Volume 2, Issue 8
Boning Up On Human Evolution
Tim White, a UC Berkeley professor of Integrative Biology, is on what he calls "a planetary mission," but the planet he's exploring is Earth, albeit a very long time ago. White and an international team of scientists are digging deep into the geological record of remote Ethiopia to find clues about this planet as it existed 6 million years ago. What was the weather like? What kinds of plants thrived? What animals roamed the terrain? And, of particular interest to paleoanthropologists like White, what did our ancestors look like before evolution transformed them into us?
Fieldwork in southern sector, Main Ethiopian Rift. Left to right: Yonas Beyene, Tesfaye Yemane, Tim White, Berhane Asfaw. (photo © Gen Suwa)
"On one hand, we're explorers of lost worlds," says White, preparing to spend the fall at a bare-bones campsite in a dry, rocky rift 180 miles northeast of Addis Ababa. "On the other hand, we're forensic scientists. We don't have a lot of clues left from what happened in prehistory, so we have to assemble those clues from the fragmentary fossil record and study each one very intensively to extract as much information as we can."
White is no novice when it comes to this kind of detective work. In the early 1970s, he worked with the well-known Leakey family of anthropologists and later collaborated with Donald Johanson to link the famed Lucy skeleton in the chain of human evolution. Since that early work, White has continued to make many of the most significant strides toward completing human ancestry's family tree.
The oldest known fossil of modern humans, dating back 160,000 years. (© 2000 David L. Brill, Brill Atlanta)
For more than two decades, White has traveled back and forth to the Middle Awash region of Ethiopia where dramatic tectonic movement and rains shift the ground to reveal long-buried fossils. While the sediment of Oldevai Gorge, Lucy's burying place, is just the depth of a football field, the Middle Awash valley goes down a mile.
"The depth of the sediment there enables us to sample evolution across vast amounts of time," White says. "We're recovering and salvaging things within a geochronological context so we know their age, the most important piece of data when painting an evolutionary picture."
While White admits that the Afar rift is a tough place to spend the winter holidays, Middle Awash has treated White and his collaborators well. For example, early into a 1994 expedition, one of his graduate students spotted a palm bone on the ground that belonged to what was at the time the oldest hominid ever found.
"At the end of the scientific trail, if you look to the left instead of the right, you could miss something,"White says. "But the well-trained paleontologist always looks both ways."
Hominid fossils belonging to Ardipithecus ramidus kadabba, found in 1997-1999. The mandible of the subspecies is at upper left, the toebone is in the right upper row, and the hand holds a fragment of collar bone. (© 1999 Tim D. White \ Brill Atlanta)
Over the next few years, White's team--including his former graduate students Yohannes Haile-Selassie, now head of physical anthropology at the Cleveland Museum, and Gen Suwa, currently a professor at the University of Tokyo--delicately excavated the remains of that 4.4 million-year-old skeleton of a species since named Ardipithecus ramidus. The fossilized arms, legs, hands, feet, pelvis, skull, and teeth recently made a brief visit to Tokyo where Suwa has developed a special computed tomography (CT) scanner to enable digital reconstructions of the fragile bones.
"At the same time we're studying what we've found, we also want to continue gathering new samples," White says. "So every autumn, we head back into the field."
Between 1997 and 2000, the group excavated early hominid fossils from at least five other individuals who nearly six million years ago called the then-wooded Afar rift home. Last year, the scientists reported that newly-discovered teeth from those hominids indicate that it's a distinct species, not a subspecies of Ardipithecus ramidus as they previously believed. Dubbed Ardipithecus kadabba, it may represent the first human species to appear after the human branch on the family tree diverged from the branch leading to modern chimpanzees and humans.
Side view of the upper and lower dentition of a contemporary female common chimpanzee (left) and a comparative view of the fossil teeth from the hominid species Ardipithecus kadabba. (Tim White/UC Berkeley, courtesy Science Magazine)
This season, White and his team returned to Ethiopia where they'll continue to dig into the bottom of the rift for more fossil, geological, and paleobotanical samples. Paul Renne, directory of the Berkeley Geochronology Center, and one of his graduate students are on site to aid in dating the various layers of sediment. The group is also focusing attention on a higher spot where in 2003 they uncovered 160,000-year-old fossilized skulls, the oldest known remains of modern humans.
"There's so much knowledge we don't have that we can capture just by being there with the right people, the right tools, and at the right time," White says. "The secrets buried in the Afar keep pulling us all back to the field."