Posted in full from
http://www.nytimes.com/2004/03/05/science/05HUMA.html
Another Branch of Human Ancestors Reported
By JOHN NOBLE WILFORD
Published: March 5, 2004
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."
--
Gregory Gadow
techbear@serv.net
http://www.serv.net/~techbear
"If you make yourself a sheep, the wolves will eat you."
-- Benjamin Franklin
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