Homo ergaster is an extinctspecies orsubspecies ofarchaic humans who lived inAfrica in theEarly Pleistocene. WhetherH. ergaster constitutes a species of its own or should be subsumed intoH. erectus is an ongoing and unresolved dispute withinpalaeoanthropology. Proponents of synonymisation typically designateH. ergaster as "AfricanHomo erectus"[2] or "Homo erectus ergaster".[3] The nameHomo ergaster roughly translates to "working man", a reference to the more advanced tools used by the species in comparison to those of their ancestors. The fossil range ofH. ergaster mainly covers the period of 1.7 to 1.4 million years ago, though a broader time range is possible.[4] Though fossils are known from across East and Southern Africa, mostH. ergaster fossils have been found along the shores ofLake Turkana in Kenya. There are later African fossils, some younger than 1 million years ago, that indicate long-term anatomical continuity, though it is unclear if they can be formally regarded asH. ergaster specimens. As achronospecies,H. ergaster may have persisted to as late as 600,000 years ago, when new lineages ofHomo arose in Africa.
Those who believeH. ergaster should be subsumed intoH. erectus consider there to be too little difference between the two to separate them into distinct species. Proponents of keeping the two species as distinct cite morphological differences between the African fossils andH. erectus fossils from Asia, as well as earlyHomo evolution being more complex than what is implied by subsuming species such asH. ergaster intoH. erectus. Additionally, morphological differences between the specimens commonly seen as constitutingH. ergaster might suggest thatH. ergaster itself does not represent a cohesive species. Regardless of their most correct classification,H. ergaster exhibit primitive versions of traits later expressed inH. erectus and are thus likely the direct ancestors of laterH. erectus populations in Asia. Additionally,H. ergaster is likely ancestral to laterhominins in Europe and Africa, such as modernhumans andNeanderthals.
Several features distinguishH. ergaster fromaustralopithecines as well as earlier and more basal species ofHomo, such asH. habilis. Among these features are their larger body mass, relatively long legs, obligatebipedalism, relatively small jaws and teeth (indicating a major change in diet) as well as body proportions and inferred lifestyles more similar to modern humans than to earlier and contemporary hominins. With these features in mind, some researchers viewH. ergaster as being the earliest true representative of the genusHomo.
H. ergaster lived on the savannah in Africa, a unique environment with challenges that would have resulted in the need for many new and distinct behaviours. EarlierHomo probably used counter-attack tactics, like modern primates, to keep predators away. By the time ofH. ergaster, this behaviour had probably resulted in the development of truehunter-gatherer behaviour, a first among primates.H. ergaster was anapex predator.[5] Further behaviours that might first have arisen inH. ergaster include male-female divisions of foraging and truemonogamous pair bonds.H. ergaster also marks the appearance of more advanced tools of theAcheulean industry, including the earliest-knownhand axes. Though undisputed evidence is missing,H. ergaster might also have been the earliest hominin tomaster control of fire.
The systematics and taxonomy ofHomo in the Early to MiddlePleistocene is one of the most disputed areas ofpalaeoanthropology.[6] In early palaeoanthropology and well into the twentieth century, it was generally assumed thatHomo 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 ofHomo). Since the late twentieth century, the diversity withinH. erectus has led some to question what exactly defines the species and what it should encompass. Some researchers, such as palaeoanthropologistIan Tattersall in 2013, have questionedH. erectus since it contains an "unwieldly" number of fossils with "substantially differing morphologies".[7]
In the 1970s, palaeoanthropologistsRichard Leakey andAlan Walker described a series of hominin fossils from Kenyan fossil localities on the eastern shore ofLake Turkana. The most notable finds were two partial skulls;KNM ER 3733 andKNM ER 3883, found atKoobi Fora. Leakey and Walker assigned these skulls toH. erectus, noting that their brain volumes (848 and 803 cc, respectively) compared well to the far younger type specimen ofH. erectus (950 cc). Another significant fossil was a fossilmandible recovered atIleret and described by Leakey with the designationKNM ER 992 in 1972 as "Homo of indeterminate species".[8]
In 1975, palaeoanthropologistsColin Groves andVratislav Mazák designated KNM ER 992 as theholotype specimen of a distinct species, which they dubbedHomo ergaster.[9] The name (ergaster being derived from theAncient Greek ἐργαστήρ,ergastḗr, 'workman') roughly translates to "working man"[10] or "workman".[11] 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 ofH. erectus in their diagnosis, inadvertently causing some of the later taxonomic confusion in regards to the species.[12]
