Saturday, 17 January 2009

Homo ergaster

Homo ergaster ("working man") is an extinct human species that is known in the fossil record in Africa from possibly > 1.9 million years ago (KNM-ER 2598) and more securely 1.7-18 million years ago (KNM-ER 3733). By 600,000 years ago, it had been supplanted by the more advanced Homo heidelbergensis (“archaic Homo sapiens”).

Homo ergaster is usually assumed to have evolved from Homo habilis and to be the first human species to have left Africa. It is widely recognised as the stem species for later humans, including ourselves. It is now thought to be distinct from Homo erectus, which persisted in Asia until as recently as 50,000 to 30,000 years ago.

Homo ergaster was described in 1975 by Colin Groves and Vratislav Mazak. The type specimen is KNM-ER 992, a mandible discovered in 1971 at Koobi Fora, East Turkana, in Kenya.

Fossil record:
Homo ergaster is known from fossils recovered in East and West Turkana, Kenya; Swartkrans, South Africa; Olduvai Gorge, Tanzania; Bouri, Ethiopia; Buia, Eritrea and possibly from Dmanisi, Georgia. Key fossils include from these locations include:

From Koobi Fora, East Turkana: KNM-ER 3733, a cranium discovered in 1975 by Bernard Ngeneo, believed to be 1.8-1.7 million years old.

KNM-ER 3883, a cranium discovered in 1976 by Richard Leakey, 1.4-1.6 million years old.

KNM-ER 1808, cranial and post-cranial remains, discovered in 1973 by Kamoya Kimeu, believed to be 1.5 million years old. These remains are thought to be from a female and show coarse bone growths suggesting hypervitaminosis A, a lethal condition arising from excessive Vitamin A consumption. This could have arisen from consumption of carnivore liver or possibly bee brood. That KNM-ER 1808 survived long enough for the pathology to show up in her bones implies that she was cared for by others of her own kind.

KNM-ER 2958, a partial occipital bone is possibly > 1.9m years old. If so, it is the earliest fossil evidence for Homo ergaster.

From West Turkana: The “Turkana Boy” (KNM-WT 15000), an almost-complete adolescent male skull and skeleton discovered in August 1984 by Kamoya Kimeu on the banks of the Nariokotome River, dated to 1.33-1.64 millon years old. One of the most important fossil finds ever made, the Turkana Boy is even more complete than “Lucy”. Based on dental irruption, he was 11-12 years old at death.

From Swartkrans: SK 847, a partial cranium noted in 1969 at the Transvaal Museum, Pretoria by Ronald Clarke, though discovered twenty years earlier by museum palaeontologists Robert Broom and John Robinson, 1.0-1.5 million years old.

The “Daka Cranium” (BOU-VP 2/66), from Bouri, Middle Awash, Ethiopia, a 1.0 million year old calvaria with a cranial capacity of 995cc, discovered 1997 by W. H. Gilbert, may belong to Homo ergaster but both it and a similar cranium from Buia, Eritrea may represent a migration from Asia back into Africa by Homo erectus.

OH9 and OH 12 from Olduvai Gorge, Tanzania. OH9, discovered by Louis Leakey in 1960 is a 1.2 million year old cranium provisionally designated Homo leakeyi. It has a cranial capacity of 1067cc. OH 12, discovered by Margaret Cropper in 1962 is an 830,000-620,000 year old fragmentary cranium.

D2700 from Dmanisi, Georgia, is a skull dated at 1.95-1.77 million years old. The brain-case is very small, just 600cc, more consistent with Homo habilis, but postcranial remains from the same site suggest Homo ergaster affinities. These hominins have been described as a separate species, Homo georgicus.

Much of our knowledge of Homo ergaster comes from study of the Turkana Boy, who stood 1.62m (5ft3) at death and would have attained at least 1.82m (6ft) had he reached adulthood.

But his brain, which was almost fully-grown, had a volume of just 880cc, only 2/3rds that of a modern human. Mature Homo ergaster had a cranial capacity of around 900cc (Klein, 1999) though it ranged from as little as 600cc (D2700) to as much as 1100cc (OH 9). Although the average is rather more than that of Homo habilis, the effective difference is only small given H. ergaster’s greater stature.

H. ergaster possessed the characteristic long, low brain-case of pre-sapiens human species; a flat and receding forehead; a bony brow-ridge over the eyes. The nose projected forward, with downward-orientated nostrils, similar to a modern human, again unlike the more ape-like H. habilis. But the jaws were massive and prognathic (jutting) and the teeth were intermediate in size between H. habilis and modern humans. They were completely chinless.

