Homo ergaster (literally “work man”) is an extinct hominin that lived in Africa about 1.9 to 1.6 million years ago. The species was named by C. Groves and V. Mazak in 1975, based on the discovery of a mandible known as “KNM-ER 992.” This fossil for which the species is named was found at a site called Koobi Fora, Kenya. It is 1.5 million years old. Important specimens assigned to this species include a very well-preserved cranium (KNM-ER 3733), which is 1.7 million years old, and the famous “Turkana Boy” (KNM-WT 15000), a nearly complete skeleton, which is 1.6 million years old.
As the name suggests, Homo ergaster was a toolmaker. Its fossil remains are typically found in association with stone tools ascribed to the Acheulean tool tradition. These tools consist of primarily hand axes (large tear-drop-shaped stone tools) and cleavers. It was once thought that the invention of these tools led to the spread of early humans outside of Africa. However, it is now known that hominins migrated out of Africa before developing Acheulean tools.
Homo ergaster is a controversial species. Some paleoanthropologists include it within the species Homo erectus, while others consider it a distinct species. Those who agree that there are two species split Homo erectus into early African and Asian species. Homo ergaster is designated as the early African species and Homo erectus is reserved for later hominins found in Asia. These differences reflect disagreements among paleoanthropologists about how many species of hominins are represented by the fossil record.
Many aspects of Homo ergaster and Homo erectus anatomy are similar. Compared with earlier hominins, both have smaller teeth, a shortened face, and projecting nasal bones. The last feature marks the first appearance of the humanlike external nose with the downward-facing nostrils. In addition, both show a significant increase in brain size compared with earlier Homo (Homo habilis). The brain size of Homo ergaster (860-910 cc) was about 75% the size of modern humans (1,000-2,000 cc).
Homo ergaster is primarily defined in terms of how it differs from Homo erectus. Relative to Homo erectus, Homo ergaster had a more rounded, higher cranium with thinner cranial bones. It also lacked a number of distinctive characters seen in Homo erectus. These include the supraorbital sulcus (depression behind the brow ridges), the sagittal keel (a ridge on top of the skull), and the occipital torus (a projection in the back of the skull).
Many paleoanthropologists believe that the differences between Homo erectus and Homo ergaster are too few to warrant assigning them to separate species. However, some recent analyses of the evolutionary relationships among the genus Homo have found that Homo ergaster differs from Homo erectus in many of the same ways that modern humans do. For this reason, other paleoanthropologists consider Homo ergaster the probable ancestor of later Homo populations (for example, Homo sapiens) and that Homo erectus represents an evolutionary dead end.
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