Posted on 03/05/2004 3:30:34 AM PST by Pharmboy
Another species has been added to the family tree of early human ancestors — and to controversies over how straight or tangled were the branches of that tree.
Long before Homo erectus, Australopithecus afarensis (Lucy, more than three million years ago) and several other distant kin, scientists are reporting today, there lived a primitive hominid species in what is now Ethiopia about 5.5 million to 5.8 million years ago.
That would make the newly recognizied species one of the earliest known human ancestors, perhaps one of the first to emerge after the chimpanzee and human lineages diverged from a common ancestor some six million to eight million years ago.
The timing of the fateful split has been determined by molecular biological research, and in recent years fossil hunters have found traces of what those earliest hominids, human ancestors and their close relatives, might have been like.
When the first fossil bones and teeth of this hominid were described three years ago, paleoanthropologists tentatively identified it as a more apelike subspecies that they named Ardipithecus ramidus kadabba. The original ramidus species was found in 1994 in 4.4-million-year-old sediments, also in Ethiopia.
But with more discoveries and a closer study, especially of the teeth, the scientists decided that the kadabba fossils from five individuals were distinctive enough to qualify as a separate species, Ardipithecus kadabba. In that case, the scientists added, kadabba was not a subspecies, but the likely direct ancestor of ramidus. But there were too few skeletal bones yet to learn much about other aspects of kadabba.
The description and interpretation of the new hominid species appear today in the journal Science. The authors of the report are Dr. Yohannes Haile-Selassie of the Cleveland Museum of Natural History, Dr. Gen Suwa of the University of Tokyo and Dr. Tim D. White of the University of California, Berkeley.
The kadabba fossils were found in the Middle Awash valley about 180 miles northeast of Addis Ababa, the Ethiopian capital. These are arid badlands now, but in the time of the early hominids the land was wooded and more hospitable.
Dr. Haile-Selassie said the shapes and wear patterns of six teeth in particular were "significant in understanding how the dentition evolved from an apelike common ancestor into the earliest hominids." They were also critical, he said, in differentiating the earlier and later species of the genus Ardipithecus.
Other scientists familiar with the research, but not involved in it, said they agreed or were at least inclined to agree with the authors' designation of a separate species for the fossils. But they were not so sure about the authors' proposal that the fossils were so similar to those of two other recently discovered early species that all three species might have actually belonged to a single genus of closely related hominids.
The other two hominid species are Sahelanthropus tchadensis, found in Chad and thought to be six million to seven million years old, and Orrorin tugenensis, a six-million-year-old specimen from Kenya. The two are primitive apelike creatures not much bigger than a modern chimp. Although the analysis of these remains is not complete, and still subject to debate, each has been classified as a separate genus and species.
In their report on kadabba, Dr. Haile-Selassie and his colleagues concluded, "Given the limited data currently available, it is possible that all of these remains represent specific or subspecific variation within a single genus."
Dr. White, one of the most experienced paleoanthropologists, emphasized this point in a telephone interview. "These earliest hominids are all very, very similar," he said. "When you look at these three snapshots we have, we are struck by the great biological similarity, not by pronounced differences, not by great lineage diversity."
But in an accompanying commentary in the journal, Dr. David R. Begun, a paleontologist at the University of Toronto, questioned this interpretation. He said it was unlikely that all three of the early hominids belonged to a single genus, noting instead that the three exhibited evidence of striking diversity.
Dr. Begun conceded that "the level of uncertainty in the available direct evidence at this time renders irreconcilable differences of opinion inevitable."
The differences, broadly speaking, take the form of two images of what the hominid family tree looks like — a ladder or a bush. A growing number of scientists, finding multiple species of hominids that overlapped in time, contend that in response to new or changed circumstances hominids evolved along many diverse lines — a bush with many branches.
Dr. Begun, in a telephone interview, emphasized that he was not disagreeing with the designation of the new species, but was "merely presenting an alternative" to the single-genus interpretation.
"The material is so fragmentary," he said, "that we really can't know, and so our differences often are a reflection of different philosophies and experience in research."
Dr. Alan Walker, an anatomist at Pennsylvania State University who specializes in hominid research but was not involved in the kadabba analysis, said that too few fossils had been discovered to justify either interpretation. He noted that it was easy to be misled by variations that are normal within the fossil collections of any single species.
"People who believe in a bushy family tree will look for bushiness in their fossils, and those who don't won't," Dr. Walker said in an interview. "We are generalizing far too much, with not very many fossils spread over a long period of time."
The voice of reason...
A prediction: sometime in the next five or ten years, we'll all be looking back to find the context of this line, due to the fact that some quote miner has decided to add Dr. Walker to the "list" of scientists who disagree with the entire theory of evolution.
His brother, Prof Haile-Unlikely, disagrees.
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