A research team led by Bay Area scientists has unveiled the oldest-known member of the human family tree: a 4.4 million-year-old female skeleton named "Ardi," who shares both chimp and modern human features.
Believed to be capable of both climbing and walking, "Ardi" reveals the early evolutionary steps that our ancestors took after we diverged from our common ancestor with chimpanzees.
"It's not a chimp. It's not a human. It shows us what we used to be," said paleoanthropologist Tim White of the University of California-Berkeley, co-director of the research group that discovered and analyzed the fossils, described in a special issue of the Oct. 2 journal Science. "It bridges a gap."
In a press briefing this morning in Washington D.C., White and his team described the Ethiopian skeleton — a creature called Ardipithecus ramidus, thought to weigh 110 pounds and stand about four feet tall — that is stunningly complete, with most of skull and teeth as well as a pelvis, hands and feet. "Ardi" and bone fragments from at least 35 related individuals were found in the 1990s. But it took 17 years for the team — 47 different scientists from ten different countries, as well as Los Alamos National Laboratory and the Berkeley Geochronology Center — to analyze and interpret the data. Until now, the earliest known specimen of human evolution was Australopithecus, a small-brain but fully bipedal "ape-man" that lived between 4 and 1 million years ago. The most famous member of this genus was the 3.2-million-year-old skeleton nicknamed "Lucy," found in 1974. The new skeleton is older and more primitive than Lucy, according to White, 59, who has collected fossils in the hot and scrubby region of Ethiopia since 1981. He wore a "Save UC" button in Washington D.C., decrying budget cuts to the university. With both chimp-like and human features, "Ardi" lived in two worlds — upright but also in trees. For instance, she was adept at efficiently climbing — but could also walk on two feet, like humans. Because she had a fully opposable grasping big toe and did not have an arched foot, she walked flat-footed. It's unlikely she could walk or run for long distances, like humans. But nor was she as agile as chimps, so could not swing through trees. She was probably a slow and careful climber. She is thought to have been more omnivorous than chimpanzees, eating nuts, insects and small mammals in the woods. Her lower face had a muzzle that juts out less than a chimpanzee's. Her face is in a more vertical position. Her head balanced atop the spine, as in later upright walkers. And her teeth lack the sharp canines of chimps. Her pelvis is large, like apes. But it is structured so it has a low center of gravity, so she could balance while walking. There are suggestions that her spine is long and curved, like humans. But she is not "the missing link," a transitional creature between today's chimps and humans. This concept has been abandoned: We did not evolve from living champs or apes, but shared a common ancestor. Nor is she this long-sought "last common ancestor." That's because she's too young; Chimps and humans are thought to have diverged between 5 and 10 million years ago. Then we went our separate ways, each taking different evolutionary trajectories. But she's important because she is the closest we have come to this unfound "last common ancestor." She belonged to a new type of early hominid that was neither chimpanzee nor fully human. The scientists have not fully connected the branches in the human evolutionary tree. Rather, they are focusing on the anatomical interpretation of "Ardi." The name Ardipithecus ramidus is derived from the Ethiopian local language, Afar. "Ardi" means "ground," and "pithecus" is Greek for ape. "Ramid" means root. A literal translation of the creature is "root of the ground ape." The fossils were found in Ethiopia's Afar Rift, about 140 miles northeast of Addis Ababa, Ethiopia's capitol. The rift is a large triangular depression, where sediments have been accumulating for millions of years. The most fossil-rich site is very extensive — transecting an ancient landscape exposed along a 41/2-mile strip — and provides a vast amount of new geological and paleobiological data. Now parched desert, the landscape was once heavily wooded. There were fresh water springs and small patches of dense forest. The site also holds fossils of fig trees, land snails, birds such as owls and parrots, small mammals like mice and bats, as well as rhinos, bears, elephants, giraffes, antelopes and two kinds of monkeys. The discovery of "Ardi" in woodlands refutes the once-popular hypothesis that early humans took their first steps in open grasslands, say scientists. The first glimpse of the creature came in 1992 when a former UC-Berkeley grad student, Gen Suwa, saw a glint in desert pebbles — the polished surface of a tooth root from a hominim molar. The team immediately got on their hands and knees and searched for more fossils. Within days, they had found a lower jaw of a child. In 1994, Berkeley graduate student Yohanes Haile-Selassie of Ethiopia crawled up an embankment in the region and found two pieces of a bone from the palm of a hand. This was soon followed by the discovery of 110 other skeletal fragments, as well as 150,000 specimens of fossil plants and animals. "This team seems to suck fossils out of the earth," anatomist C. Owen Lovejoy of Kent State University, also part of the team, told the journal Science. But the bones were in terrible condition, trampled and scattered. White called them "road kill." So entire blocks of fossil-rich rock were moved to the National Museum of Ethiopia in Addis Ababa. It took years to remove the clay from the fragile fossils. Many high-tech instruments were used in the analysis, such as mass spectrometers to measure the age of the rock and micro-CT scanners to study the inner anatomy of bones and teeth. Electron microscopes revealed the tiny surface details of the fossils. The lava and ashes were shipped from Africa to California for dating analysis. At Los Alamos National Laboratory, Giday WoldeGabriel helped produce a detailed chemical profile of the major constituents of the glass shards found in the volcanic ash surrounding the fossils. The Berkeley Geochronology Center did paleomagnetic lab work, using a dating technique that determines how much time has elapsed since the eruption of volcanic ash. "Because they were sandwiched between two volcanic horizons with virtually indistinguishable dates, the thousands of fossils collected at Aramis are among the best calibrated in the world, at 4.4 million years ago," said project geochronologist Paul Renne of the Berkeley Geochronology Center. UC-Berkeley was critical to the fossil-mining operation that discovered "Ardi," said White. White came to UC-Berkeley because of its reputation in the exploration of human origins. He became interested in the field as a teenager after reading the 1965 Time/Life Nature Library book, "Early Man," written by famed Berkeley anthropologist Francis Clark Howell. Once at Berkeley, White studied under Howell and esteemed African archeologist J. Desmond Clark. About one-quarter of the project's 47 authors were educated or trained at UC-Berkeley, he said. The university also trained the first generation of African scholars in the field. Africans now play a critical role in studying and preserving this cradle of humankind. "We are very proud of Cal," said White. "This is what universities can achieve — and especially the public museums that support and produce this research, which are under great stress today."
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