The New York Times
November 19, 1996
2.3-Million-Year-Old Jaw Extends Human Family
By JOHN NOBLE WILFORD
An upper jaw found in the Hadar badlands of northern
Ethiopia is being hailed as
the most convincing and earliest definitively dated
fossil of the genus Homo, to
which living humans belong.
The 2.33-million-year-old jaw extends the established age of
the human family line by
400,000 years, closer to the time of the first evidence of
toolmaking and to
environmental upheavals that may have been decisive in human
evolution. The fossil
was found in sediments with a scattering of crude stone
tools, the earliest association
between Homo remains and such artifacts.
Of greatest potential importance, paleontologists said, is
the rare glimpse the discovery
provides into what has been a kind of dark age of
evolutionary change, the period
between three million and two million years ago. Scientists
are all but certain that
sometime in that epoch the genus Homo evolved from the more
apelike
australopithecines, yet the fossil record in Africa for that
period has been frustratingly
spotty.
The new discovery was announced Monday by a team of American,
Canadian,
Ethiopian and Israeli scientists. A detailed description and
interpretation of the fossil is
being published in the December issue of The Journal of Human
Evolution.
Until a skull and other bones are found, however, the
scientists said they would not be
able to determine the relationship of their find to any of
the known Homo species, all of
which are dated at 1.9 million years and after, or whether it
is a previously unknown
species representing a transitional step. In that case, it
might even be a more direct
ancestral line leading to modern humans.
In any case, the well-preserved jaw, the researchers write in
the journal article,
represents "the oldest association of hominid remains with
stone tools and possibly the
earliest well-dated occurrence of the genus Homo." This, they
concluded, "promises to
add new insights on hominid paleobiology and behavior in this
poorly understood time
period."
The leaders of the team are Dr. William H. Kimbel, Dr. Donald
C. Johanson and Dr.
Robert C. Walter, all of the Institute of Human Origins in
Berkeley, Calif. Kimbel is the
institute's science director. Johanson is a paleontologist
best known for the discovery in
the 1970s at a nearby Hadar site of the "Lucy" skeleton, a
3.18-million-year-old
ancestral hominid later designated Australopithecus
afarensis. Walter is a geologist
who was in charge of dating the jaw, based on an analysis of
volcanic ash just above it.
Other palentologists familiar with the work said the jaw was
just what they had long
sought to begin filling in the fossil record for early Homo.
"The jaw is definitely Homo and the date is good," said Dr.
Philip Rightmire of the State
University of New York at Binghamton. "It's the first good
clear confirmation of Homo
earlier than two million years ago."
In recent years, paleontologists had found possible Homo
traces this old or slightly
older in Kenya and Malawi, though most scientists were
unconvinced. They were not
sure that the three-inch skull fragment from Kenya was from
an early Homo or an
australopithecine. The date for the jaw from Malawi was not
well established.
Dr. Andrew Hill, a Yale University paleontologist, who
identified the Kenya fragment as
a 2.4-million-year-old Homo, said: "It's nice to have another
specimen and from another
part of the anatomy. I think we now know Homo existed
probably as early as 2.5 million
years ago."
Like other scientists, Dr. Nicholas Toth of Indiana
University, who specializes in early
stone tools, said the discovery did not answer the question
of whether the making and
use of stone tools began only after the emergence of the
larger-brained Homo genus or
was also a talent of the australopithecines that existed
then. Since the earliest evidence
of tools is dated at 2.6 million years ago, he said, it is
possible that small-brained
hominids were the first toolmakers, with brain expansion
coming later as one
consequence of stone technology.
The discovery team said they could not say with assurance
that the stone flakes and
chopping tools found at the Hadar site were made by the Homo
species represented by
the jaw.
Ethiopian fossil hunters made the discovery on the first day
of their field season in
November 1994. They spotted two halves of a maxilla, or upper
jaw, that had eroded
out of the side of a barren hill near a dry stream bed.
"The instant we fit the two halves together we knew we
weren't dealing with an apelike
Australopithecus," said Kimbel, who was at the site at the
time of the discovery.
A species like Australopithecus afarensis, which lived from
3.9 million years ago to at
least 3 million, had a long, narrow palate and projecting
face, giving its skull an apelike
visage. But the Hadar jaw is broader, with a parabolic dental
arch. Other characteristics
suggested that the individual had a short, flat nose, more
like Homo than
Australopithecus, and did not have a projecting, apelike face.
With no more to go on, Kimbel said in an interview, it was
impossible to say if the jaw
belonged to either of the two previously known early Homo
species in East Africa,
generally classified as Homo habilis and Homo rudolfensis, or
the later Homo erectus.
The evolutionary path from Homo erectus to Homo sapiens is a
subject of much debate,
but anatomically modern Homo sapiens sapiens is thought to
have arisen from 200
million to 100 million years ago.
Kimbel said that, if anything, the jaw bore more resemblance
to Homo habilis. "That is
based primarily on the fact that habilis is likewise more
primitive than any other Homo,"
he explained.
Dr. Eric Delson, a paleontologist at the American Museum of
Natural History and the
City University of New York, said, "It's conceivable that
this jaw could be something
different, neither habilis nor rudolfensis."
This prospect is especially intriguing. Since paleontologists
have reservations about H.
habilis or H. rudolfensis as species on the direct ancestral
line to modern humans,
Delson said, it could be "possibly extremely important" if
the jaw proved to be a distinct
species that might be on the mainline of human evolution.
Other investigations at Hadar have provided more evidence of
drastic environmental
change in this period. Abundant fossils of antelopes
recovered at the site indicate that
the region was fairly open and grassy, with some water not
far away. In the time of A.
afarensis, before three million years ago, Hadar was a
wetter, more densely wooded
area teeming with impalas and other woodland mammals.
Some paleontologists, notably Dr. Elisabeth Vrba of Yale,
suspect that the origin of the
Homo lineage could have been influenced by the onset of a
colder global climate,
beginning about 2.7 million years ago. At about that time, it
is thought that A. afarensis,
the Lucy species, split into at least two lineages, A.
africanus and A. aethiopicus, and a
third group may have appeared as well -- Homo.
"The Hadar fossil," Johanson said, "helps plug a gap in the
evidence for the early
evolution of our own lineage and perhaps will help us begin
to forge links back in time
to A. afarensis."
Other paleontologists in the last two years have begun
filling in another crucial gap in
the hominid fossil record. This is the period after the
hominids split from the apes, about
seven to five million years ago, and before the first known
evidence for A. afarensis,
about 3.9 million years ago.
Last year Dr. Meave Leakey of the Kenya National Museum in
Nairobi and Dr. Alan
Walker of Pennsylvania State University uncovered fossils in
Kenya of a
4.1-million-year-old species, designated A. anamensis. And
Dr. Tim D. White of the
University of California at Berkeley identified an even more
apelike hominid, the
4.4-million-year-old Ardipithecus ramidus, from fossils found
in Ethiopia. Both show
early evidence for the transition to upright walking.
Bill Hamilton
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William E. Hamilton, Jr, Ph.D. | Staff Research Engineer
Chassis and Vehicle Systems | General Motors R&D Center | Warren, MI
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