Temporal range: 1.9–1.3 Ma Early Pleistocene
|Skull KNM-ER 3733, discover: Bernard Ngeneo, 1975. (Kenya)|
Groves and Mazák, 1975
Homo ergaster (meaning "working man") or African Homo erectus is an extinct chronospecies of the genus Homo that lived in eastern and southern Africa during the early Pleistocene, that is, between 1.8 million and 1.3 million years ago. It is one of the earliest hominins, which are those hominids that comprise the original members and species of the human clade after splitting from the line of the chimpanzees. (And Homo ergaster is variously thought to be ancestral to, or as sharing a common ancestor with, or as being the same species as, Homo erectus).
Interpreting Homo ergaster inevitably leads to Homo erectus, particularly regarding the taxonomy issues that persist within the scientific community of classifying the two species and separating their two lineages—if indeed they represent two separate lineages rather than one. Some palaeoanthropologists consider H. ergaster to be a variety of H. erectus, that is, the so-called African Homo erectus. Others call H. ergaster the direct ancestor of H. erectus, which then emigrated out of Africa into Eurasia and branched into a distinct species. Still others dispense with the specific epithet ergaster and make no such distinctions among fossils assigned to erectus.
The binomial name was published in 1975 by Groves and Mazák. The specific epithet, "ergaster", is derived from the Ancient Greek ἐργαστήρ "workman", in reference to the advanced lithic technology developed by the species, thereby introducing the Acheulean industry.
Discovery and representative fossils
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.
Classification and distinctions
Paleoanthropologists debate the defining of H. ergaster and H. erectus as separate species. Some consider H. ergaster to be a variety of H. erectus, the so-called African Homo erectus; they propose that H. ergaster emigrated early-out-of-Africa into Eurasia, branching into a distinct species.—and they offer the labels "Homo erectus sensu stricto" (strict sense) for the Asian species and "Homo erectus sensu lato" (broad sense) for the greater species comprising both Asian and African populations. Two major notions for classifying the two species still divide the scientific community; they are: 1) H. ergaster and H. erectus are the same species; or, 2) H. erectus is indeed an Asian species distinct from African H. ergaster. Thus, although "Homo ergaster" has gained some acceptance as a valid taxon, ergaster and erectus are often identified as separate (that is, African or Asian) populations of the larger species H. erectus (sensu lato).
Some scientists dispense altogether with using the specific epithet ergaster, making no distinction, for example, between Turkana Boy and Peking Man as H. erectus fossils; see Interpreting evolution—Springer graph-model. Sura et al (2007) declared that Homo erectus "was a likely source of multiple events of gene flow to the Eurasian continent". In 2003, Anton found the taxonomic issues surrounding Asian vs. African H. erectus somewhat "intractable", and reported: "The H. ergaster question remains famously unresolved" and that same condition persists till today.
The latest discoveries at Dmanisi, Georgia, (see Dmanisi skull) suggest that all the contemporary groups of early Homo in Africa—which include Homo habilis, H. rudolfensis, H. ergaster, and H. erectus—are of the same species, which evolved at least 1.9 million years ago in Africa and later expanded through Eurasia as far as China and Java, where it is documented from about 1.2 mya—and which species is best named H. erectus.Thus, debate continues about the classification, ancestry, and progeny of H. ergaster, but it is still widely accepted to be the direct ancestor of later hominins such as Homo heidelbergensis, Homo neanderthalensis, Asian Homo erectus, and Homo sapiens.
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; that is, had the number of taxa within the Homo genus been seriously overstated? The major implication of the analyses of the Dmanisi skull and the other finds at Dmanisi is that all the earliest varieties of Homo are of one species, and are best classified as Homo erectus. This implies that H. ergaster is subsumed under the taxon H. erectus, and that the two species have much the same ancestry and progeny, and should be classified accordingly.
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.
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 putative ancestor of Homo, and the direct ancestor of H. ergaster. But, habilis's status as a legitimate species within "Homo", is particularly contentious. Apparently, habilis and ergaster coexisted in East Africa for almost a half-million years, which likely indicates that, rather than an anagenetic connection between them, they diverged from a common ancestor.
It is unclear what genetic influence H. ergaster had on later hominins. Recent genetic analysis has generally supported the recent-Out-of-Africa hypothesis, and that same kind of analysis may in time designate H. ergaster as the ancestor to all later hominins.
Origin and extinction
H. ergaster is believed to have diverged from the lineage of H. habilis by 1.8 million years ago. Soon thereafter H. erectus split from ergaster and then emigrated from Africa; or, the inverse may have occurred: there diverged from ergaster a group that emigrated from Africa and then developed as H. erectus. These early descendants of H. habilis may have been discovered at Dmanisi, Georgia as Homo erectus georgicus.
In 2013, a fragment of fossilized jawbone was discovered in the Ledi-Geraru research area in the Afar depression, Ethiopia. The fossil seems to be intermediate between Australopithecus and H. habilis; and, because it is dated to around 2.8 million years ago, it is considered the earliest known evidence of the genus Homo.
H. ergaster remained stable in Africa for about 500 thousand 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 similar H. heidelbergensis in the same region may indicate a hole in the record or that an intermediate species has yet to be discovered.
Use of tools
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.
Use of language
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. And it has been established that the 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.
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By the 1980s, the growing numbers of H. erectus specimens, particularly in Africa, led to the realization that Asian H. erectus (H. erectus sensu stricto), once thought so primitive, was in fact more derived than its African counterparts. These morphological differences were interpreted by some as evidence that more than one species might be included in H. erectus sensu lato (e.g., Stringer, 1984; Andrews, 1984; Tattersall, 1986; Wood, 1984, 1991a, b; Schwartz and Tattersall, 2000) ... Unlike the European lineage, in my opinion, the taxonomic issues surrounding Asian vs. African H. erectus are more intractable. The issue was most pointedly addressed with the naming of "H. ergaster" on the basis of the type mandible KNM-ER 992, but also including the partial skeleton and isolated teeth of KNM-ER 803 among other Koobi Fora remains (Groves and Mazak, 1975). Recently, this specific name was applied to most early African and Georgian H. erectus in recognition of the less-derived nature of these remains vis à vis conditions in Asian H. erectus (see Wood, 1991a, p. 268; Gabunia et al., 2000a). It should be noted, however, that at least portions of the paratype of " H. ergaster " (e.g., KNM-ER 1805) are not included in most current conceptions of that taxon. The " H. ergaster " question remains famously unresolved (e.g., Stringer, 1984; Tattersall, 1986; Wood, 1991a, 1994; Rightmire, 1998b; Gabunia et al., 2000a; Schwartz and Tattersall, 2000), in no small part because the original diagnosis provided no comparison with the Asian fossil record
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