The earlier species, A. anamensis, had large canine teeth and a narrow jaw. When Lucy appeared, compared to A. anamensis, the jaw had widened, the canines had become smaller, and the molars had grown. Such changes suggest that the A. afarensis chewed, not tore, its food.
Previously, the researchers had found teeth dating from about 3.5 million years ago, but their 2007 field work yielded more complete jaws.
Generally, the bones could help clarify a long-standing debate in human evolution: How many branches of human ancestor existed millions of years ago? Did some evolve into new forms only to die out and disappear?
"People are prepared to accept that there's diversity," said Chris Stringer, a research leader at London's Natural History Museum, who was not involved in the latest work. "But of course what is uncertain is how widespread it was through the last six million years."
"If we had a complete fossil record, would we see branching events right through that period of time ... or were there only specific times of branching events?" Stringer said.
The Afar Depression—a sunken area in the Horn of Africa where the new jawbone was found— has produced some of the most significant fossil finds in history.
For example, the region has yielded some of the earliest examples of human ancestors ever, which lived 5.8 million years ago, as well as an early anatomically modern human from 160,000 years ago.
(Related: "Fossil Find Is Missing Link in Human Evolution, Scientists Say" [April 13, 2006].)
Three years of research in the area where the latest bones were found have yielded more than 1,900 vertebrate specimens. They include human ancestor species from many time periods, as well as other animals such as mice, elephants, monkeys, rhinoceros, primitive horses, and fish.
A partial skeleton of a human ancestor was discovered in 2005, and Haile-Selassie's team continue to excavate what's left of it. Haile-Selassie said his team would need some time to study the bones and come up with conclusions about the relationship between A. anamensis and A. afarensis.
"Two years down the line we may be able to say something about it," Haile-Selassie said. "Patience is needed."
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