Los Alamos geologist Giday WoldeGabriel has left his footprints across miles of Ethiopia’s Afar Rift, establishing the temporal and spatial contexts for amazing fossil finds. One of those finds, “Ardi,” Ardipithecus ramidus, made headlines in late 2009 as the world’s oldest hominid skeleton and the source of new information about human origins and evolution.
Ethiopian-born geologist Giday WoldeGabriel, a staff member at Los Alamos National Laboratory, leads a double professional life.
As a member of the Earth and Environmental Sciences Division,WoldeGabriel is the Laboratory’s resident expert in the geology of rifts—places where Earth’s shell is being stretched and thinned, as it is in the Rio Grande Rift, where Los Alamos makes its home in northern New Mexico. He uses geochronology and geochemistry to help the Laboratory build a threedimensional geological model of the formations that underlie Los Alamos, its immediate surroundings on the Pajarito Plateau and the entire Española Basin.
The distant tents under acacia trees are home for the Middle Awash team during its field sessions in the Afar Rift. The rift is the year-round home of the seminomadic Afar people (foreground), who herd goats and cattle.
Simultaneously, as co-leader and lead geologist of the world-renowned international Middle Awash paleoanthropological research team, he annually takes his skills to Ethiopia’s Afar Rift. There WoldeGabriel’s geological field surveys have led the team to some of the world’s most exciting hominid finds, including the nearly complete skeleton of an ancient lady named “Ardi,” Ardipithecus ramidus.
The Middle Awash team discovered Ardi in 1994 in its study area, a 35-by-45-mile tract around Ethiopia’s Awash River. She became a worldwide sensation on October 2, 2009, when team members published the results of their 15-year study of her in a special issue of Science. On December 18, 2009, Science declared her to be “Science Breakthrough of the Year.” Time magazine followed suit, naming her 2009’s top scientific discovery.
What’s so special about Ardi? A nearly complete hominid skeleton is an extreme rarity, and Ardi is the oldest one yet found, 4.4 million years old. She’s also the most significant skeleton since 1974’s 3.2-million-yearold Lucy, Australopithecus afarensis. Her significance stems from more than her age. Her skeleton reveals surprising information about how she looked and moved, changing our view of human evolution.
Scientists long assumed that the ancestor we share with chimpanzees (thought to have existed around 8 million years ago) would be like a chimpanzee, right down to knuckle walking: the body’s weight resting forward on the knuckles of the hands. Ardi has ended that notion. The bones of her feet and pelvis show she was bipedal, an upright walker, and her hands were not adapted for knuckle walking. No one expected these characteristics in a hominid so close in time to the common ancestor. Project members now believe that knuckle walking evolved separately in apes after hominids branched off. It’s a radically new theory.
Ardi is one of an unparalleled number of prestigious finds for the Middle Awash team. Members have found seven of the world’s known hominid species in their study area. Three of those, Australopithecus garhi, Ardipithecus ramidus, and Ardipithecus kadabba were first-time discoveries, yet to be found anywhere else. The team’s hominid discoveries have been featured often on the covers of Science and Nature.
Such acclaim is born of the team’s meticulous care at retrieving each fossil and piecing together its story—a story told not just by bones, but also by the ground that held them. For 18 years, WoldeGabriel has made accurate storytelling possible through a unique geological study of the Middle Awash that incorporates satellite and aerial photos, on-the-ground field work, and high-tech laboratory analysis of collected rock samples.
He has deciphered the land’s complex code to produce a detailed geologic picture that another of the team’s leaders, Tim White (University of California, Berkeley) calls “the project’s bedrock.” It enables the team to keep finding promising sites time after time. Integrated with paleontology, archaeology, and studies of the paleoenvironment, it becomes a critical part of a superior, multidisciplinary approach that makes all the difference in the team’s ability to place fossils accurately in time. Other explorers made the area’s first hominid find in the 1970s, an “archaic” Homo sapiens cranium, but they lacked the best geologic data and so misdated the find. WoldeGabriel’s studies redated that hominid to 600,000 years, almost twice the initial perception of its age.