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 archaeologistKamoya Kimeu.[11] The fossils were described by Leakey and Walker, alongside paleoanthropologists 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 representingH. erectus.[13] Turkana Boy was the first discovered comprehensively preserved specimen ofH. ergaster/erectus found and constitutes an important fossil in establishing the differences and similarities between earlyHomo and modern humans.[14] Turkana Boy was placed inH. ergaster by paleoanthropologist Bernard Wood in 1992,[11] and is today, alongside other fossils in Africa previously designated asH. erectus, commonly seen as a representative ofH. ergaster by those who supportH. ergaster as a distinct species.[15]
H. ergaster is easily distinguished from earlier and more basal species ofHomo, notablyH. habilis andH. 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 genusHomo, notably relatively long legs which would have made them obligately bipedal. The teeth and jaws ofH. ergaster are also relatively smaller than those ofH. habilis andH. rudolfensis, indicating a major change in diet.[16] In 1999, palaeoanthropologists Bernard Wood and Mark Collard argued that the conventional criteria for assigning species to the genusHomo were flawed and that early and basal species, such asH. habilis andH. rudolfensis, might appropriately be reclassified as ancestralaustralopithecines. In their view, the true earliest representative ofHomo wasH. ergaster.[17]
Since its description as a separate species in 1975, the classification of the fossils referred toH. 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 onH. erectus in 1990, continued to prefer a more inclusive and comprehensiveH. erectus. Overall, there is no doubt that the group of fossils composingH. erectus andH. 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.[9] This long-running debate remains unresolved, with researchers typically using the termsH. erectus s.s. (sensu stricto) to refer toH. erectus fossils in Asia and the termH. erectus s.l. (sensu lato) to refer to fossils of other species that may or may not be included inH. erectus, such asH. ergaster,H. antecessor andH. heidelbergensis.[19]
For obvious reasons,H. ergaster shares many features withH. erectus, such as large forward-projecting jaws, large brow ridges and a receding forehead.[20] Many of the features ofH. ergaster are clearly more primitive versions of features later expressed inH. erectus, which somewhat obscures the differences between the two.[21] 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 ofH. ergaster, and the even more massivebrow ridges and faces of AsianH. erectus.[20]
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 variousH. erectus subspecies, and including fossils attributed toH. ergaster, found that the intraspecific variation withinH. erectus was greater than expected for a single species when compared to modernhumans andchimpanzees, but fell well within the variation expected for a species when compared togorillas, and even well within the range expected for a single subspecies when compared toorangutans (though this is partly due to the greatsexual dimorphism exhibited in gorillas and orangutans).[22] Baab concluded thatH. 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.[23] In 2015, paleoanthropologists David Strait, Frederick Grine and John Fleagle listedH. ergaster as one of the seven "widely recognized" species ofHomo, alongsideH. habilis,H. rudolfensis,H. erectus,H. heidelbergensis,H. neanderthalensis andH. sapiens, noting that other species, such asH. floresiensis andH. antecessor, were less widely recognised or more poorly known.[18]
Comparing various African fossils attributed toH. erectus orH. ergaster to Asian fossils, notably the type specimen ofH. erectus, in 2013, Ian Tattersall concluded that referring to the African material asH. ergaster rather than "AfricanH. erectus" was a "considerable improvement" as there were many autapomorphies distinguishing the material of the two continents from one another.[24] Tattersall believes it to be appropriate to use the designationH. 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 theH. ergaster material represents the fossils of a single clade ofHomo, 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 asOH 22 fromOlduvai andKNM 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.[24]
The most "iconic" fossil ofH. ergaster is the KNM ER 3733 skull, which is sharply distinguished from AsianH. 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.[25] 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.[26] 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.[14]
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 paleoanthropologist Jeffrey H. Schwartz, as suggesting that Turkana Boy and the rest of theH. ergaster material does not represent the same taxon. Schwartz also noted none of the fossils seemed to representH. erectus either, which he believed was in need of significant revision.[27] In 2000, French palaeoanthropologist Valéry Zeitoun suggested that KNM ER 3733 and KNM ER 3883 should be referred to two separate species, which she dubbedH. kenyaensis (type specimen KNM ER 3733) andH. okotensis (type specimen KNM ER 3883), but these designations have found little acceptance.[28]
