A nearly complete, million-year-old fossilized skull found in the Afar desert of Ethiopia represents the latest clue in the mystery of the evolutionary role of Homo erectus. The discovery, made by a NSF-funded international team led by paleoanthropologist Tim D. White of University of California, Berkeley, supports the theory that one million years ago, Homo erectus was a single species scattered widely throughout Asia, Europe and Africa, and not divided into separate ancestral species.
The fossil remains were found in the Bouri Formation of the Middle Awash project in Ethiopia, a site which has produced several other important hominid fossils. But the new fossils provide the most complete evidence of Homo erectus in that country.
"Both earlier (2.5 myr) and younger fossils are known," says Dr. White, "but we now have strong evidence that the African and Eurasian populations of Homo erectus were one species 1 million years ago."
The discovery is exceptional in that it produced an extremely well preserved and largely intact specimen for analysis, and allowed scientists to arrive at an accurate age via isotopic dating procedures.
The research team's analysis of the new fossil skull showed that it is so similar to its Asian and European contemporaries that it is impossible to cleanly segregate Homo erectus crania between the different continents, and therefore that Homo erectus was probably a single species with gene flow across its known range from Java to Italy to Ethiopia. The description and analysis of the skull were published in Nature, March 2002, by Tim D. White, professor of integrative biology and co-director of UC Berkeley's Laboratory for Human Evolutionary Studies, Berhane Asfaw of the Rift Valley Research Service in Addis Ababa, Ethiopia, and UC Berkeley graduate student W. Henry Gilbert, who found the skull.
Homo erectus originated at an unknown time in an as yet unknown region. Its first known appearance in Africa is placed at 1.7 million years ago. It then spread widely into new habitats. The species eventually "split" into Asian and AfroEuropean branches, possibly because of the major global climatic effects of the Ice Age about 950,000 years ago. The western branch apparently led to Homo sapiens in Africa, and neanderthals in Europe. The neanderthals and the Asian branch of the Homo erectus family tree both eventually died out.
"By 0.5myr there appear to have been two lineages," Dr. White says.
The Pleistocene sedimentary layer in which the current fossilized objects were found is known as the Dakanihylo ("Daka") member of the Bouri formation. Comprised of the silty sand representative of stream and shallow lakeside deposits, the site is estimated by project geologists to have been deposited approximately one million years ago.
The diverse fauna in evidence at the site were representative of a predominantly savannah-type environment, similar in many ways to the current-day African savannah. The vertebrate fauna identified (a total of over 700 specimens) included a large number of bovid fossils, which would indicate a grassland habitat, as well as a large number of hippo fossils, corresponding to a water-margin habitat.
"These are exciting times for this time frame (1myr)," explains Dr. White, "with new discoveries by colleagues in former Soviet Georgia (Dmanisi) as well as others here in Africa (Konso, Rudolf basin, etc.).
The discovery has shed important light on the ancestry of present-day Homo sapiens. But the question of the point of origin of Homo erectus, and the eventual migratory route taken -- either from Africa to Eurasia or vice versa -- remain a mystery.
"Work continues at the Bouri site, on younger rocks," Dr. White says. "The overall Middle Awash project is only 20 years young, and will continue indefinitely."
The on-going work of Dr. White and his team in the Middle Awash Valley is funded by a continuing grant from the Physical Anthropology program of the NSF. The Middle Awash research project is a multidisciplinary, international effort to help illuminate the origin and evolution of human ancestors and close relatives during the past six million years. It gives priority to placing archaeological and paleontological discoveries within accurate time-stratigraphic and paleoenvironmental contexts.
"This project is truly international in scope, with researchers from 14 countries participating to date." Dr. White explains. "It incorporates Ethiopian scholars at the highest level. The research would not have been possible without NSF support."
The Middle Awash is a tectonically active segment of the African rift system. This study area offers a patchwork of sediments exposed by tectonic action and erosion, and calibrated by interbedded volcanic horizons. This unique set of geological circumstances allows the project to simultaneously explore different time horizons, and to generate data bearing on different aspects of human origins and evolution. The study area has yielded fossil hominoid remains spanning five million years; a longer record than is available anywhere else on earth. The research team led by Dr. White is confident that ongoing work there will continue to allow a deeper understanding of human origins and evolution.
For more information, please see:
Posted: January 31, 2003
All photos and illustrations are copyright© of their respective owners and may not be used without permission.