"It has the complete skull, the mandible, and the whole brain case. Lucy doesn't have much of a head."
"The most impressive difference between them is that this baby has a face," Zeresenay added. (Ethiopians' first names are their formal names.)
That face, no bigger than a monkey's, was spotted peering from a dusty slope in December 2000. Its smooth brow and short canine teeth identified it as a hominin, a group that encompasses humans and their ancestors.
Zeresenay's research at Dikika was funded in part by the National Geographic Society.
The find, reported tomorrow in the journal Nature, is also featured in the November issue of National Geographic magazine. (National Geographic News and National Geographic magazine are both parts of the National Geographic Society.)
(Check out a special "Dikika Baby" Web site with the entire National Geographic article, unpublished photographs, and a video interview with the author.)
The fossil child, who died at nursing age, offers important clues to the development of early humans, says Spoor, of UCL.
"It will teach us how our early ancestors grew up," he said. "The only way you can evolve from one type of species into another is by growing up in a different way, because that's how you change."
For instance, a prolonged, dependent childhood allowed later human species to grow larger brains, which need more time to develop after birth.
"As far as we can tell, it is not yet happening [with Lucy's baby]," Spoor said.
While the adult A. afarensis is thought to have had a brain slightly larger than a chimpanzee's, the hominin child's brain appears to have been smaller than an average chimp brain of the same age.
"For the first time we have insights that they may have grown their brains a little bit slower than your average chimp," Spoor said.
"If you take more time to form your brain, it may well be that you make more intricate connections inside," the researcher added. "Or it may not be a positive thingperhaps you live on poorer food or are a bit behind."
Spoor favors the latter explanation in the case of these early hominins.
"They haven't progressed over great apes at all," he said. "They've just changed their locomotion for whatever reason, but they were not necessarily any more clever than chimps were."
The new fossil also supports the theory that A. afarensis walked upright on two legs, but it hints that human ancestors hadn't completely left the trees by that time.
The skeleton's ape-like upper body includes two complete shoulder blades similar to a gorilla's, so it could have been better at climbing than humans are.
"This was a bit of a surprise, and controversial," Spoor said.
Some researchers will say the feature was inherited from an ancestor and reveals little about this hominin's lifestyle, Spoor adds.
"Other people will say it shows they are still using their arms for climbing quite a lot," he said.
"The question is not whether they spent all day swinging around in the trees. But it may be true there was still some climbing aspect, for instance, for building nests at night or to forage in the trees."
A Record Find
Louise Humphrey, a paleontologist at the Natural History Museum in London who wasn't part of Zeresenay's team, describes the find as "an extremely valuable addition to the hominin fossil record.
"The fossil also preserves parts of the skeleton not previously documented for A. afarensis," she added.
These included a hyoid bone in the throat area that later went on to form part of the human voice box.
"Detailed analysis of the skeleton will reveal a lot more about the [locomotion] and foraging behavior of this young hominin," Humphrey said.
How the child died is unclear, though it appears the body was rapidly covered by sand and gravel during a flood.
"It was buried just after it died," Zeresenay said. "That's why we found an almost complete skeleton, so maybe [drowning] could be the cause of its demise."
Like Lucy and many other hominin fossils, the child was uncovered in the low-lying northern end of Africa's Great Rift Valley.
Researchers say the region was once much less arid. Hominins shared the area's lush woods and grasslands with extinct species of elephants, hippos, crocodiles, otters, antelopes, and other animals whose fossils have been found nearby.
For these remains to be preserved and discovered, Zeresenay says, they needed to be covered in sediments and then exposed by tectonic activity, as has happened in the Great Rift Valley.
"These deposited environments were subsequently exposed by tectonics for us to go there and find the hominins," he added.
The Ethiopian paleoanthropologist says several more years of painstaking work will be needed to remove the remaining hard sandstone encasing much of the fossil child's skeleton.
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