| Homo ergaster Fossil range: Pleistocene | |
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 Skull KNM-ER 3733 discovered by Bernard Ngeneo in 1975 (Kenya) | |
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Simiiformes | |
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Catarrhini | |
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Hominoidea | |
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Hominina |
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H. ergaster | |
Homo ergaster[1] is the name giving to certain fossils of extinct hominin that lived in eastern and southern Africa beginning about 1.9 million years ago during the late Pliocene epoch. Long-standing debate about the classification of H. ergaster has categorized it as a subspecies of Homo erectus, a separate species of African erectus, a broader-defined species (including H. erectus and H. heidelbergensis), or in strict species definition as temporal form of Homo sapiens. Thus dates for its extinction are often subjective to the definition of the hominin.[2] Its very likely that Homo Ergaster is the same species as Homo Erectus and thus may not count as a valid species anymore. Derived features separating it from earlier species include reduced sexual dimorphism (around 25%), a smaller, more orthognathic (less protruding) face, a smaller dental arcade, and a larger cranial capacity (700 - 850 cm³). It is estimated that H. ergaster stood at 1.9 meters (6.2 ft) tall. Remains have been found in Tanzania, Ethiopia, Kenya, and South Africa.
Taxonomy[]
Research history[]
The systematics and taxonomy of Homo in the Early to Middle Pleistocene is one of the most disputed areas of palaeoanthropology. In early palaeoanthropology and well into the twentieth century, it was generally assumed that H. sapiens was the end result of gradual modifications within a single lineage of hominin evolution. As the perceived transitional form between early hominins and modern humans, H. erectus, originally assigned to contain archaic human fossils in Asia, came to encompass a wide range of fossils covering a large span of time (almost the entire temporal range of Homo). Since the late twentieth century, the diversity within H. erectus has led some to question what exactly defines the species and what it should encompass. Some researchers, such as palaeoanthropologist Ian Tattersall in 2013, have questioned H. erectus since it contains an "unwieldly" number of fossils with "substantially differing morphologies".
In the 1970s, palaeoanthropologists Richard Leakey and Alan Walker described a series of hominin fossils from Kenyan fossil localities on the eastern shore of Lake Turkana. The most notable finds were two partial skulls; KNM ER 3733 and KNM ER 3883, found at Koobi Fora. Leakey and Walker assigned these skulls to H. erectus, noting that their brain volumes (848 and 803 cc respectively) compared well to the far younger type specimen of H. erectus (950 cc). Another significant fossil was a fossil mandible recovered at Ileret and described by Leakey with the designation KNM ER 992 in 1972 as "Homo of indeterminate species".
In 1975, palaeoanthropologists Colin Groves and Vratislav Mazák designated KNM ER 992 as the holotype specimen of a distinct species, which they dubbed Homo ergaster. The name (ergaster being derived from the Ancient Greek ἐργαστήρ, ergastḗr, 'workman') roughly translates to "working man" or "workman". Groves and Mazák also included many of the Koobi Fora fossils, such as KNM ER 803 (a partial skeleton and some isolated teeth) in their designation of the species, but did not provide any comparison with the Asian fossil record of H. erectus in their diagnosis, inadvertently causing some of the later taxonomic confusion in regards to the species.
A nearly complete fossil, interpreted as a young male (though the sex is actually undetermined), was discovered at the western shore of Lake Turkana in 1984 by Kenyan archaeologist Kamoya Kimeu. The fossils were described by Leakey and Walker, alongside paleanthropologists Frank Brown and John Harris, in 1985 as KNM-WT 15000 (nicknamed "Turkana Boy"). They interpreted the fossil, consisting of a nearly complete skeleton, as representing H. erectus. Turkana Boy was the first discovered comprehensively preserved specimen of H. ergaster/erectus found and constitutes an important fossil in establishing the differences and similarities between early Homo and modern humans. Turkana Boy was placed in H. ergaster by paleanthropologist Bernard Wood in 1992, and is today, alongside other fossils in Africa previously designated as H. erectus, commonly seen as a representative of H. ergaster by those who support H. ergaster as a distinct species.
