Fossil Human Teeth Fan Diversity Debate
The discovery in Ethiopia's Middle Awash region of a handful of
nearly six-million-year-old teeth is adding fuel to a longstanding
debate among scholars of human evolution. At issue is whether the base
of our family tree is as streamlined as a saguaro, or as shaggy as a
When it comes to classifying fossils, paleoanthropologists generally
fall into two camps. There are the splitters, who parse the human fossil
record into numerous genera and species; and the lumpers, who recognize
fewer, more variable taxa. Both factions agree that several hominid
species co-existed during the later stages of human evolution, between
three million and 1.5 million years ago. The number of forms that shared
the landscape shortly after humans diverged from chimpanzees, on the
other hand, is vigorously disputed.
|In a report detailing the new findings, published today
in the journal Science, Yohannes Haile-Selassie of the Cleveland
Museum of Natural History and his colleagues assign the ancient teeth to
a new hominid species, Ardipithecus kadabba. In light of the
discovery, the team argues, remains previously attributed to the
subspecies A. ramidus kadabba should now be considered part of
the new species, which is older and more primitive than A. ramidus.
Particularly important in their analysis are the upper canine and
lower third premolar that turned up. All fossil and modern apes,
particularly males, have large, tusklike canines that are continually
honed against the lower third premolars, which keeps them sharp for
fighting (mostly over access to mates). Humans, in contrast, have
smaller, more incisorlike canines, which scientists have interpreted as
indicative of increased male cooperation. For their part, the A.
kadabba canine and premolar exhibit a mix of apelike and hominidlike
traits, prompting Selassie to speculate that this species might be the
first on the human line after the chimp-human split.
|A. kadabba is not the lone contender for the title
of earliest member of the human lineage. Two other putative hominids
dating to the late Miocene epoch--Sahelanthropus tchadensis from
Chad and Orrorin tugenensis from Kenya--surfaced in 2002 and
2000, respectively. But Selassie and his collaborators suggest that the
teeth of these specimens indicate that they are very similar to A.
kadabba. On the basis of the available evidence, they contend, all
three may belong to the same genus, or even species.
A contrary view comes from David R. Begun of the University of
Toronto, who counters that the A. kadabba, Sahelanthropus and Orrorin
dentitions differ in important ways. "Rather than a single lineage,
the late Miocene [hominid] fossil record may sample an adaptive
radiation, from a source either in Eurasia or yet undiscovered in
Africa, the first of several radiations during the course of human
evolution," he writes in an accompanying commentary. But the level
of uncertainty about the fragmentary fossils known thus far makes it
impossible to reconcile these differences of opinion between lumpers and
splitters. "The solution is in the mantra of all
paleontologists," he concludes. "We need more fossils!"
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