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 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 indicates that the common ancestors of people and chimpanzees did not resemble chimps, as many scientists have assumed, says 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.
In 1992, teeth thought to belong to Ardipithecus were found in Ethiopia's Afar Rift. A hand bone from Ardi turned up in 1994. Excavations over three years unearthed the rest of Ardi's bones, and fieldwork from 1981 to 2004 yielded fossils of other individuals.
It took years to remove Ardi's fossils from hardened sediment and to conduct comparisons with other fossil apes 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 about 50 kilograms, or 110 pounds, and stood 120 centimeters tall, or nearly four feet. Lucy, a 3.2-million-year-old partial skeleton of the hominid species Australopithecus afarensis, weighed about half as much as Ardi and was about 15 centimeters shorter. Lucy was found in 1974, and the new Ardipithecus finds offer unprecedented new avenues for testing hypotheses about the evolution of Lucy's kind.
A relatively small skull and reduced canine teeth suggest that Ardi was female. Her brain case and face share many features with skull pieces from Sahelanthropus, a 6- to 7-million-yearold hominid in Chad (SN: 7/13/02, p. 19). Ardi's brain size was close to that of both Sahelanthropus and modern chimps.
Ardi's hands, arms, feet, pelvis and legs collectively indicate that her species moved capably in the trees, on hands and feet. Ardi's kind lacked skeletal traits for hanging from branches, adeptly climbing tree trunks or knuckle-walking, thus distinguishing Ardipithecus from modern African apes.
Small faces and canine teeth indicate that Ardipithecus males rarely fought, proposes team member Owen Lovejoy of Kent State University in Ohio. In apes and monkeys with large canines, males frequentlywield 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, he suggests.
The features seen in Ardipithecus teeth "tell us that humans have been evolving toward what we are today for at least 6 million years," Lovejoy said at a news conference on October 1. "This is one of the most revealing hominid fossils that I could have imagined."
Bower, Bruce. "Ancient hominids get a new look with analysis of ardipithecus fossil: fossils suggest creature didn't resemble any living primate." Science News 24 Oct. 2009: 9. Academic OneFile. Web. 20 Nov. 2009.
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