[iagi-net-l] Fossil Human Teeth Fan Diversity Debate

[Rovicky Dwi Putrohari]

March 05, 2004   (Scientific American)

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 shrub.

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!" --Kate Wong

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