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Image: PATRICIA J. WYNNE
The arid badlands of Ethiopia's Afar region have long been a favorite hunting ground for paleoanthropologists. The area is perhaps best known for having yielded "Lucy," the 3.2-million-year-old skeleton of a human ancestor known as Australopithecus afarensis. Now researchers have unveiled another incredible find, from a site called Dikika, just four kilometers from where Lucy turned up. It is the skeleton of an A. afarensis child who lived 3.3 million years ago. No other hominin of such antiquity--including Lucy--is as complete as this one. Moreover, as the earliest juvenile hominin ever found, the Dikika fossil provides a rare opportunity to study growth processes in our ancient relatives.
Fossil hunters led by paleoanthropologist Zeresenay Alemseged, now at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, discovered the remains--believed to be those of a three-year-old girl--in 1999. Most of the upper part of the skeleton was entombed in sandstone when it was found. It has taken Alemseged five years to remove enough of the cementlike matrix to expose the key elements, and many more bones remain obscured by the sediment. Still, the specimen has already yielded precious insights into a species that most researchers agree gave rise to our own genus, Homo. Alemseged and his colleagues describe the fossil and its geological and paleontological context in two papers that will be published September 21 in Nature.
The skeleton consists of a virtually complete skull, the entire torso and parts of the arms and legs. Many of the bones were still in articulation. The exceptional preservation of the baby, as well as that of other animals found at the site, indicates to team geologist Jonathan G. Wynn of the University of South Florida that her body was buried shortly after death by a flood event. Whether she perished in the flood or before the flood is unknown.
Although she was only three when she died, the Dikika youngster already possessed the distinctive characteristics of her species. Some of the most intriguing ones pertain to locomotion.
Scholars agree that A. afarensis was a creature that got around capably on two legs. But starting in the 1980s, a debate over whether the species was also adapted for life in the trees emerged. The argument centered on the observation that whereas A. afarensis has clear adaptations to bipedal walking in its lower body, its upper body exhibits a number of primitive traits better suited to an arboreal existence, such as long, curved fingers for grasping tree branches. One camp held that A. afarensis had transitioned fully to terrestrial life, and that the tree-friendly features of the upper body were just evolutionary baggage handed down from an arboreal ancestor. The other side contended that if A. afarensis had retained those traits for hundreds of thousands of years, then tree climbing must have still formed an important part of its locomotor repertoire.
Like adult A. afarensis, the Dikika baby had long, curved fingers. But the fossil also brings new data to the debate in the form of two shoulder blades, or scapulae--bones previously unknown for this species. According to Alemseged, the shoulder blades of the child look most like those of a gorilla. The upward-facing shoulder socket is particularly apelike, contrasting sharply with the laterally facing socket modern humans have. This, Alemseged says, may indicate that the individual was raising its hands above its head--something primates do when they climb.
Further hints of arboreal tendencies reside in the baby's inner ear. Using computed tomographic imaging, the team was able to glimpse her semicircular canal system, which is important for maintaining balance. The researchers determined that the infant's semicircular canals resemble those of African apes and another australopithecine, A. africanus. This, they suggest, could indicate that A. afarensis was not as fast and agile on two legs as we modern humans are. It could also mean that A. afarensis was limited in its ability to decouple its head and torso, a feat that is said to play a key role in endurance running in our own species.