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Photo
from cover of Nature. Photo © David L. Brill \ Brill Atlanta
Fossil skull from
Ethiopia indicates human ancestor, Homo erectus, was single, widespread
species 1 million years ago
20 March 2002
Bob
Sanders, Media Relations
Video clips of the
fossil discovery
BERKELEY -
A million-year-old Homo
erectus skull found in Ethiopia indicates that this human ancestor
was a single species scattered widely throughout Asia, Europe and Africa,
not two separate species, according to an international group of scientists
who discovered the skull in 1997.
Some archaeologists
and anthropologists have argued that African and European populations
were a different species, Homo ergaster, distinct from the strictly
Asian Homo erectus.
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Henry Gilbert, a Ph.D. Candidate at the University of California
at Berkeley, discovered this Homo erectus skull. With a porcupine
quill, and other tools intended not to scratch the fossil, he is carefully
removing the silty matrix that entombed the specimen for a million
years. The newly discovered fossil and its surrounding sediment was
encased in a plaster "jacket" to enable safe transportation. At the
time of this picture, the upper half of that jacket had been removed
for Gilbert to continue the excavation in the lab. The roof of the
specimen's right eye socket is visible at this stage. The face appears
to have been chewed away by a large carnivore prior to burial and is
missing. Photo, © David L. Brill \ Brill Atlanta
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It took University
of California, Berkeley, researchers and their colleagues more than two
years to clean and reassemble the crushed skull, which is described by
the Ethiopian and American team in the March 21 issue of Nature.
The fossil was described by Berhane Asfaw of the Rift Valley Research
Service in Addis Ababa, Ethiopia, paleoanthropologist Tim D. White, professor
of integrative biology and co-director of UC Berkeley's Laboratory for
Human Evolutionary Studies, and UC Berkeley graduate student W. Henry
Gilbert, who found the skull.
"This fossil is a
crucial piece of evidence showing that the splitting of Homo erectus
into two species is not justified," said White. "This African fossil is
so similar to its Asian contemporaries that it's clear Homo erectus
was a truly successful, widespread species throughout the Old World."
The Ethiopian and
American scientists also conclude in their paper that the onset of the
Ice Ages about 950,000 years ago likely split the Homo erectus
populations and led to their divergent evolution. The African population
of Homo erectus probably gave rise to modern Homo sapiens,
the European branch perhaps became the Neandertals, or Homo neanderthalensis,
while the Asian population went extinct.
Homo erectus
first appeared about 1.8 million years ago and, based on the fossil evidence,
quickly populated Africa, Asia and Europe. Though it is unclear whether
the species arose in Africa or Asia, a million years later the widespread
populations were still similar enough to be considered a single species,
White argued. The largest number of Homo erectus specimens are
from Asia, including the first specimen - "Java Man."
"What we are saying
in this paper is that the anthropological splitting common today is giving
the wrong impression about the biology of these early human ancestors,"
he said. "The different names indicate an apparent diversity that is not
real. Homo erectus is a biologically successful organism, not a
whole series of different human ancestors, all but one of which went extinct."
By the time Homo
erectus disappeared some 400,000 years ago, its various populations
had clearly diverged, since 500,000-year-old fossils from Asia, including
"Peking Man," differ significantly from African fossils of the same age,
particularly in the size of the cranium.
The new Homo erectus
fossils were discovered in the Middle Awash region of the Afar Rift in
eastern Ethiopia, which has been the source of many fossil human ancestors,
ranging from half a million years old to more than 6 million years old.
First targeted as a study site by French geologist Maurice Taieb in the
1960s, the Middle Awash has been explored since 1981 by a team assembled
by the late UC Berkeley anthropologist J. Desmond Clark, to whom the paper
is dedicated. It has yielded a nearly continuous record of human occupation
that dramatically demonstrates the evolution of modern humans from ape-like
ancestors.
"In the Middle Awash,
we see a chain of ancestors that is powerful evidence for evolution,"
White said. "As we step back in time, we see more and more primitive technology
and anatomy, all the way back to six million years ago, where we see almost
the anatomy of an ape."
Before his death last
month, Clark published an extensive monograph on the primitive stone tools,
including hand axes and cleavers, found around the village of Bouri in
the Middle Awash and used by the Homo erectus population associated
with the new fossil finds.