Classification[]
H. ergaster is easily distinguished from earlier and more basal species of Homo, notably H. habilis and H. rudolfensis, by a number of features that align them, and their inferred lifestyle, more closely to modern humans than to earlier and contemporary hominins. As compared to their relatives, H. ergaster had body proportions more similar to later members of the genus Homo, notably relatively long legs which would have made them obligately bipedal. The teeth and jaws of H. ergaster are also relatively smaller than those of H. habilis and H. rudolfensis, indicating a major change in diet. In 1999, palaeoanthropologists Bernard Wood and Mark Collard argued that the conventional criteria for assigning species to the genus Homo were flawed and that early and basal species, such as H. habilis and H. rudolfensis, might appropriately be reclassified as ancestral australopithecines. In their view, the true earliest representative of Homo was H. ergaster.
Since its description as a separate species in 1975, the classification of the fossils referred to H. ergaster has been in dispute. H. ergaster was immediately dismissed by Leakey and Walker and many influential researchers, such as palaeoanthropologist G. Philip Rightmire, who wrote an extensive treatise on H. erectus in 1990, continued to prefer a more inclusive and comprehensive H. erectus. Overall, there is no doubt that the group of fossils composing H. erectus and H. ergaster represent the fossils of a more or less cohesive subset of closely related archaic humans. The question is instead whether these fossils represent a radiation of different species or the radiation of a single, highly variable and diverse, species over the course of almost two million years. This long-running debate remains unresolved, with researchers typically using the terms H. erectus s.s. (sensu stricto) to refer to H. erectus fossils in Asia and the term H. erectus s.l. (sensu lato) to refer to fossils of other species that may or may not be included in H. erectus, such as H. ergaster, H. antecessor and H. heidelbergensis.
For obvious reasons, H. ergaster shares many features with H. erectus, such as large forward-projecting jaws, large brow ridges and a receding forehead. Many of the features of H. ergaster are clearly more primitive versions of features later expressed in H. erectus, which somewhat obscures the differences between the two. There are subtle, potentially significant, differences between the East African and East Asian fossils. Among these are the somewhat higher-domed and thinner-walled skulls of H. ergaster, and the even more massive brow ridges and faces of Asian H. erectus.
The question is made more difficult since it regards how much intraspecific variation can be exhibited in a single species before it needs to be split into more, a question that in and of itself does not have a clear-cut answer. A 2008 analysis by anthropologist Karen L. Baab, examining fossils of various H. erectus subspecies, and including fossils attributed to H. ergaster, found that the intraspecific variation within H. erectus was greater than expected for a single species when compared to modern humans and chimpanzees, but fell well within the variation expected for a species when compared to gorillas, and even well within the range expected for a single subspecies when compared to orangutans (though this is partly due to the great sexual dimorphism exhibited in gorillas and orangutans). Baab concluded that H. erectus s.l. was either a single but variable species, several subspecies divided by time and geography or several geographically dispersed but closely related species. In 2015, paleanthropologists David Strait, Frederick Grine and John Fleagle listed H. ergaster as one of the seven "widely recognized" species of Homo, alongside H. habilis, H. rudolfensis, H. erectus, H. heidelbergensis, H. neanderthalensis and H. sapiens, noting that other species, such as H. floresiensis and H. antecessor, were less widely recognised or more poorly known.
Variation in the fossil material[]
Comparing various African fossils attributed to H. erectus or H. ergaster to Asian fossils, notably the type specimen of H. erectus, in 2013, Ian Tattersall concluded that referring to the African material as H. ergaster rather than "African H. erectus" was a "considerable improvement" as there were many autapomorphies distinguishing the material of the two continents from one another. Tattersall believes it to be appropriate to use the designation H. erectus only for eastern Asian fossils, disregarding its previous use as the name for an adaptive grade of human fossils from throughout Africa and Eurasia. Though Tattersall concluded that the H. ergaster material represents the fossils of a single clade of Homo, he also found there to be considerable diversity within this clade; the KNM ER 992 mandible accorded well with other fossil mandibles from the region, such as OH 22 from Olduvai and KNM ER 3724 from Koobi Fora, but did not necessarily match with cranial material, such as KNM ER 3733 and KNM ER 3883 (since neither preserves the jaw), nor with the mandible preserved in Turkana Boy, which has markedly different dentition.