The rib-cage is barrel-shaped rather than conical, the pelvis is narrower and in body proportions, very much like modern humans, with lower limbs indicating a full striding gait. Homo ergaster was a fully-committed terrestrial biped, unlike Homo habilis, which retained relatively longer arms, shorter legs and a conical ribcage: remnants of an arboreal past now finally abandoned.

The narrower pelvis increases the energy efficiency of muscles involved in bipedal movement, but this forced the lower part of the ribcage to narrow. To retain the same lung capacity, the upper part of the ribcage expanded to give it its modern barrel shape. The down side is that the female birth canal narrowed. This in turn would have restricted antenatal brain growth. The very long postnatal dependency of modern humans might have had its origins with Homo ergaster.

These changes would have also forced a reduction in the size of the digestive tract, which could only have happened in conjunction with higher quality food. This suggests consumption of more meat, tubers, bulbs, etc and the possible use of fire for cooking, but there is no hard evidence for either.

Homo ergaster colonised dryer, more seasonal African environments, where there was relatively little surface water or shade. Its physique is not unlike that of present-day humans living in equatorial East Africa, who have slim bodies and long limbs; a body-shape that gives a higher surface area to volume ratio than the stockier build of, say, an Eskimo and is more efficient at dissipating (as opposed to conserving) heat.

Under hot conditions, the projecting, human-like nose would act as a condenser, preventing moisture from being exhaled and so wasted. It is likely that Homo ergaster was almost naked, like a modern human, but unlike any previously-existing hominin. This would have greatly aided heat dissipation. Like modern Africans, Homo ergaster was almost certainly dark-skinned, to protect against skin cancer. (Scarre, 2005; Cameron & Groves, 2004).

Because the natal brain size was less than that of modern humans, the birth canal, though still problematic, was smaller than that of modern humans. Consequently Homo ergaster may have been a more efficient biped than ourselves, and was probably a superb all-round runner that would have left our best athletes trailing in its wake. Enhanced middle and long-distance running abilities would have given them an edge when hunting; conversely when the tables were turned, sprinting abilities would have helped them to escape predators.

As noted above, the widely-accepted view is that Homo ergaster evolved from Homo habilis. Although Homo rudolfensis is also a possible ancestor, it seems likely that this taxon is off the line of human evolution and should be removed from Homo altogether (Cameron & Groves, 2004).

Another possibility is that both Homo habilis and H. ergaster both evolved from a common ancestor around 2.3 million years ago. Spoor et al (2007) note that both species were sympatric in the Turkana Basin for approximately 400,000 years. They believe that this is a more likely explanation than the alternative, which is a Homo ergaster split from an earlier population of Homo habilis, with the Turkana Basin being a region of secondary contact between the two species. Cameron and Groves (2004) take the opposite view, pointing out that Homo ergaster does not appear in the fossil record until much later than Homo habilis.

If the second scenario is correct, and Homo ergaster split in a sudden “punctuated” evolutionary event, a possible cause would be the increase in seasonal rainfall and aridity that occurred across Africa with the onset of the Pleistocene, 1.8 million years ago.

On the other hand it is possible that from 3 to 2 million years ago in Africa there existed a “bush” or complex of closely-related but distinct hominin species, and that Homo habilis and H. ergaster could have emerged from totally separate branches. If this is correct, then there are still many species of hominin remaining undiscovered.

Relationship to Homo erectus:
It used to be the general view that the “Version 2.0 humans” living in Africa and Europe until 600,000 years ago and Asia until possibly 50,000-30,000 years ago belonged to a single species, Homo erectus. But this view has fallen out of fashion. On average, the African skulls tend to be higher-domed and thinner-walled than those from East Asia, and they have less massive faces and brow-ridges. Accordingly the African hominins are usually now classified as Homo ergaster, with the designation H. erectus reserved for Asian fossils. Many sources do still lump both together as Homo erectus or use the term “African Homo erectus” in preference to H. ergaster.

The view that Homo ergaster left Africa and evolved into Homo erectus, while Africa remained exclusively populated by stay-at-home H. ergaster, is almost certainly an oversimplification.

The presence of the 1.95-1.77 million years old ergaster-like skulls at Dmanisi show that humans had left Africa by then, but whether or not these hominins can be included in Homo ergaster or were ancestral the Homo erectus in Asia remains a matter for debate.

The discovery of an erectus-like hominin at Ceprano, Italy, in 1994 and its similarity to the Olduvai hominin OH 9 (Clarke, 2000) has led to the view that there was a migration back into Africa by Homo erectus. In addition to OH 9, the more recent OH 12, the “Daka Cranium” (BOU-VP 2/66), from Bouri and the similar cranium from Buia have been proposed as examples of “Into Africa” Homo erectus rather than Homo ergaster (Cameron & Groves, 2004). The partial skullcap OH 9 has massive brow-ridges, thick walls and angular rear profile typical of East Asian Homo erectus, but in other characteristics it is like Homo ergaster. The Buia and Daka skulls show only minor differences from earlier Homo ergaster (Scarre, 2005).

