Fossils of Homo ergaster, also known as the "Work Man," have been dispersed remotely throughout Africa from the Olduvai Gorge to the East Turkana region. Dating between 1.9 and 1.4 million years ago, H. ergaster is believed to be the ancestor of H. erectus. However, their distinction as separate species is an ongoing debate amongst anthropologists.
Homo ergaster is considered the African counterpart to the Eurasian Homo erectus. Morphologically, they bear similarities such as a low cranial vault and keeling along the midline along the top of the cranium. They also share similar morphologies in their occipital bone, which displays a rounded torus and a cranial base that is wider than the top of the cranium (bell-shaped). Compared to modern humans, the brain case is more sloping and flattened along the sides compared to the arching sides seen among modern Homo. Bergmann and Allen conclude that hominids thatlive in hot and arid environments will typically have thin, elongated torsos with long limbs to dissapiate the heat. These traits are observable in KNM-WT 15000, a H. ergaster specimen found at the Turkana site in Kenya. H. ergaster is associated with Acheulean stone tools, suggesting a higher cognitive ability amongst their species.
The differences present between H. ergaster and H. erectus are extreme enough to justify two separate species. H. erectus displays thick cranial bones, whereas H. ergaster is more gracile. H. ergaster also lacks an angular torus across the occipital bone, and a defined sulcus behind the browridge. Another modern feature is the double-arched browridge, unlike earlier ancestors with a single, sloping browridge. The nasal aperture also suggests that 3733 had a more human-like nose than ape-like, for the first time in hominid history.
H. ergaster also exhibites a larger body size postcranially than earlier Homo species, standing possibly six feet tall as a mature adult, and presents sexual dimorphism as a species. The type specimen for H. ergaster is KNM-ER 992 (for more information, visit http://efossils.org/page/boneviewer/Homo%20ergaster/KNM-ER%20992), discovered in 1971 by Richard Leakey and dating to approximately 1.5 million years ago. The adult mandible exhibits more modern and gracile morphologies than australopithecines, but more robusticity than its descendants.