Although frequently assumed to have originated inEast Africa, the origins ofH. ergaster are obscured by the fact that the species marks a radical departure from earlier species ofHomo andAustralopithecus 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 thatH. ergaster originated there without further fossil discoveries.[29] It is assumed thatH. ergaster evolved from earlier species ofHomo, probablyH. habilis. Though populations ofH. 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.[10]
The oldest known specimen ofH. erectus s.l. in Africa (i.e.H. ergaster) isDNH 134, a skull recovered in theDrimolen Palaeocave System in South Africa, dated to 2.04 to 1.95 million years ago. The skull is also the oldest knownH. erectus s.l. specimen overall, showing clear similarities to KNM ER 3733, and demonstrates that earlyH. ergaster coexisted with other hominins such asParanthropus robustus andAustralopithecus sediba.[30]
There are also younger specimens ofH. ergaster; notably, Turkana Boy is dated to about 1.56 million years ago.[10] 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 asH. 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 AsianH. erectus in its massive brow ridge, but the others only show minor differences to earlierH. ergaster skulls.[31]
TheH. erectus in Asia, as well as later hominins in Europe (i. e.H. heidelbergensis andH. neanderthalensis) and Africa (H. sapiens) are all probably lineages descended fromH. ergaster.[10] BecauseH. ergaster is thought to have been ancestral to these laterHomo, it might have persisted in Africa until around 600,000 years ago, when brain size increased rapidly andH. heidelbergensis emerged.[32]
Traditionally,H. erectus was seen as the hominin that first left Africa to colonise Europe and Asia. IfH. ergaster is distinct fromH. erectus, this role would apply toH. ergaster instead.[24][10] Very little concrete information is known on when and whichHomo first appeared in Europe and Asia, since Early Pleistocene fossil hominins are scarce on both continents, and that it would have beenH. ergaster (or "earlyH. erectus") that expanded, as well as the particular manner in which they did, remains conjecture.[20] The presence ofH. erectus fossils in East Asia means that a human species, most likelyH. ergaster, had left Africa before 1 million years ago,[33] the assumption historically having been that they first migrated out of Africa around 1.9 to 1.7 million years ago.[20] Discoveries in Georgia and China push the latest possible date further back, before 2 million years ago, also casting doubt on the idea thatH. ergaster was the first hominin to leave Africa.[34]
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 ofH. ergaster might have allowed them to travel to and colonise territories that no one had ever occupied before.[33] It is unclear ifH. 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 beforeH. ergaster.[19]
The general assumption is that hominins migrated out of the continent either across the southern end of theRed Sea or along theNile Valley, but there are no fossil hominins known from either region in the early Pleistocene. The earliestHomo fossils outside Africa are theDmanisi skulls from Georgia (dated to 1.77–1.85 million years old,[34] representing either earlyH. ergaster or a new taxon,H. georgicus), three incisors fromUbeidiya in Israel (about 1.4 to 1 million years old) and the fossils ofJava Man (H. erectus erectus, more than five thousand miles away).[29] The dating of key AsianH. erectus specimens (including Java Man) is not entirely certain, but they are all likely to be 1.5 million years old or younger.[20] Ubeidiya is also the oldest firmly confirmed site ofAcheulean tools (one of the tool industries associated withH. ergaster) outside Africa, the tools recovered there closely resembling older tools discovered in East Africa.[33]
The earliest fossil evidence ofHomo in Asia are the aforementioned Dmanisi skulls, which share many traits withH. ergaster in Africa, suggesting thatH. ergaster might have expanded out of Africa as early as 1.7–1.9 million years ago.[35] In addition toH. ergaster-like traits, the Dmanisi skulls possess a wide assortment of other traits, some of which are similar to traits in earlier hominins such asH. habilis, and the site notably lacks preserved hand axes (otherwise characteristic ofH. ergaster), which means that hominins might have spread out of Africa even earlier thanH. ergaster.[19] The skullD2700 (Dmanisi skull 3) in particular resemblesH. 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 ofH. ergaster (orH. erectus) might most appropriately be expanded to contain fossils that would otherwise be assigned toH. habilis or that two separate species of archaic humans left Africa early on.[31] In addition to the Dmanisi fossils, stone tools manufactured by hominins have been discovered on theLoess Plateau in China and dated to 2.12 million years old, meaning that hominins must have left Africa before that time.[34]
