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Evolution's Child: Fossil puts youthful twist on Lucy's kind

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10:53am, September 20, 2006
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As fossil hunters crossed a dusty slope of Ethiopia's Dikika region on Dec. 10, 2000, one noticed a child's face bones poking out of the ground. Now, after years of painstaking work to remove the ancient individual's skull and some of the other bones from sandstone, researchers have announced that this discovery represents the oldest and most complete fossil child in our evolutionary family.

The nearly complete skeleton, missing only the pelvis and a few other bones, comes from a 3-year-old Australopithecus afarensis female who died about 3.3 million years ago, say Zeresenay Alemseged of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues.

"This [new fossil] is something you find once in a lifetime," says Zeresenay.

The Dikika child's skeleton has not yet been entirely removed from the surrounding rock. Zeresenay's team plans to compare it with Lucy, the 3.2-million-year-old partial skeleton of an adult female A. afarensis, which was unearthed in 1974.

The researchers found the child's skeleton between previously dated volcanic-ash layers. A flood apparently covered the child's body in sand and pebbles, protecting it during fossilization. Comparisons of the youngster's teeth with those of people and chimpanzees yielded the estimate of age.

The Dikika girl displays a brain size about equal to that of a 3-year-old chimpanzee, the investigators report in the Sept. 21 Nature. However, the ancient child's brain grew more slowly than brains of chimps or other apes do, comparisons with adult A. afarensis and ape skulls reveal.

The parts of the specimen that have so far been removed from the sandstone include the hyoid, a small neck bone. The Dikika girl's hyoid resembles hyoids of living nonhuman apes, suggesting that she possessed air sacs in her neck as apes do, says coauthor Fred Spoor of University College London. The function of air sacs in apes is unclear.

The shape of the Dikika girl's thighs and shins indicate that she walked upright, even at age 3, the researchers hold. However, several apelike lower-body traits support the view—hotly debated over the past 25 years—that A. afarensis sometimes climbed in trees, perhaps to make nests or to avoid predators, Spoor says.

For instance, as in other A. afarensis specimens and chimps, the new specimen's finger bones are long and curved. Computerized tomography images of the inner ear show semicircular canals, crucial for maintaining balance, like those of chimps.

Finally, and surprisingly, the Dikika girl's shoulder blades and those of gorillas look much alike, the researchers assert. Spoor regards that scapula as evidence of an "all-purpose" shoulder that allowed for climbing.

A. afarensis "was unlikely to have been restricted to walking on two feet," remarks anthropologist Bernard Wood of George Washington University in Washington, D.C.

The Dikika girl's scapula indeed resembles scapulas of gorillas, although the implications of this trait for tree climbing await complete analysis of the skeleton, says anthropologist Daniel E. Lieberman of Harvard University.

"This skeleton is really important because it's so complete," comments anthropologist Tim D. White of the University of California, Berkeley. "But it's a black box until it's been fully cleaned and properly analyzed."

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