Berkeley -- A new species of human ancestor and the earliest traces of animal butchery have been discovered in Ethiopia. Both finds date to 2.5 million years ago.

In this week's issue of Science (April 23, 1999) Ethiopian anthropologist Dr. Berhane Asfaw and colleagues from the United States and Japan describe fossils of what may be a direct human ancestor and an evolutionary link between the ape-man of Africa, Australopithecus, and the genus Homo.

In a companion paper other members of the research team, including well known University of California, Berkeley, archaeologist J. Desmond Clark, announce evidence that antelopes and horses were butchered with the world's earliest stone tools.

The discoveries, made between 1996 and last December, come from the Middle Awash study area in the Afar desert of Ethiopia. The new fossils were dated by the argon-argon radioisotopic method.

The discoveries refocus attention on the eastern part of Africa, a crucible of human evolution, according to one of the team leaders, Tim White, a professor of integrative biology at UC Berkeley. In 1992, the same research team found Ardipithecus, the earliest known hominid, at the nearby Middle Awash site of Aramis.

The scientists say that the new discoveries are important for three reasons. First, they add a new potential ancestor to the human family tree. They also show that the thigh bone (femur) had elongated by 2.5 million years ago, a million years before the forearm shortened, to create our familiar human proportions.

Finally, evidence of large mammal butchery shows that the earliest stone technologies were aimed at getting meat and marrow from large mammals. This signals a dietary revolution that may have eventually paved the way toward an invasion of new habitats and continents.

"We cannot yet conclusively link the new species with the butchery or the more modern limb proportions," White said. (To view video footage of the Afar region of Ethiopia and an interview with Tim White, link to a web site at http://media2.bmrc.berkeley.edu/projects/public_affairs/white/.)

Background

Theories abound in the contentious field of human origins research, but a basic outline has emerged from genetic and anatomical studies of modern apes and humans. Fossils found over the last 75 years in Africa have validated and extended this picture (Figure 1).

The evolutionary lines leading to humans and the African chimpanzees and gorillas split more than five million years ago. Research in Africa since the 1920s has identified several successive fossil species leading to humans. As more fossils have been found, the family tree has become more complex, sprouting several side branches including the robust Australopithecus "ape-men" of southern and eastern Africa.

The earliest known fossil hominids belong to the genus Ardipithecus and date to 4.4 million years ago. The descendent ape-man genus Australopithecus was first recognized in South Africa in the 1920s. Several species are now known. Some, like A. aethiopicus (the "black skull" at 2.6 million years ago), A. robustus (1.8 million years ago) and A. boisei (2.3-1.3 million years ago) specialized and went extinct. Other earlier forms like the "Lucy" species Australopithecus afarensis (3.6-2.9 million years ago) and its precursor Australopithecus anamensis (3.9-4.2 million years ago) were less specialized and therefore suitable ancestors of the earliest generally recognized species in our own genus, Homo.

That species, Homo habilis, named by Louis Leakey and colleagues in the early 1960s, is widely recognized as the maker of the earliest stone tools from the bottom of Olduvai Gorge in Tanzania. These deposits are dated to 1.8 million years ago.

Some researchers think that "Homo habilis" fossils actually represent two different species. But the relationships between the earlier ape-men (particularly A. africanus) and their stone-tool-wielding early Homo descendants have been impossible to decipher because of the lack of an adequate east African fossil record between 2 million and 3 million years ago.

The new Ethiopian fossils reported this week help fill this gap. According to Asfaw, the Ethiopian co-leader of the team, they are certain to stimulate new debate and additional research. But they also complicate our family tree.

The discoveries: Where?

The discoveries are the latest in a series from the Afar depression of eastern Ethiopia where three great rift valleys intersect in the Horn of Africa. In recent years, the Middle Awash research area has become a focal point for studies of human origins and evolution, surpassing such renowned sites as Tanzania's Olduvai Gorge in both the number of ancestor fossils found and the time span of the deposits.

Project co-leader Tim White of UC Berkeley says "With the publication of these new results, and with a record spanning five million years, Ethiopia's Middle Awash has become the world's most important single site for studying human origins and evolution."

The ancient environment

Today, the Middle Awash is a harsh desert -- a dry, remote land inhabited by the Afar people. But through millions of years of human evolution its landscapes saw dramatic changes as a result of earth movements, volcanism, erosion, climate change and biological evolution.

Investigation of fossils and sediments from near the Afar village called "Bouri" (which means "dust" in the Afar language) paints a different picture of this place 2.5 million years ago. Fossils indicate that herds of grazing antelope occupied grassy plains surrounding a freshwater lake, hunted by saber-toothed cats and a variety of extinct animal species. Among them was the big-toothed creature that this international team of scientists is now saying could be the ancestor of later humans.

The discoveries: partial skeleton and cutmarks

The new fossils featured in this week's issue of Science were found near the Afar village of Bouri beginning in 1996. As with most discoveries of this type, these were made by skilled workers searching the surface of eroding sediments lain down alongside an ancient freshwater lake.

White found the first of the key fossils in 1996 -- arm and leg bones of a small hominid eroding from a low hill. Excavation at this location revealed more pieces of the poorly preserved partial skeleton, as well as antelope bone fragments bearing the telltale marks left by stone implements. The individual is estimated to have stood less than five feet (1.45 meters) tall when alive.

The scientists are uncertain whether this hominid was the tool user, or even what species the limb bones represent, since no teeth were recovered from this individual.

