seen, and I'd seen them all," White told me. "Gen and I just looked at each other. We didn't have to say anything. is was something way more primitive." e team set up a perimeter and began sweep- ing the area clean. WoldeGabriel went to work on the geology. He gured out that the hominid- bearing deposits were sandwiched between two volcanic ash layers, the Gàala ("camel") tu be- low and the Daam Aatu ("baboon") tu above. e dates of these tu s proved indistinguish- able---4.4 million years for both. is meant the volcanic eruptions had captured between them a focused lens of time---perhaps as little as a thousand years. And everywhere the deposits outcropped along a ve-and-a-half-mile arc, there were fossils---monkeys, antelope, rhinos, bears, birds, insects, fossilized wood, and other plant parts, even fossil dung beetle brood balls. ey called the place Aramis, the Afar name for a nearby dry streambed. "At this place, at that time, all the conditions were met," said White, spreading his arms out wide. "Everything was good." e next year the team began exploring an Aramis exposure less than a mile to the west. More hominid fossils turned up---an unworn upper canine; a pearly, eye-catching molar; more teeth; then an arm bone. But even more impor- tant than the hominid bones was the slam dunk evidence for the ecological context the creature THE FIRST INKLING WAS AN ENIGMATIC MOLAR WITH JUST ENOUGH DETAIL TO REVEAL IT WAS HOMINID. had inhabited. For almost a century scientists had assumed that our ancestors rst began walk- ing on two legs when they moved out of the for- est, where our relatives the apes still live, and onto open grasslands---to move more e ectively across long distances, perhaps, or to see above savanna grass. But an overwhelming percentage of the mammal bones at Aramis belonged to woodland-dwelling monkeys and antelope.