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STEPPING OUTAn artist’s interpretation shows how a 4.4-million-year-old female Ardipithecus may have looked.2009, J.H. Matternes

It’s been 4.4 million years since a female now nicknamed Ardi lived in eastern Africa, but she still knows how to make an entrance.

Analyses of her partial skeleton and the remains of at least 36 of her comrades, described in 11 papers in the Oct. 2 Science, provide the first comprehensive look at an ancient hominid species. Ardipithecus ramidus evolved a few million years after humanity’s evolutionary family diverged from a lineage that led to chimpanzees, but it is not clear exactly how this species is related to other early hominids.

Ardi’s skeleton, which includes a skull with teeth, arms, hands, pelvis, legs and feet, indicates that the common ancestors of people and African apes (which include chimpanzees and gorillas) did not resemble chimpanzees, as many scientists have assumed, says anthropologist and project director Tim White of the University of California, Berkeley. Ardi displays an unexpected mix of apelike and monkeylike traits suitable for both tree climbing and upright walking. Overall, Ardipithecus looks unlike any living primate, White adds. Early hominids evolved in distinctive ways, so modern apes and monkeys provide poor models of a creature such as Ardi, in his view.

“Ardipithecus is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence,” White says.

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TOOTH TELLERSTeeth from a human (upper left) and Ardipithecus ramidus (upper middle) include small canines relative to those of a chimp (upper right). Corresponding cheek teeth contain thick enamel, in red, for humans (lower left), enamel of intermediate thickness in Ardipithecus (lower middle) and thin enamel for chimps (lower right).
Science/AAAS

Fossils of this ancient hominid display an unpredicted mosaic of traits suitable for two-legged walking and tree climbing, remarks anthropologist Alan Walker of Pennsylvania State University in University Park. “It now seems that the last common ancestor of chimpanzees and humans was much less chimplike than previously thought,” Walker says.

In 1992, teeth attributed to Ardipithecus were found in Ethiopia’s Afar Rift. A hand bone from Ardi turned up in 1994. Excavations over the next three years unearthed the rest of Ardi’s bones. Fieldwork from 1981 to 2004 yielded fossils of other individuals, as well as remains of animal and plant species from Ardi’s time.

It took years to remove Ardi’s fossils from hardened sediment — strata that White described as a time capsule with traces of a vanished world — and to conduct comparisons with other fossil and modern apes. Analyses of argon isotopes in volcanic ash layers sandwiching the new finds provided an estimate that Ardi lived 4.4 million years ago.

White’s team calculates that Ardi weighed roughly 50 kilograms, or 110 pounds, and stood 110 centimeters tall, or nearly four feet. Lucy, a 3.2-million-year-old partial skeleton from the hominid species Australopithecus afarensis, weighed about half as much as Ardi and was about 15 centimeters shorter. Fossil hunters found Lucy at another Ethiopian site in 1974. These new Ardipithecus finds offer unprecedented new avenues for testing hypotheses about the evolution of Lucy's kind.

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LINES OF DESCENTView larger version | Ardipithecus evolved shortly after the appearance of the first hominids and may have set the stage for Lucy’s kind.Science/AAAS

A relatively small skull and reduced canine teeth suggest that Ardi was female. Her brain case and face share many features with skull pieces of Sahelanthropus, a 6-million-year-old hominid found in Chad (SN: 7/13/02, p. 19).

Ardi’s brain size was close to that of Sahelanthropus and of modern chimps.

Ardi’s hands, arms, feet, pelvis and legs collectively indicate that her species moved capably in the trees, on hands and palms. Ardi’s kind lacked skeletal traits for hanging from branches, adeptly climbing tree trunks or knuckle-walking, thus distinguishing them from modern African apes.

On the ground, Ardi walked flat-footed with opposable, grasping big toes. The rest of her foot was stiff compared with chimps’ flexible feet. Ardipithecus apparently could not walk or run for long distances, the researchers say.

Ardi’s limb proportions, with long arms and legs, resemble those of monkeys more than apes. Her hands and back display traits that offered much more flexibility than observed in chimps, allowing her to twist into different positions while clambering through trees.

Remains of fig trees, snails, monkeys and other species found near the new fossils suggest Ardipithecus lived in a forested area. And Ardipithecus teeth were suited to an all-purpose diet in that habitat, the researchers say.

Small faces and canine teeth indicate that Ardipithecus males rarely fought, proposes anthropologist and team member Owen Lovejoy of Kent State University in Ohio. In apes and monkeys with large canines, males frequently wield their sharp teeth in battles over status and access to females.

Lovejoy hypothesizes that Ardipithecus males, like their counterparts in Lucy’s species (SN: 6/11/05, p. 379), formed families with specific females. Males cemented relationships with mates by sharing food with them, he suggests.

Changes in Ardipithecus teeth “tell us that humans have been evolving toward what we are today for at least 6 million years,” Lovejoy said in a press conference on October 1. “This is one of the most revealing hominid fossils that I could have imagined.”

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