Homo erectus, an ancient human ancestor that lived from 1.8 million to 35,000 years ago, is said by theorists of human evolution to have lived alongside Homo sapiens (modern humans) in Indonesia, surviving most other Homo erectus populations that became extinct by 500,000 years ago. Perhaps not so, according to an international team of researchers.
     The work was conducted by a team of scientists (the SoRT, or Solo River Terrace Project) under the direction of anthropologists Etty Indriati of Gadja Mada University, Indonesia, and Susan Antón of New York University. It involved geological surveys, site trenching, archaeological excavations, and analyses of animal remains related to two sites, Ndangong and Jigar, composed of terraces formed by sediment deposits along the Solo river in Indonesia.
     In 1996, scientists dated Homo erectus fossils found at these sites to about 35,000 - 50,000 years ago, based on the dating of associated animal fossil teeth. This dating placed the early human finds contemporaneous with other Homo sapiens finds in Indonesia, suggesting that late-surviving Homo erectus individuals and Homo sapiens (who arrived in Indonesia about 40,000 years ago) shared the same environment at the same time. The SoRT team expedition, however, arrived at different results. Their findings indicate that Homo erectus was extinct in the area by at least 143,000 years ago, and more probably by at least 550,000 years ago, long before the arrival of Homo sapiens.
     This is significant because scholarly critics of an earlier expedition finds have suggested that the sites may have contained a mixture of the fossil remains of younger animals and older homin (early human) fossils, which cast doubt in some minds about the validity of the dates assigned to the homin remains. However, evidence found from the newer excavations indicated that the sediments were actually deposited over a very short time period, and that intermixing of sediments from different time periods, and thus by extension the artifacts and fossils contained within them, did not occur.
     The project team applied three different dating techniques to the finds at the sites. All three depended upon rates of radioactive decay ; the first two, applied to fossil teeth yielded dates approaching 143,000 years. The third methodology was applied to pumice material, a light, porous volcanic rock found within the sediments. The results of this application yielded relatively precise dates around 550,000 years.
     Project scientists posit different plausible theories or possibilities that might account for the enormous gap between the dates obtained, but all agree on one thing - they provide a minimum and a maximum date for a time range that clearly and significantly predates those suggested by the earlier study. According to SoRT, Homo erectus could not have inhabited Indonesia any later than about 143,000 years ago. "Thus," says Indriati, "Homo erectus probably did not share habitats with modern humans."
     The 'Out of Africa' model, perhaps the most widely held theory among evolutionists today, suggests posits that archaic Homo sapiens (an earlier version of modern humans) evolved to anatomically modern humans solely in Africa between 200,000 and 150,000 years ago, and that members of anatomically modern humans left Africa by between 125,000 and 60,000 years ago, replacing earlier human populations such as Neanderthals and Homo erectus over time. Geographic and time period overlapping would be a natural occurrence under this scenario.  The 'Muliti-regional Hypothesis', on the other hand, does not predict such an occurrence. It holds that humans first arose near the beginning of the Pleistocene two million years ago and that evolution occurred within a single species, which included a variety of forms such as Homo erectus and Neanderthals. This species evolved into the currently diverse anatomically modern human populations known today by a combination of adaptations within various regions of the world and gene flow between those regions.

Edited from Popular Archaeology (29 June 2011)