Homo ergaster's dispersal is limited to the continent of Africa, and fossil remains have been found in East Africa at Olduvai Gorge and East Turkana, and in South Africa at Swartkrans. Scientists continue to debate whether H. ergaster and Homo erectus are the same species. While some researchers make no distrinction, others argue that H. erectus evolved from the H. ergaster ancestor in Asia, and H. ergaster may be the first human ancestor to populate Europe1. Most African fossils date H. ergaster between 1.9 and 1.4 million years ago (Ma). If the European fossils are included, then the geologic age range of H. ergaster would extended to almost 780,000 years ago (Ka). H. ergaster is also chronologically associated with the invention of hand axes and the Acheulean stone tool technology, which probably required enhanced cognitive ability.1
Broadly speaking, H. ergaster (and its close phyletic Asian relative H. erectus) share a set of novel cranial features that separate them from earlier Homo habilis (from which H. ergaster may have evolved), including a more rounded brain case, a relatively small prognathic face, a pronounced brow ridge, and the development of an occipital torus.1 This species also exhibits reduced postcanine dentition and mandibular robusticity, and an increased brain size to ~900 cc.
An increase in brain size may be accounted for by an increase in body size. Postcranially, H. ergaster had a larger body size than earlier Homo species, exhibited significant sexual dimorphism, and had longer hindlimbs relatively to their forelimbs. An adult H. ergaster may have stood 6 feet or taller. Based on the known poscranial material, the H. ergaster hip is anatomically similar to that of modern humans, indicating that this species was an obligatory biped.2 Inferences about the body plan of H. ergaster have been made from several fossil specimens, including KNM-WT 15000, a nearly complete fossilized skeleton of an adolescent boy that dates to 1.6 Ma. The type specimen for H. ergaster is KNM ER 992 which was found at Koobi Fora and dates to approximately 1.5 Ma.
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