The most "iconic" fossil of H. ergaster is the KNM ER 3733 skull, which is sharply distinguished from Asian H. erectus by a number of characteristics, including that the brow ridges project forward as well as upward and arc separately over each orbit and the braincase being quite tall compared to its width, with its side walls curving. KNM ER 3733 can be distinguished from KNM ER 3883 by a number of features as well, notably in that the margins of KNM ER 3883's brow ridges are very thickened and protrude outwards but slightly downwards rather than upwards. Both skulls can be distinguished from the skull of Turkana Boy, which possesses only slightly substantial thickenings of the superior orbital margins, lacking the more vertical thickening of KNM ER 3883 and the aggressive protrusion of KNM ER 3733. In addition to this, the facial structure of Turkana Boy is narrower and longer than that of the other skulls, with a higher nasal aperture and likely a flatter profile of the upper face. It is possible that these differences can be accounted for through Turkana Boy being a subadult, 7 to 12 years old. Furthermore, KNM ER 3733 is presumed to have been the skull of a female (whereas Turkana Boy is traditionally interpreted as male), which means that sexual dimorphism may account for some of the differences.
The differences between Turkana Boy's skull and KNM ER 3733 and KNM ER 3883, as well as the differences in dentition between Turkana Boy and KNM ER 992 have been interpreted by some, such as paleanthropologist Jeffrey H. Schwartz, as suggesting that Turkana Boy and the rest of the H. ergaster material does not represent the same taxon. Schwartz also noted none of the fossils seemed to represent H. erectus either, which he believed was in need of significant revision. In 2000, French palaeoanthropologist Valéry Zeitoun suggested that KNM ER 3733 and KNM ER 3883 should be referred to two separate species, which she dubbed H. kenyaensis (type specimen KNM ER 3733) and H. okotensis (type specimen KNM ER 3883), but these designations have found little acceptance.
Evolutionary history[]
Evolution and temporal range[]
Although frequently assumed to have originated in East Africa, the origins of H. ergaster are obscured by the fact that the species marks a radical departure from earlier species of Homo and Australopithecus in its long limbs, height and modern body proportions. Though a large number of Pleistocene tools have been found in East Africa, it can not be fully ascertained that H. ergaster originated there without further fossil discoveries. It is assumed that H. ergaster evolved from earlier species of Homo, probably H. habilis. Though populations of H. ergaster outside of Africa have been inferred based on the geographical distribution of their descendants and tools matching those in East Africa, fossils of the species are mainly from East Africa in the time range of 1.8 to 1.7 million years ago. Most fossils have been recovered from around the shores of Lake Turkana in Kenya.
The oldest known specimen of H. erectus s.l. in Africa (i.e. H. ergaster) is DNH 134, a skull recovered in the Drimolen Palaeocave System in South Africa, dated to 2.04 to 1.95 million years ago. The skull is also the oldest known H. erectus s.l. specimen overall, showing clear similarities to KNM ER 3733, and demonstrates that early H. ergaster coexisted with other hominins such as Paranthropus robustus and Australopithecus sediba.
There are also younger specimens of H. ergaster; notably, Turkana Boy is dated to about 1.56 million years ago. A handful of even younger African skulls make the case for long-term anatomical continuity, though it is unclear if they can appropriately be formally regarded as H. ergaster specimens; the "Olduvai Hominid 9" skull from Olduvai Gorge is dated to about 1.2 to 1.1 million years ago and there are also skulls from Buia (near the coast of Eritrea, dated to ~1 million years old), the Bouri Formation in Ethiopia (dated to between 1 million and 780,000 years old) and a fragmentary skull from Olorgesailie in Kenya (dated to between 970,000 and 900,000 years ago). The Olduvai skull is similar to Asian H. erectus in its massive brow ridge, but the others only show minor differences to earlier H. ergaster skulls.