About 275 meters away, erosion of the same geological layer had exposed the skullcap of another hominid individual that was found by UC Berkeley graduate student Yohannes Haile-Selassie in 1997. When the scientists had carefully removed the specimen and examined it, they were shocked. It demonstrated a combination of bony and dental features completely unforeseen and unanticipated. The scientists therefore gave it a new name, Australopithecus garhi. The word "garhi" means "surprise" in the Afar language.

Pieces of six additional hominid individuals were found among the thousands of animal fossils from the 2.5-million-year-old sediments. Like the limb bones, these fragments of arm, jaw, skullcap and teeth may also belong to A. garhi.

The fossils: recovery and cleaning

A massive recovery effort was mounted because of the importance of hominid remains. The cranium had broken into many pieces when it eroded from the slope. The pieces had scattered down the steep, rocky hillside. Because of the dense carbonate encrustation covering the bones, recovery and cleaning took many months.

The team screened the loose surface material and hauled tens of thousands of pebbles, stones and fossils to the nearby Awash River for water washing. This was necessary to spot bone fragments within the encrusting matrix.

When the left half of the upper jaw was recovered, only the tops of the teeth peeked out from beneath the encasing crust. When put together, the upper jaw, or palate, was virtually complete. It took weeks to painstakingly remove the hard matrix with a miniature air-driven jackhammer (an airscribe) at the Paleoanthropology Laboratory in Addis Ababa. All Ethiopian original fossils are permanently kept there.

The geology

The geology of the Middle Awash region has been investigated in detail as part of the paleoanthropological research by a group of Ethiopian, American and European geologists led by project co-leader Giday WoldeGabriel of Los Alamos National Laboratory in New Mexico.

In a second, companion paper in this week's issue of Science, the geologists name a new stratigraphic formation, the Bouri Formation, with three subunits (the Herto, Daka and Hata Members). The fossils are from the Hata Member and come from sediments deposited along the edge of an ancient lake. Volcanic ash and lava flows from nearby and distant volcanoes are interbedded with the fossil-bearing horizons, providing spatial and temporal constraints on the geological processes and the fossils themselves.

How old?

The layer of volcanic ash just below the fossils was the product of an eruption 2.5 million years ago as determined by the argon-argon radioisotopic dating system at the Berkeley Geochronology Center. These dates were independently confirmed by analysis of the accompanying animal fossils and studies of the earth's magnetic signal trapped in the ancient sediments.

The new species and its evolutionary significance

The new hominid species Australopithecus garhi was not anticipated by the researchers.

"This cranium was a big surprise to all of us," said Asfaw, the Ethiopian lead author of the paper naming the new species.

Its teeth are larger than the "Lucy" species A. afarensis. Its braincase, face and palate are more primitive than Homo. It lacks the specialized cranial characters of the robust ape-men of eastern and southern Africa.

"The new species is most like its ancestor afarensis," said White. "The face projects forward, the braincase is crested and small, but the premolars and molars are enormous. This combination of features has never been seen before, and that's why we named a new species."

The scientists can't tell whether this species used stone tools, but the proximity of cutmarked antelope bones provides circumstantial support for this idea.

The new species joins a growing gallery of ancestors that has emerged from Africa in the last 30 years. Scientists are currently split over how many branches of early hominids existed &emdash; even researchers who agree on the number of branches arrange them differently on their family trees.

The published paper provides several different alternative family trees, but Asfaw and colleagues conclude that it is too early to choose among the various alternatives. They say that identifying A. garhi as a new species is just the first step in solving the puzzle and that more fossils will be needed to choose the most accurate tree -- and to reveal whether human evolution between 2 million and 3 million years ago was "jerky" or relatively smooth.

The scientists say it is very possible A. garhi was the direct ancestor of Homo, including modern humans.

Meat and marrow eating

Ethiopian archaeologist Sileshi Semaw (now at Indiana University) announced two years ago that he had found the world's earliest stone tools at 2.5 million years. His site of Gona sits immediately north of the Middle Awash. He and his colleagues could only speculate about which human ancestor made these earliest tools and what they were used for.

The Middle Awash discoveries now answer the question of function. At the same time, they identify Australopithecus garhi as the best candidate for toolmaker.

Antelope fossils found on the surface and in excavations show crisp, curvilinear cutmarks that could only have been made by stone tools. Some of the antelope limb bones show evidence of having been bashed open by hammerstones. These telltale traces show that by 2.5 million years ago hominids were exploiting food resources unavailable to any other primate.

The scientists say that this new evidence indicates a very early breakthrough in the human career. Methods of acquiring large quantities of high quality dietary resources (meat and marrow fat) were important because they would greatly improve our ancestors' ability to provide for themselves and their offspring.

"The development of stone tool technology allowed this dietary revolution," said White. "This is the earliest evidence of a key adaptation that let our ancestors spread beyond Africa."

Fieldwork and prospects

The results published this week are part of the larger Middle Awash research project, a broad investigation of human origins and evolution involving geology, paleontology and archaeology. The research team will return to the study area in November for further work.

Support

Research in the Middle Awash is conducted under permit from the Ethiopian government through the Centre for Research and Conservation of the Cultural Heritage (CRCCH) of the Ministry of Information and Culture.

The research effort in the Middle Awash receives its primary support from the U.S. National Science Foundation. Additional funding comes from the Institute of Geophysics and Planetary Physics of the University of California's Los Alamos National Laboratory and a variety of additional institutions identified in the published papers.

The team

The interdisciplinary research team involves archaeologists, geologists and paleontologists from about 40 different institutions in 13 different countries.