|KNM-ER 3733 (1.6 Million years ago, discovered 1975 at Koobi Fora, Kenya)|
Homo ergaster, also Homo erectus ergaster or African Homo erectus is an extinct chronospecies of the genus Homo that lived in eastern and southern Africa during the early Pleistocene, between about 1.9 million and 1.4 million years ago.
The binomial name was published in 1975 by Groves and Mazák. The specific epithet, "ergaster", is derived from the Ancient Greek ergast?r - "workman", in reference to the advanced lithic technology developed by the species, thereby introducing the Acheulean industry.
KNM-ER 2598, a "H. erectus-like" occipital bone stands as the earliest evidence for H. erectus in Africa at approximately 1.9 million years ago (contemporary with Homo rudolfensis). There is a fossil gap between 1.9 and 1.6 million years ago, KNM-ER 3733 is the oldest known H. ergaster skull dated to about 1.6 million years ago. Its survival past 1.4 million years ago is uncertain, again due to a fossil gap, the next available African fossils allowing reliable morphological analysis are those of Homo rhodesiensis (African H. heidelbergensis), at 0.6 million years ago.
South African palaeontologist John T. Robinson discovered in 1949 a mandible of a new hominin in southern Africa, which he named Telanthropus capensis and which today is classified as Homo ergaster. That taxon was first applied to a mandible found near Lake Rudolf (now Lake Turkana), Kenya, by Colin Groves and Vratislav Mazák in 1975; dubbed KNM-ER 992, it became the type-specimen of the species. A near-complete skeleton of H. ergaster, KNM-WT 15000, or "Turkana Boy", was discovered in 1984 at Lake Turkana by Kamoya Kimeu and Alan Walker. It is dated to 1.6 million years ago (mya) and is one of the most complete early hominin fossils found to date.
Although "Homo ergaster" has gained some acceptance as a valid taxon since its proposal in 1975, ergaster and erectus since the 1980s have increasingly come to be seen as separate (that is, African or Asian) populations of the larger species H. erectus. Some consider H. ergaster to be a variety of H. erectus ("African Homo erectus") that migrated out of Africa, branching into various Eurasian subspecies. In this scenario, the labels "Homo erectus sensu stricto" (strict sense) refers to the Eurasian varieties and "Homo erectus sensu lato" (broad sense) for the greater species comprising both Asian and African populations.  The notation "Homo erectus/ergaster" was also used to express the uncertainty on whether the two designations should be considered synonyms. The question was described as "famously unresolved" as of 2003.[note 1] Suwa et al (2007) concluded that Homo erectus "was a likely source of multiple events of gene flow to the Eurasian continent".
The discoveries of the Dmanisi skulls in the South Caucasus since 2005 have re-opened this question. Their great morphological diversity suggests that the variability of Eurasian H. erectus already includes the African fossils dubbed H. ergaster. The discovery of Dmanisi skull 5 in 2013, dated to 1.8 million years ago, now dates evidence of H. erectus in Eurasia as of virtually the same age as evidence for H. ergaster in Africa, so that it is unclear if the speciation of H. erectus/ergaster from H. habilis took place in Africa or Asia. This has reinforced the trend of considering H. ergaster as synonymous with H. erectus, a species which would have evolved just after 2 million years ago, either in Africa or West Asia, and later dispersed throughout Africa and Eurasia.
Even before the discoveries at Dmanisi there was broad concern about the amount of speciation as a norm described throughout the course of human evolution. A question was raised as to whether the number of taxa within the genus Homo had been seriously overstated.
H. ergaster may be distinguished from H. erectus by its thinner skull-bones and lack of an obvious supraorbital foramen, and from H. heidelbergensis by its thinner bones, more protrusive face, and lower forehead. Derived features separating it from earlier non-Homo species include reduced sexual dimorphism, a smaller, more orthognathous (less protrusive) face, a smaller dental arcade, and a larger cranial capacity (that is, 700-900 cm³ in earlier H. ergaster-specimens, and 900-1100 in later specimens). Remains have been found in Tanzania, Ethiopia, Kenya, and South Africa.
H. erectus/ergaster (H. erectus sensu lato) is the predecessor of Homo heidelbergensis (in Africa also known as Homo rhodesiensis), which arose after about 0.8 million years ago. Late populations of morphological H. erectus were mostly replaced by H. heidelbergensis by 0.5 million years ago. Homo neanderthalensis and Homo sapiens are in turn derived from H. heidelbergensis at about 0.3 million years ago.
There are broad divisions in the scientific community re interpreting the development of the earliest species of genus Homo. H. habilis is generally accepted as the ancestor of Homo, and the direct ancestor of H. ergaster. However, habilis's status as a legitimate species within "Homo" is particularly contentious. Apparently, habilis and ergaster coexisted in East Africa for almost half a million years, which likely indicates that, rather than an anagenetic connection between them, they diverged from a common ancestor.[note 2]
The derivation of H. erectus (H. ergaster) from Homo habilis is thought to have taken place shortly after two million years ago. By 1.8 million years ago, both African (H. ergaster) and Asian (H. erectus georgicus) variants were present. These early descendants of H. habilis may have been discovered at Dmanisi, Georgia.
H. ergaster remained in Africa for about 500,000 years before disappearing from the fossil record after 1.4 million years ago; no identifiable cause has been attributed to the disappearance. The much-later evidence of the derived Homo heidelbergensis in the same region may indicate a gap in the fossil record, with intermediate variants of the period 1.4 to 0.6 million years ago still to be discovered.
H. ergaster used more diverse and sophisticated stone tools than its predecessor, H. habilis. H. ergaster refined the inherited Oldowan technology, then developed the first Acheulean bifacial axes. While the use of Acheulean tools began ca. 1.6 million years ago, the line of H. erectus diverged some 200,000 years before the general innovation of Acheulean technology.
Sexual dimorphism in H. ergaster is greatly reduced from its australopithecine ancestors (around 20%), but still is greater than the dimorphism in modern humans. Diminished dimorphism is speculated to be a sign of reduced competition for mates between males.
Not only was H. ergaster like modern humans in body, but also more in organisation and sociality than any earlier species. It is conceivable that H. ergaster was the first hominin to harness fire: whether as the containment of natural fire, or as the lighting of artificial fire, is still a matter of contention. It is now assumed that H. erectus did have control of fire, as did every other hominin sharing a common ancestor with H. ergaster.
Based on Turkana Boy's cervical vertebrae, which were far narrower than in later humans, it was thought that H. ergaster was restricted in the physical ability to regulate breathing and thereby to produce complex sounds. Later finds, however, disclosed that cervical vertebrae in Dmanisi, which are some 300,000 years older than those of Turkana Boy, were well within the normal range of human vertebrae. It has been established that Turkana Boy probably suffered from a disease of the spinal column that resulted in narrower cervical vertebrae than in modern humans (or in the older Dmanisi finds). While the Dmanisi finds have not been established definitively as H. ergaster, they are older than Turkana Boy (the only definitive H. ergaster vertebrae on record), thereby suggesting kinship to H. ergaster.