The H. erectus in Asia, as well as later hominins in Europe (i. e. H. heidelbergensis and H. neanderthalensis) and Africa (H. sapiens) are all probably lineages descended from H. ergaster. Because H. ergaster is thought to have been ancestral to these later Homo, it might have persisted in Africa until around 600,000 years ago, when brain size increased rapidly and H. heidelbergensis emerged.
Expansion out of Africa[]
Traditionally, H. erectus was seen as the hominin that first left Africa to colonise Europe and Asia. If H. ergaster is distinct from H. erectus, this role would apply to H. ergaster instead. Very little concrete information is known on when and which Homo first appeared in Europe and Asia, since Early Pleistocene fossil hominins are scarce on both continents, and that it would have been H. ergaster (or "early H. erectus") that expanded, as well as the particular manner in which they did, remains conjecture. The presence of H. erectus fossils in East Asia means that a human species, most likely H. ergaster, had left Africa before 1 million years ago, the assumption historically having been that they first migrated out of Africa around 1.9 to 1.7 million years ago. Discoveries in Georgia and China push the latest possible date further back, before 2 million years ago, also casting doubt on the idea that H. ergaster was the first hominin to leave Africa.
The main reason for leaving Africa is likely to have been an increasing population periodically outgrowing their resource base, with splintering groups moving to establishing themselves in neighboring, empty territories over time. The physiology and improved technology of H. ergaster might have allowed them to travel to and colonise territories that no one had ever occupied before. It is unclear if H. ergaster was truly uniquely capable of expanding outside Africa; australopithecines had likely colonised savannah grasslands throughout Africa by 3 million years ago and there are no clear reasons as to why they would not have been able to expand into the grasslands of Asia before H. ergaster.
The general assumption is that hominins migrated out of the continent either across the southern end of the Red Sea or along the Nile Valley, but there are no fossil hominins known from either region in the Early Pleistocene. The earliest Homo fossils outside Africa are the Dmanisi skulls from Georgia (dated to 1.77–1.85 million years old, representing either early H. ergaster or a new taxon, H. georgicus), three incisors from Ubeidiya in Israel (about 1.4 to 1 million years old) and the fossils of Java Man (H. erectus erectus, more than five thousand miles away). The dating of key Asian H. erectus specimens (including Java Man) is not entirely certain, but they are all likely to be 1.5 million years old or younger. Ubeidiya is also the oldest firmly confirmed site of Acheulean tools (one of the tool industries associated with H. ergaster) outside Africa, the tools recovered there closely resembling older tools discovered in East Africa.
The earliest fossil evidence of Homo in Asia are the aforementioned Dmanisi skulls, which share many traits with H. ergaster in Africa, suggesting that H. ergaster might have expanded out of Africa as early as 1.7–1.9 million years ago. In addition to H. ergaster-like traits, the Dmanisi skulls possess a wide assortment of other traits, some of which are similar to traits in earlier hominins such as H. habilis, and the site notably lacks preserved hand axes (otherwise characteristic of H. ergaster), which means that hominins might have spread out of Africa even earlier than H. ergaster. The skull D2700 (Dmanisi skull 3) in particular resembles H. habilis in the small volume of its braincase (600 cc), the form of the middle and upper face and the lack of an external nose. The mixture of skulls at Dmanisi suggests that the definition of H. ergaster (or H. erectus) might most appropriately be expanded to contain fossils that would otherwise be assigned to H. habilis or that two separate species of archaic humans left Africa early on. In addition to the Dmanisi fossils, stone tools manufactured by hominins have been discovered on the Loess Plateau in China and dated to 2.12 million years old, meaning that hominins must have left Africa before that time.