An alternative hypothesis historically has been thatHomo evolved in Asia from earlier ancestors that had migrated there from Africa, and then expanded back into Europe, where it gave rise toH. sapiens. This view was notably held byEugène Dubois, who first describedH. 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 earlierHomo in Africa meant thatHomo itself did not originate in Asia, the idea thatH. erectus (orH. ergaster) in particular did, and then expanded back into Africa, has occasionally resurfaced.[36] Various fossil discoveries have been used to support it through the years, including a massive set of jaws from Indonesia which were perceived to be similar to those of australopithecines and dubbedMeganthropus (now believed to be an unrelatedhominidape). The discovery ofH. floresiensis in 2003, which preserved primitive foot and wrist anatomy reminiscent of that ofH. habilis andAustralopithecus again led to suggestions of pre-erectus hominins in Asia, though there are no known comparable foot or wrist bones fromH. erectus which makes comparisons impossible.[37] The idea thatH. 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 knownH. ergaster/H. erectus fossils.[30]
The only well-preserved post-cranial remains ofH. 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.[38] Thetibia (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.[39] The slim and long build of Turkana Boy may be explained byH. 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.[40]
H. ergaster individuals were significantly taller than their ancestors. WhereasLucy, a famousAustralopithecus 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.[38] AdultH. 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.[39]
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 insteadH. 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.[40] Though sweating is the generally accepted explanation for hairlessness, other proposed explanations include a reduction ofparasite load[41] andsexual selection.[42] It is doubtful if australopithecines and earlierHomo were sufficiently mobile to make hair loss an advantageous trait, whereasH. 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.[43]
Alternatively and despite this, the loss of body hair could have occurred significantly earlier thanH. 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 acquiredpubic 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.[44] It is also possible that the loss of body hair occurred at a significantly later date. Genetic analysis suggests that high activity in themelanocortin 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 harmfulUV radiation.[45]
Differences to modern humans would have been readily apparent in the face and skull ofH. 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 inH. habilis, about 500 cc below the average of modern humans. The 130 cc increase fromH. habilis becomes much less significant than what could be presumed when the larger body size of Turkana Boy andH. ergaster is considered.[46] With allH. 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 ofH. ergaster immediately narrowed behind the eye sockets (post-orbital constriction).[39]
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 andH. 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.[46] 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 andH. habilis, they were still significantly larger than those of modern humans. Since the jaw slanted sharply backwards, it is probable that they were chinless.[40]
The overall structure of Turkana Boy's skull and face is also reflected in otherH. ergaster skulls, which combine large and outwardly projecting faces with brow ridges, receding foreheads, large teeth and projecting nasal bones.[40] 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.[47]
H. ergaster possessed a significantly larger body mass in comparison to earlier hominins such as earlyHomo,Australopithecus andParanthropus.[16] 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).[48] 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.[16] The increased body mass also means that parents would have been able to carry their children to an older age and larger mass.[48]
Though reducedsexual dimorphism has often been cited historically as one of the radical differences betweenH. ergaster and earlierHomo and australopithecines,[16][29] it is unclear whether australopithecines were significantly more sexually diamorphic thanH. ergaster or modern humans.[49] Skeletal evidence suggests that sexes inH. ergaster differed no more in size than sexes in modern humans do,[47] 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 earlierAustralopithecus afarensis.[49] 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.[50]
The dimensions of a 1.8 million years old adult femaleH. ergaster pelvis fromGona, Ethiopia suggests thatH. 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%).[51] Further conclusions about the growth and development in earlyHomo can be drawn from theMojokerto child, a ~1.4–1.5-million-year-old ~one-year-old AsianH. erectus, which had a brain at about 72–84% the size of an adultH. erectus brain, which suggests a brain growth trajectory more similar to that of othergreat apes than of modern humans.[52] Both the Gona pelvis and the Mojokerto child suggest that theprenatal growth ofH. 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.[51] The faster development rate suggests thataltriciality (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.[52] The faster development rate might also indicate that the expected lifespan ofH. ergaster andH. erectus was lower than that of later and modern humans.[53]