WoldeGabriel joined the Middle Awash team in 1992, becoming one of the team’s more than 70 scientists from 19 different countries. He shares team leadership with White, a paleontologist; Berhane Asfaw, also a paleontologist, from Ethiopia’s Rift Valley Research Service; and Yonas Beyene, archeologist, from Ethiopia’s Authority for Research and Conservation of the Cultural Heritage. The team operates under a permit from the Ethiopian government.
A Rip in the Earth’s Crust
The Middle Awash study area sits in the southwestern portion of the Afar Depression, a sunken area in the Horn of Africa. A sizzling desert, the Afar Depression is at the northern end of the fossil-famous East African Rift (the Great Rift Valley) and so is known to geologists as the Afar Rift. Intermittently active volcanoes mark the horizon, and cracks and fissures split the ground, annually widening the rift an eighth of an inch—sometimes more. In September 2005 a volcanic eruption pushed magma close to the surface in a huge, ground-splitting wedge that, in one week, opened a new fissure 38 miles long and 26 feet wide.
WoldeGabriel explains that the volcanoes are a sign of gargantuan movements deep underground: magma rising and shouldering aside tectonic plates. The magma’s inexorable push created the East African Rift and is particularly pronounced in the Afar Depression, where the Afar Rift meets rifts at the bottom of the Red Sea and the Gulf of Aden (see “The Tectonics of a Fossil Repository,” above). Water from one or both of these will eventually spill down the East African Rift, separating the bulk of Africa from its eastern side.
Meanwhile, the Afar Rift and, within it, the Middle Awash study area are rich fossil-hunting ground, and not coincidentally. “If it weren’t for the tectonics and the resulting rifts,” says WoldeGabriel, “we wouldn’t be there.”
As the rift opened, the land within it subsided and the bordering highlands regulated its climate, making it an ideal place for life to proliferate. At the same time, sediments washed in and covered the bodies of dead animals; then vast layers of ash from periodic volcanic eruptions locked down the sediments and the bones they contained. Subsidence, sedimentation, and volcanic eruption—the pattern repeated itself over and over, creating a multitude of deeply buried, alternating sedimentary and volcanic layers. (See “The Tectonics of a Fossil Repository,” p. 13.)
As the land sank lower—more than 2,000 feet lower today than in Ardi’s time—the surrounding high ground held in more and more heat until what had been cool, wet forest became today’s hot, dry desert. At the same time, tectonics pushed blocks of the buried layers upward. These now exist as discontinuous ridges, weathered into undulating hills, their layers revealed only where a fault has made a vertical break or where seasonal streams have sliced into a hillside.
Fossilization is a chance occurrence, so only onequarter of the sedimentary layers now exposed in the Middle Awash contain fossils, but those that do are a treasure trove. “It’s the most prolific paleoanthropological area ever discovered,” says WoldeGabriel, “with the longest hominid record yet available.”
The Middle Awash team has recovered about 20,000 vertebrate fossils, including hominids, from more than 200 different sites. The hominids, 160,000 years to 5.8 million years old, were found in 13 different layers.
Leading the search for fossil-bearing sediments is part of WoldeGabriel’s job. The search begins from a distance—and from a great height. Satellite photos, Google Earth images, and aerial shots from airplanes reveal locations the team may want to visit.
“It’s important to discriminate,” says WoldeGabriel, “before you climb the next ridge into the next valley.” Because, in the end, the hard work is done on foot. “You have to physically walk the site to be sure the things are what you think they are.”
Excavating Ardi’s fragile bones required infinite patience and the utmost delicacy. Here her lower jaw is revealed with the use of a dental tool. Many bones had to be treated with a stabilizing solution.
Geologists call data gathered on the spot “ground truth,” and the truth the team needs is the difference between sediments, something that has to be learned up close. Lacustrine sediments, laid down in standing water, yield aquatic fossils—crocodiles, hippos, and fish—but contain no hominids. For those, you need the fluvial sediments of streams and rivers, deposited in floodplains along deltas and lake margins.
The difference reveals itself in grain size. Fluvial sediments are generally coarser than lacustrine, and grain size diminishes with slope gradient. The largest materials, rocks and gravel, are moved by high-energy water on steep ground and are deposited where the ground levels out. There they solidify into conglomerate, a rock composed of tumbled stone fragments. Conglomerate is a signal to WoldeGabriel.