An alternative hypothesis historically has been that Homo evolved in Asia from earlier ancestors that had migrated there from Africa, and then expanded back into Europe, where it gave rise to H. sapiens. This view was notably held by Eugène Dubois, who first described H. erectus fossils in the 19th century and considered the fossils of Java Man, at the time undeniably the earliest known hominin fossils, as proof of the hypothesis. Though the discovery of australopithecines and earlier Homo in Africa meant that Homo itself did not originate in Asia, the idea that H. erectus (or H. ergaster) in particular did, and then expanded back into Africa, has occasionally resurfaced. Various fossil discoveries have been used to support it through the years, perhaps most famously a massive set of jaws from Indonesia which were perceived to be similar to those of australopithecines and dubbed Meganthropus (now believed to be an unrelated hominid ape). The discovery of H. floresiensis in 2003, which preserved primitive foot and wrist anatomy reminiscent of that of H. habilis and Australopithecus again led to suggestions of pre-erectus hominins in Asia, though there are no known comparable foot or wrist bones from H. erectus which makes comparisons impossible. The idea that H. ergaster/H. erectus first evolved in Asia before expanding back into Africa was substantially weakened by the dating of the DNH 134 skull as approximately 2 million years old, predating all other known H. ergaster/H. erectus fossils.
Anatomy[]
Build and appearance[]
The only well-preserved post-cranial remains of H. ergaster come from the Turkana Boy fossil. Unlike the australopithecines, Turkana Boy's arms were not longer relative to their legs than the arms of living people and the cone-shaped torso of their ancestors had evolved into a more barrel-shaped chest over narrow hips, another similarity to modern humans. The tibia (shin bone) of Turkana Boy is relatively longer than the same bone in modern humans, potentially meaning that there was more bend in the knee when walking. The slim and long build of Turkana Boy may be explained by H. ergaster living in hot and arid, seasonal environments. Through thinning of the body, body volume decreases faster than skin area and greater skin area means more effective heat dissipation.
H. ergaster individuals were significantly taller than their ancestors. Whereas Lucy, a famous Australopithecus fossil, would only have been about 1 m (3 ft 3 in) tall at her death, Turkana Boy was about 1.62 m (5 ft 4 in) tall and would probably have reached 1.82 m (6 ft) or more if he had survived to adulthood. Adult H. ergaster are believed to have ranged in size from about 1.45 to 1.85 m (4 ft 9 in to 6 ft 1 in) tall.
Because of being adapted to a hot and arid climate, H. ergaster might also have been the earliest human species to have nearly hairless and naked skin. If instead H. ergaster had an ape-like covering of body hair, sweating (the primary means through which modern humans prevent their brains and bodies from overheating) would not have been as efficient. Though sweating is the generally accepted explanation for hairlessness, other proposed explanations include a reduction of parasite load and sexual selection. It is doubtful if australopithecines and earlier Homo were sufficiently mobile to make hair loss an advantageous trait, whereas H. ergaster was clearly adapted for long-distance travel and noted for inhabiting lower altitudes (and open, hot savannah environments) than their ancestors. Australopithecines typically inhabited colder and higher altitudes 1,000–1,600 m (3,300–5,200 ft), where nighttime temperatures would have gotten significantly colder and insulating body hair may have been required.
Alternatively and despite this, the loss of body hair could have occurred significantly earlier than H. ergaster. Though skin impressions are unknown in any extinct hominin, it is possible that human ancestors were already losing their body hair around 3 million years ago. Human ancestors acquired pubic lice from gorillas about 3 million years ago, and speciation of human from gorilla pubic lice was potentially only possible because human ancestors had lost most of their body hair by this early date. It is also possible that the loss of body hair occurred at a significantly later date. Genetic analysis suggests that high activity in the melanocortin 1 receptor, which produces dark skin, dates back to about 1.2 million years ago. This could indicate the evolution of hairlessness around this time, as a lack of body hair would have left the skin exposed to harmful UV radiation.
Skull and face[]
Differences to modern humans would have been readily apparent in the face and skull of H. ergaster. Turkana Boy's brain was almost fully grown at the time of his death, but its volume (at 880 cc) was only about 130 cc greater than the maximum found in H. habilis, about 500 cc below the average of modern humans. The 130 cc increase from H. habilis becomes much less significant than what could be presumed when the larger body size of Turkana Boy and H. ergaster is considered. With all H. ergaster skulls considered, the brain volume of the species mostly varied between 600 and 910 cc, with some small examples only having a volume of 508–580 cc. Since their brain was smaller than that of modern humans, the skull of H. ergaster immediately narrowed behind the eye sockets (post-orbital constriction).