Jaw ofHomo ergaster (KNM ER 992 in the top-right, labelled asHomo erectus in the image) compared to jaws of other members of the genusHomo
It is frequently assumed that the larger body and brain size ofH. ergaster, compared to its ancestors, would have brought with it increased dietary and energy needs.[48] In 2002, palaeoanthropologists Leslie C. Aiello and Jonathan C. K. Wells stated that the average resting metabolic requirements ofH. ergaster would have been 39% higher than those ofAustralopithecus afarensis, 30% higher in males and 54% higher in females.[54] However, the torso proportions ofH. ergaster implies a relatively small gut,[54] which means that energy needs might not necessarily have been higher inH. ergaster than in earlier hominins. This is because the earlier ape (and australopithecine) gut was large and energy-expensive since it needed to synthesize fat through fermenting plant matter, whereasH. ergaster likely ate significantly more animal fat than their predecessors. This would have allowed more energy to be diverted to brain growth, increasing brain size while maintaining the energy requirements of earlier species.[55][56]
If they had increased energy requirements,H. ergaster would have needed to eat either vastly more food than australopithecines, or would have needed to eat food of superior quality. If they ate the same type of foods as the australopithecines, feeding time would then have had to be dramatically increased in proportion to the extra calories required, reducing the timeH. ergaster could use for resting, socialising and travelling. Though this would have been possible, it is considered unlikely, especially since the jaws and teeth ofH. ergaster are reduced in size compared to those of the australopithecines, suggesting a shift in diet away from fibrous and difficult-to-chew foods. Regardless of energy needs, the small gut ofH. ergaster also suggests a more easily digested diet composed of food of higher quality.[54]
It is likely thatH. ergaster consumed meat in higher proportions than the earlier australopithecines.[54] Meat was probably acquired through a combination of ambushes, active hunting and confrontational scavenging.H. ergaster must not only have possessed the ability ofendurance running, but must also have been able to defend themselves and the carcasses of their prey from the variety of contemporary African predators. It is possible that a drop in Africancarnivoran species variety around 1.5 million years ago can be ascribed to competition with opportunistic and carnivorous hominins.[57]
On its own, meat might not have been able to fully sustainH. ergaster. Modern humans can not sufficiently metabolize protein to meet more than 50% of their energy needs and modern humans who heavily rely on animal-based products in their diet mostly rely on fat to sustain the rest of their energy requirements.[54] Multiple reasons make a fully meat-based diet inH. ergaster unlikely, the most prominent being that Africanungulates (the primary prey available) are relatively low in fat and that high meat diets demand increased intake of water, which would have been difficult in an open and hot environment. Modern Africanhunter-gatherers who rely heavily on meat, such as theHadza andSan peoples, also use cultural means to recover the maximum amount of fat from the carcasses of their prey, a method that would not have been available toH. ergaster.[58]
H. ergaster would thus likely have consumed large quantities of meat, vastly more than their ancestors, but would also have had to make use of a variety of other food sources, such as seeds, honey, nuts, invertebrates,[58] nutritious tubers, bulbs and other underground plant storage organs.[40] The relatively small chewing capacity ofH. ergaster, in comparison to its larger-jawed ancestors, means that the meat and high quality plant food consumed would likely have required the use of tools to process before eating.[59]
Diagram of fossil trackways from two sites nearIleret, Kenya attributed toHomo ergaster/Homo erectus
H. ergaster lived on the African savannah, which during the Pleistocene was home to a considerably more formidable community of carnivorans than the present savannah. Hominins could probably only have adapted to life on the savannah if effective anti-predator defense behaviours had already evolved. Defense against predators would likely have come throughH. ergaster living in large groups, possessing stone (and presumably wooden) tools and effective counter-attack behaviour having been established. In modern primates that spend significant amounts of time on the savannah, such as chimpanzees andsavannah baboons, individuals form large, multi-male, groups wherein multiple males can effectively work together to fend off and counter-attack predators, occasionally with the use of stones or sticks, and protect the rest of the group. It is possible that similar behaviour was exhibited in earlyHomo. Based on the male-bonded systems withinbonobos and chimpanzees, and the tendency towards male bonding in modern foragers, groups of earlyHomo might have been male-bonded as well. Because of the scarcity of fossil material, group size in earlyHomo cannot be determined with any certainty. Groups were probably large, it is possible groups were above the upper range of known group sizes among chimpanzees and baboons (c. 100 individuals or more).[60] In 1993, palaeoanthropologists Leslie C. Aiello and R. I. M. Dunbar estimated that the group size ofH. habilis andH. rudolfensis, based onneocortex size (as there is a known relationship between neocortex size and group size in modern non-human primates),[61] would have ranged from about 70–85 individuals.[62] With the additional factor of bipedalism, which is energetically cheaper than quadrupedalism, the maximum ecologically tolerable group size may have been even larger.[60] Aiello's and Dunbar's group size estimates in regards toH. ergaster was 91–116 individuals.[62]
