— She could walk, but not talk (Image: ZeresenayAlemseged/ARCCH)
The stunningly complete skeleton of a three-year-old girl who lived 3.3 million years ago has been uncovered in Ethiopia. The child belongs to the species Australopithecus afarensis like the famous "Lucy", who was discovered in 1974. The young age of the so-called Dikika child promises new insights into the growth of early humans.
The new find is the most complete and important skeleton of an immature Pliocene hominin ever found, says Tim White of the University of California, Berkeley, who worked on the Lucy discovery. "The gist of the current paper is, 'Eureka, we have it'," he says.
A team led by Zeresenay Alemseged of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, spotted the first bones south of the Awash river on 10 December 2000. The team spent four field seasons scouring the area for every scrap of the skeleton.
Lucy was also found in the Awash region, which is famed for its early human fossils. Many anthropologists think A. afarensis was ancestral to the genus Homo, though its exact position in the human family tree remains a matter for debate.
Alemseged's team believes that a flood rolled the child's body into a ball and buried it in sand soon after her death, before the bones could be weathered or pulled apart by scavengers.
Analysis of the skeleton has barely begun because the upper parts, including the skull, shoulder blades, collarbones, ribs and backbone, are still largely encased in a block of hard sandstone. However, a CT scan of the skull revealed tooth development matching that of a three-year-old, the team reports in Nature (vol 443, p 296).
"At least 50 per cent of the skeleton is there, but more importantly we have the face and brain endocast, and the whole skull, telling us clearly how the [child] looked," says Alemseged. He estimates the brain size was 330 cubic centimetres, between 63 and 88 per cent of the size of an adult of the species. This hints at brain growth slower than in chimpanzees, whose brains have reached 90 per cent of adult volume by age three. A. afarensis may therefore have begun evolving the slower brain development characteristic of modern humans.
The exposed leg bones show the child walked bipedally like Lucy. In contrast, the shoulder blade "in some ways resembles young gorillas", says collaborator Bill Kimbel of Arizona State University. That supports the inference from Lucy's long arms that she was a better climber than modern humans. During the girl's lifetime the environment was a mosaic of forest and savannah, so the species may have gathered food and slept in trees, but walked from place to place.
Another key discovery is a hyoid bone, which is found in the throat and in humans is involved in speech. Until now, only one fossil hyoid has ever been found, and it was from a Neanderthal. The Dikika hyoid resembles an ape's, suggesting speech had not begun to evolve in A. afarensis.
Alemseged believes much information can be gained once the skeleton is freed from its stone casing. "A clear picture will emerge of how baby human ancestors were built, and how they grew up," he says.