The brain case was long and low, and Turkana Boy's forehead was flat and receding, merging at an angle with the brow ridge above their eyes. A noticeable difference between Turkana Boy and the australopithecines and H. habilis would have been their nose, which would have been similar to that of modern humans in projecting forwards and having nostrils oriented downwards. This external nose may have also been an adaptation towards a warmer climate, since the noses of modern humans are usually cooler than their central bodies, condensing moisture that would otherwise have been exhaled and lost during periods of increased activity. The face of Turkana Boy would have been longer from top to bottom than that of modern humans, with the jaws projecting farther outwards (prognathism). Though the jaws and teeth were smaller than those of the average australopithecine and H. habilis, they were still significantly larger than those of modern humans. Since the jaw slanted sharply backwards, it is probable that they were chinless.
The overall structure of Turkana Boy's skull and face is also reflected in other H. ergaster skulls, which combine large and outwardly projecting faces with brow ridges, receding foreheads, large teeth and projecting nasal bones. Though Turkana Boy would have been no more than 12 years old when he died, their stature is more similar to that of a modern 15-year-old and the brain is comparable to that of a modern 1-year-old. By modern standards, H. ergaster would thus have been cognitively limited, though the invention of new tools prove that they were more intelligent than their predecessors.
Body mass and sexual dimorphism[]
H. ergaster possessed a significantly larger body mass in comparison to earlier hominins such as early Homo, Australopithecus and Paranthropus. Whereas australopithecines typically ranged in weight from 29–48 kg (64–106 lbs), H. ergaster typically ranged in weight from 52–63 kg (115–139 lbs). It is possible that the increased body size was the result of life in an open savannah environment, where increased size gives the ability to exploit broader diets in larger foraging areas, increases mobility and also gives the ability to hunt larger prey. The increased body mass also means that parents would have been able to carry their children to an older age and larger mass.
Though reduced sexual dimorphism has often been cited historically as one of the radical differences between H. ergaster and earlier Homo and australopithecines, it is unclear whether australopithecines were significantly more sexually diamorphic than H. ergaster or modern humans. Skeletal evidence suggests that sexes in H. ergaster differed no more in size than sexes in modern humans do, but a 2003 study by palaeoanthropologists Philip L. Reno, Richard S. Meindl, Melanie A. McCollum and C. Owen Lovejoy suggested that the same was also true for the significantly earlier Australopithecus afarensis. Sexual dimorphism is difficult to measure in extinct species since the sex of fossils is usually not determinable. Historically, scientists have typically measured differences between the extreme ends (in terms of size and morphology) of the fossil material attributed to a species and assumed that the resulting ratio applies to the mean difference between male and female individuals.
Growth and development[]
The dimensions of a 1.8 million years old adult female H. ergaster pelvis from Gona, Ethiopia suggests that H. ergaster would have been capable of birthing children with a maximum prenatal (pre-birth) brain size of 315 cc, about 30–50 % of adult brain size. This value falls intermediately between that of chimpanzees (~40 %) and modern humans (28%). Further conclusions about the growth and development in early Homo can be drawn from the Mojokerto child, a ~1.4–1.5 million year old ~1-year old Asian H. erectus, which had a brain at about 72–84% the size of an adult H. erectus brain, which suggests a brain growth trajectory more similar to that of other great apes than of modern humans. Both the Gona pelvis and the Mojokerto child suggest that the prenatal growth of H. ergaster was similar to that of modern humans but that the postnatal (post-birth) growth and development was intermediate between that of chimpanzees and modern humans. The faster development rate suggests that altriciality (an extended childhood and a long period of dependency on your parents) evolved at a later stage in human evolution, possibly in the last common ancestor of Neanderthals and modern humans. The faster development rate might also indicate that the expected lifespan of H. ergaster and H. erectus was lower than that of later and modern humans.
Culture[]
References[]
(Possiblility of H. Ergaster being the same species as H> Erectus)- https://humanorigins.si.edu/evidence/human-fossils/species/homo-erectus#:~:text=Early%20African%20Homo%20erectus%20fossils,the%20size%20of%20the%20torso.
http://www.enzimuseum.org/the-stone-age/stone-age-ancestors/homo-ergaster
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