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.
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 many fossil skulls are. 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.