Berhane Asfaw argues that “the Daka cranium confirms previous suggestions that geographic subdivision of early H. erectus into separate species lineages is biologically misleading, artificially inflating early Pleistocene species diversity. Rather, the Daka calvaria is consistent with the hypothesis of a widespread, moderately polymorphic and polytypic species [i.e. Homo erectus] at 1.0Myr” Asfaw et al (2002).

Subsequent evolutionary career:
The traditional view is that around 600,000 years ago, Homo ergaster (or African Homo erectus) evolved into the larger-brained “archaic Homo sapiens” from which two distinct subspecies, the Neanderthals and modern Homo sapiens (us), eventually arose. The currently popular view represents only a slight updating of this paradigm in that “archaic Homo sapiens” is now seen as a distinct species, Homo heidelbergensis. This species migrated into Europe, where it evolved into Homo neanderthalensis (the Neanderthals). Meanwhile, H. heidelbergensis in Africa evolved into Homo sapiens about 200,000 years ago. While it does seem likely that African Homo erectus (whatever this may be) did evolve into larger-brained hominins it is problematic as to whether there was just one successor species or several; from which of these Homo sapiens eventually arose; of if indeed the progenitor species for Homo sapiens was African or a European species that migrated back into Africa.

The earliest Homo ergaster remains, including the Dmanisi hominins, are associated with the Oldowan (Mode I) tool tradition. At about 1.65 million years ago these primitive tools give way to the teardrop shaped “hand-axes” of the Acheulian (Mode II) tradition, named for Saint-Acheul in Northern France, where the first examples were found at in the mid-19th Century. The oldest known Acheulian tools are dated to 1.65 million years ago from West Turkana, Kenya and 1.5-1.4 million years ago from Konso, Ethiopia, East Turkana, Kenya and Peninj, near Olduvai Gorge, Tanzania (Scarre, 2005).

The Acheulian hand-axe tradition endured with little change until it was finally abandoned 250,000 years ago. It has been described as displaying a “variable sameness” that strikes “even enthusiasts as monotonous”. The tradition does however represent a considerable advance on its predecessor as it is necessary for the maker to preconceive the form of the finished tool from a block of raw material. The 3D symmetry often shown by the axes indicate their makers were intent on imposing form on the artefact rather than just creating a sharp edge as in the Oldowan tradition. This is very difficult and requires forward planning (Mithen, 1996).

Despite the name, the function of these often beautifully-crafted hand-axes remains conjectural. At some sites such as Melka Kunture in Ethiopia, Olorgesailie in Kenya, Isimila in Tanzania and Kalambo Falls, hand-axes occur in large numbers and appear to have been discarded soon after manufacture, with no sign of wear, suggesting that they were never used. Another major puzzle is that they are often too large to be useful (see, for example, the fine example in the Natural History Museum in Kensington).

One theory (Kohn & Mithen, 1999) proposes that the axes were made to impress prospective mates. When a female saw a large, symmetrical axe, she might conclude that its maker possessed the right attributes to father successful offspring. The axe, having served its purpose (or not) would then be discarded. This – like the elaborate bower of the male bower bird – would be an example of the extended phenotype of a species playing a role in sexual selection (Dawkins, 1982).

Regardless of whether or not the mate selection hypothesis is true, the hand-axes almost certainly were used as tools. Experiments have shown them to be effective for butchery purposes; some may have been used as a discus for bringing down prey; others could have been used for chopping and scraping wood. Klein and Edgar (2002) liken the Acheulian hand-axe to a Swiss army knife.

Another issue with the hand-axes is that while they are ubiquitous in Africa and western Eurasia, they are not found east of Northern India. This was first noted by American archaeologist Hallam Movius in 1948. The “Movius Line” has stood the test of time and two theories have been proposed to explain it. One is that the ancestors of those living east of the Movius Line left Africa before the hand-axes were invented. The other possibility is that the migrants from Africa passed through a region lacking suitable materials to make the axes, and by the time they emerged from it, the tradition had been forgotten.

Recent discoveries support the first possibility. Hominin remains such as the Mojokarta Child [Homo erectus] from Java have now been dated to 1.81 million years ago and the Dmanisi hominins are at least 1.77 million years old, predating the earliest-known Acheulian hand-axes.


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© Christopher Seddon 2009

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