Social and counter-attack behaviour of earlierHomo probably carried over intoH. ergaster, where they are likely to have developed even further.H. ergaster was probably the first primate to move into the niche of social carnivore (i. e.hunter-gatherer).[60] Such behaviour would probably have been the result of counter-attacks in the context of competition over nutritious food with other carnivores and would probably have evolved from something akin to the opportunistic hunting sometimes exhibited by chimpanzees. The switch to predation in groups might have triggered a cascade of evolutionary changes which changed the course of human evolution. Cooperative behaviours such as opportunistic hunting in groups, predator defense and confrontational scavenging would have been critical for survival which means that a fundamental transition in psychology gradually transpired. With the typical "competitive cooperation" behaviour exhibited by most primates no longer being favored through natural selection and social tendencies taking its place, hunting, and other activities, would have become true collaborative efforts. Because counter-attack behaviour is typically exhibited in males of modern primates, social hunting in archaic humans is believed to have been a primarily male activity. Females likely conducted other types of foraging, gathering food which did not require hunting (i.e. fruits, nuts, eggs, etc.).[63]
Reconstruction ofH. ergaster, American Museum of Natural History
With hunting being a social activity, individuals probably shared the meat with one another, which would have strengthened the bonds both between the hunters themselves and between the hunters and the rest of theH. ergaster group. Females likely shared what they had foraged with the rest of the group as well. This development could have led to the development of male-female friendships into opportunisticmonogamous pair bonds. Since sexual selection from females probably favored males that could hunt, the emerging social behaviour resulting from these new behaviours would have been carried over and amplified through the generations.[63]
The only direct evidence ofH. ergaster group composition comes from a series of sites outside of Ileret in Kenya, where 97 footprints made around 1.5 million years ago by a group of at least 20 individuals have been preserved. Based on the size of the footprints, one of the trackways appears to have been a group entirely composed of males, possibly a specialised task group, such as a border patrol or a hunting or foraging party. If this assessment is correct, this would further suggest a male-female division of responsibilities. In modernhunter-gatherer societies who target large prey items, male parties are typically dispatched to bring down these high-risk animals, and, due to the low success rate, female parties tend to focus on more predictable foods.[64]
EarlyH. ergaster inherited theOldowan culture of tools from australopithecines and earlierHomo, though they quickly learnt to strike much larger stone flakes than their predecessors and contemporaries. By 1.65 million years ago,H. ergaster had created the extensively flaked artefacts and earlyhand axes that mark the Acheulean culture,[10] and by 1.6–1.4 million years ago, the new tool industry was widely established in East Africa.[65] Acheulean tools differ from Oldowan tools in that the core forms of the tools were clearly deliberate. Whereas the shape of the core forms in Oldowan tools, which were probably used mostly as hammers to crack bones for marrow, appears to not have mattered much, the hand axes of the Acheulean culture demonstrate an intent to produce narrow and sharp objects, typically in teardrop, oval or triangular shapes.[32] Once in place, the Acheulean industry remained unchanged throughoutH. ergaster's existence and later times, with tools produced near its end about 250,000 years ago not being significantly different from tools produced 1.65 million years ago.[66]
The oldest Acheulean assemblages also preserve core forms similar to those in Oldowan tools, but there are no known true intermediate forms between the two, suggesting that the appearance of Acheulean tools was an abrupt and sudden development. The most significant development that led to the Acheulean tools was likely early hominins learning the ability to strike large flakes, up to 30 cm (1 ft) or more in length, from larger boulders, from which they could manufacture new tools such as hand axes.[65] Though "hand axe" implies that all hand axes were used for chopping and were hand-held, they came in a variety of different shapes and size and probably served several different functions. Carefully shaped and symmetric examples may have been hurled at prey akin to moderndiscuses, more casually made examples may simply have served as portable sources for sharp flakes and some could have been used for scraping or chopping wood. Additionally, hand axes are effective butchering tools and were possibly also used for dismembering carcasses of large animals.[66]