Gilbert first noticed
the skull or calvaria, which is missing the lower face and teeth, during
a survey near Bouri on Dec. 27, 1997, while scouring the ground in 110-degree
midday heat.
"It was pretty breathtaking,"
Gilbert said. "I got lucky."
Though crushed, the
skull was not fragmented and scattered as are many fossil skulls. White
and Gilbert excavated the rock encasing the skull and transported it back
to Addis Ababa, where the UC Berkeley team spent two years extracting
it from the encrusting rock matrix.
White was disappointed
to find that the lower face was gone. Because of peculiar scratches on
the skull, he thinks the individual may have been killed by a large lion
or hyena, which probably ate the face and gnawed on the skull in an attempt
to extract the brain.
Despite the lack of
the lower part of the skull and the teeth, the calvaria displayed obvious
characteristics of Homo erectus: a shallow forehead sloping back
from massive brow ridges, and an elongated, less spherical brain case.
Asfaw, White and Gilbert compared the specific size and shape of these
features to those of other Homo erectus fossils and found them
to share characteristics with contemporary Homo erectus fossils
from Asia and Africa.
"Before this time,
we really haven't had a good comparison between African and Asian forms
from the same time window," Gilbert said. "We've had early African forms
and late Asian forms, and people have used the differences between them
to generalize about all African and Asian specimens. Now that we have
a later African form for comparison, we are finding that they are very
similar in a lot of the features that people were formerly using to separate
early African from late Asian ones.
"One of the biggest
impacts this calvaria will have on the field is making Homo erectus
look more like a single species again."
Six other Homo
erectus fossils, apparently from separate individuals, also were found
in the area, including three thighbones (femurs) and a shin bone (tibia).
All were from the same sedimentary layer, the Dakanihylo or "Daka" member
of the Bouri formation, which is dated at 1 million years ago.
Gilbert's primary
interest and the subject of his thesis is the diverse fauna of the site,
which included numerous species of pigs, bovids similar to the wildebeest
or gnu, several types of elephants, hippos, a giant hyena and a large
cat.
"The fauna there a
million years ago was in many ways very similar to a modern African fauna,"
he said. "This is actually the first site in East Africa with an extremely
high diversity of alcelaphine bovids, antelope adapted to grazing and
broad expanses of savanna. What we think this means is, we are dealing
with an open, more savanna-like environment."
Other co-authors of
the Nature paper are Yonas Beyene of the Ethiopian Ministry of
Information and Culture, who was involved with the archaeological field
work at the site; Elizabeth Vrba of the Department of Geology and Geophysics
at Yale University, who described many of the bovids from the site; and
geologists William K. Hart of Miami University in Oxford, Ohio, Paul R.
Renne of the Berkeley Geochronology Laboratory and UC Berkeley's Department
of Earth and Planetary Science, and Giday WoldeGabriel of Los Alamos National
Laboratory, who together worked on the field and laboratory geological
investigations of the million-years-old sedimentary rocks.
The work was supported
primarily by the National Science Foundation, with additional funds from
the Institute of Geophysics and Planetary Physics at Los Alamos National
Laboratory and the Department of Geology at Miami University.
 Because
of differences in skull anatomy, some scientists had divided early Homo
fossils into two separate species between 1.5 and 0.5 million years.
The differences between Asian and African fossils were deemed so great
that they would not have been able to interbreed to produce viable offspring.
Here, Dr. Berhane Asfaw and Henry Gilbert compare the newly discovered
Daka Homo erectus with the Asian and African specimens. The newly
discovered specimen is so similar to the Asian fossils that the scientists
concluded that these early hominids constituted a widespread single
species that ranged from Java to Italy to Ethiopia a million years ago.
Asfaw is holding the original Daka fossil with its base toward the photographer,
and is pointing to its massive browridges. Gilbert is holding a cast
a "Peking man" fossil from China in his right hand and a cast of an
earlier Kenyan representative of Homo erectus in his left hand.
Asfaw is pointing out the massive browridges, one of the many features
shared by the newly discovered Ethiopian specimen and the Chinese and
Indonesian representatives of Homo erectus. These anatomical
similarities bridge the continental gaps and unify the species. Photo,
© David L. Brill \ Brill Atlanta |
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