Contentious canines. Ancient Ardipithecus teeth, which bear a striking resemblance to chimpanzee teeth (inset), could shake up the human family tree.

Gnawing Away at the Human Family Tree

The discovery of six fossil teeth may prune some branches off the human family tree. For the first time, anthropologists can directly compare three different species of protohumans that lived approximately 5 million years ago. The apelike teeth from the three groups are so similar that the researchers suggest the species all belong to one genus, not three,

Fossils of the earliest humans are few and far between. A femur here, a toe bone there, and a few teeth were all the evidence that researchers had to describe three early human types: Sahelanthropus, Orrorin, and Ardipithecus. Based on these bones, and the spread of distance and time between the fossils, anthropologists tentatively suggested that they each belonged to a separate genus. But with limited evidence, the analysis was shaky.

The six new Ardipithecus teeth found in Middle Awash, Ethiopia, finally provide a way to compare all three species. For the first time, researchers have specimens of the same type--canine teeth, in this case--from all three hominids. Yohannes Haile-Selassie, of the Cleveland Museum of Natural History in Ohio, and colleagues describe the teeth in the 5 March issue of Science.

The canines and lower premolars look conspicuously apelike. They have a distinctive orientation that might have allowed the canine to hone itself against the lower premolar--considered a defining characteristic of apes. A canine tooth from Orrorin, found previously, is shaped similarly to the newfound Ardipithecus tooth, but it isn't worn enough to tell whether the owner honed it. Conversely, a canine from Sahelanthropus was too worn to know. However, the shape and thickness of the enamel of all three species' teeth is very similar.

According to Haile-Selassie, the natural variation among modern-day humans is nearly as large as the variation among the canines of these three protohuman species, suggesting that all three belong in the same genus.

The differences are so small that the three groups could even belong in the same species, says Clark Howell, an anthropologist at the University California, Berkeley. But not everyone agrees. David Begun, an anthropologist at the University of Toronto, says that differences in enamel thickness and canine shape are the standard features used for classification. And, in this case, he says, "the differences warrant maintaining the status quo" of three separate genera.

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