There are preserved hand axes that are too unwieldy and large to be used for any apparent practical purpose. The use of these larger hand axes, and for some discovered collections of hundreds of hand axes without obvious signs of use, is speculative and conjectural. An idea that has been popular in the popular press, and frequently cited in academia,[67] is that large and impressive hand axes might have been emblems used for attracting mates, with makers of large axes showing strength, coordination and determination, qualities that may have been regarded as attractive.[66] Palaeoanthropologists April Nowell and Melanie Lee Chang noted in 2009 that though this theory is "both intriguing and emotionally appealing", there is little evidence for it and it is untestable.[68] They considered it more probable that variations in hand axe morphology over the course of hundreds of thousands of years was the result of various different factors rather than a single, overarching factor in sexual selection.[69]
AsHomo migrated into open savannah environments, encounters with natural fires must have become more frequent and significant.[70] It is possible thatH. ergaster were the earliest humans to master the control of fire, which they may have used for cooking purposes. Cooking renders both meat and plant foods more digestible, which might have been important since the guts ofH. ergaster were reduced in size compared to those of their ancestors.[40] ThoughH. ergaster/H. erectus is frequently assumed to have been the earliestHomo to control fire, concrete evidence is somewhat lacking in the fossil record, perhaps partly due to the difficulty for actual evidence of fire usage to be preserved.[71][72] Two of the earliest sites commonly claimed to preserve evidence of fire usage are FxJj20 at Koobi Fora and GnJi 1/6E nearLake Baringo, both in Kenya and both dated as up to 1.5 million years old. The evidence at FxJj20 consists of burned sediments and heat-altered stone tools, whereas GnJi 1/6E preserves largeclasts of baked clay, associated with stone tools and faunal remains. Though it is difficult to exclude a natural origin for the fire residue evidenced, the sites remain strong candidates for early fire use.[72][73]
Several sites, preserving more widely accepted evidence of fire usage, have been dated to 1 million years ago or earlier, postdating the emergence and last generally accepted record ofH. ergaster.[40] These sites include cave sites, such asWonderwerk andSwartkrans in South Africa, and open sites, such asKalambo Falls in Zambia. The site Gesher Benot Ya'aqov in Israel, dated to about 700,000 years ago, preserves widely accepted evidence of fire usage through burnt materials and burnt flint microartefacts being preserved at numerous levels.[40][73] From around 400,000 years ago and onwards, traces of fire become even more numerous in sites across Africa, Europe and Asia.[74]
Thespinal cord of Turkana Boy would have been narrower than that of modern humans, which means that the nervous system ofH. ergaster, and their respiratory muscles, may not have been developed enough to produce or control speech.[39] In 2001, anthropologists Bruce Latimer and James Ohman concluded that Turkana Boy was afflicted byskeletal dysplasia andscoliosis, and thus would not have been representative of the rest of his species in this respect.[75] In 2006, when anthropologistMarc Meyer and colleagues described aH. erectus s.l. specimen from Dmanisi, Georgia, dated to 1.78 million years old. The fossil preserves the oldest knownHomo vertebrae and the spine found falls within the range of modern human spines, suggesting that the individual would have been capable of speech. Meyer and colleagues concluded that speech was probably possible withinHomo very early on and that Turkana Boy probably suffered from some congenital defect, possiblyspinal stenosis.[76]
In 2013 and 2014, anthropologist Regula Schiess and colleagues concluded that there was no evidence of anycongenital defects in Turkana Boy, and, in contrast to the 2001 and 2006 studies, considered the specimen to be representative of the species.[77][78]
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Rogers, Alan R.; Iltis, David; Wooding, Stephen (2004). "Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair".Current Anthropology.45 (1):105–108.Bibcode:2004CurrA..45..105R.doi:10.1086/381006.S2CID224795768.
Schiess, Regula; Häusler, Martin (2013). "No skeletal dysplasia in the Nariokotome boy KNM-WT 15000 (Homo erectus) – a reassessment of congenital pathologies of the vertebral column".American Journal of Physical Anthropology.150 (3):365–374.Bibcode:2013AJPA..150..365S.doi:10.1002/ajpa.22211.PMID23283736.
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