“It means you’ve found the ideal fossil environment,” he says, “not in the conglomerate itself—the big rocks break up animal remains—but in the layers farther downstream on a flatter plane.” The stones in conglomerate show the way because their tips often overlap and point in the direction of flow. On a mild slope, like a lake’s edge or a floodplain, water’s sedimentary load is reduced in both volume and grain size and becomes a gentle covering for animal remains. It’s there that fossils may be found.
The pieces of Ardi’s skeleton were found where she died, at a single site. Each flag marks a fragment’s position. The local Afar people enjoy watching and are sometimes paid to help with general labor. Team leaders’ careful diplomacy with these independent people wins the team access to fossil sites.
Ardi was found in an ancient floodplain at Aramis (named for a nearby village) in the Central Awash Complex (CAC), west of the Awash River.
A broad 8-by-10-mile dome, the CAC is the hardened heart of a group of extinct volcanoes, now weathering 100 to 400 feet above its surroundings and wearing slabs of the rocks it displaced. Those slabs contain millions of years of rock layers, still mostly buried but, as WoldeGabriel suspected, exposed where erosion and faulting have cut into the uplifted ground. When
If stacked in a continuous column, the Aramis layers would be about 1,000 feet high, but not all the layers show at every site; despite some overlap, each site is a slightly In such a complex landscape, WoldeGabriel couldn’t chance a mistake in pinpointing the exact tuffs marking Ardi’s sediments.
The tuff layers bracketing the Ardipithecus ramidus sediments are marked here with dotted lines. The tuff layers are almost contemporaneous, the top one being 4.416 million years old, plus or minus 0.031 million years, and the bottom one 4.419 million years old, plus or minus 0.068 million years. The sediments between them were deposited over only a few thousand years and are 18 to 20 feet thick in places but nearly nonexistent in others, where erosion washed them away before deposition of the top tuff layer.
So he identified and collected samples pristine enough to be chemically fingerprinted for their unique percentage of common elements: silicon, titanium, iron, aluminum, magnesium, manganese, calcium, potassium, sodium, and phosphorus. The specific percentage mix allows him to correlate all of a single tuff’s exposures, wherever they appear.
WoldeGabriel labeled each sample with an exact GPS reading and, back in Los Alamos, had the samples sliced thin enough for light to pass through. Discrete fragments of volcanic glass in the slices were then examined with an electron microprobe to reveal the constituent elements. In addition, collaborator William Hart of Miami University in Oxford, Ohio, and his team of graduate students and postdoctoral fellows, joined WoldeGabriel in identifying the less-common trace and rare-earth elements.
The Middle Awash team visits its study area yearly, driving from Ethiopia’s capital, Addis Ababa. Visits are timed for October through January.
No End in Sight
The team was back in Ethiopia early in 2010, searching for potential new sites and scouring Aramis again for more pieces of Ardi’s species. Returning to the same ground is necessary because each rainy season reveals more fossils but just as easily washes them away. “It’s a fragile erosional system,” says White. What you miss seeing one year may be gone the next. “You get one chance to do this right.” This year the team turned up a new Ardipithecus ramidus bone that confirms bipedality.
WoldeGabriel returned briefly to the area’s oldest rocks, west of Aramis on the border of the rift, and will walk that ground again and again in the future. Years ago in this portion of the rift, he led then-graduate-student Haile-Selassie in search of a new fossil site and followed a promising sedimentary layer until Haile-Selassie made the find that eventually earned him his doctorate. He found fragments of a more-primitive example of Ardi’s genus, Ardipithecus kadabba, also bipedal and as much as 5.8 million years old.
Fossilized seeds (shown here), wood, and silica parts of plants are evidence of a long-departed forest. Ardi’s world was filled with insects, gastropods, diverse birds, and mammals, both small (shrews, mice, and bats) and large (for example, bears, rhinos, elephants, monkeys, and antelopes).
How much this earlier specimen has to tell is still unknown. So far, the fragments are sparse, but the hunt will continue. And WoldeGabriel’s practiced eye and deep knowledge of the land will remain at the center of the work.
In this issue...