Homo ergaster

Introduction

Homo ergaster is represented by fossils such as “Nariokotome Boy” - KNM-WT 15000 (view skeleton). It lived during the Lower Pleistocene epoch (dated at between 1.51 and 1.56 million years old), with fossils dated between 1.8 and 1.2 million years old. Some scientists have split H. erectus into three separate species, based on the geographic region in which specimens have been found: H. ergaster (Africa), H. erectus (Asia), and H. heidelbergensis (Europe). Homo ergaster and H. erectus comparison However, the discovery of H. ergaster fossils outside Africa are forcing a reanalysis of this species. H. ergaster has thinner cranial bones, higher cranial vault, a less robust face and lighter frame than H. erectus (Heslip, 2001). Note the more human appearance of the more ancient H. ergaster, with less heavy browridges and a more globe-shaped braincase.

The use and control fire, a milestone in human development, occurred 1 to l.5 million years ago. Control of fire may have enabled Homo erectus to migrate out of Africa. H. ergaster fossil finds  are not associated with evidence of fire use.  Modern humans share the same differences as H. ergaster with the Asian H. erectus, leading to the possibility that H. ergaster is the ancestor of later Homo populations (Heslip, 2001). 

Tapeworms leave evolutionary clues   

As a clue to a part of our evolutionary history, scientists have studied the evolutionary relationships of our host-specific taeniid tapeworms: Taenia saginata, T. asiatica and T. solium and those of other species (Hoberg et al, 2001). This is done by looking at genetic and host differences among tapeworm species. A tapeworm's life cycle is adapted to its parasitic existence. For taeniids, this centres upon a predator-prey relationship, with a carnivore carrying the adult tapeworm and a herbivore hosting the infective larvae. Evolutionary relatives of our tapeworm are usually found in intestines of carnivores such as lions, hyenas or African wild dogs.

Adult worms shed eggs which are then ingested by an intermediate host, usually a particular species of herbivore, such as a domestic cow, some antelope or a pig (domestic or wild). The tapeworm larvae that develops, living in the host’s flesh, pass back into the definitive predator host when it eats the infected intermediate host! They are adapted to this African predatory association.

Animal domestication arose about 10,000 years ago. However, the genetic sequence divergence between the two taeniid tapeworms most closely related to the human-infecting species: T. saginata and T. asiatica, gives a divergence date of between 780,000 and 1.71 million years ago. As their ancestor probably already lived in humans, their presence was not due to the domestication of pigs and cattle, but due to some much earlier behaviour. This is a type of smoking gun to our carnivore origins.


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As the early hominids,  Homo habilis and H. ergaster, switched to an increasingly carnivorous diet in Africa, they would have encountered the prey and carnivores carrying these parasites.  T. solium, a human-specific tapeworm, is closely related to tapeworms such T. hyaenae (brown hyenas, spotted hyenas and African hunting dogs), T. crocutae (spotted hyenas and African hunting dogs), T. gonyamai (lions and cheetahs) and T. madoquae (jackals). Our hominid ancestors must have occupied a similar ecological niche to these species, relying on similar prey as food. This is good proof that early Homo hunted and scavenged animal carcasses and is supported by archaeological evidence such as the presence of stone tools, cutmarks and hominid-induced breakage patterns on the fossil bones at hominid sites.

That the human-specific T. solium and T. asiatica have intermediate hosts that are not antelopes, provides further pieces to our evolutionary puzzle. The intermediate hosts of T. solium are humans, other primates, hares or rabbits, hyraxes, members of the dog family, and wild or domestic pigs (Shipman, 2002).  Fossil evidence shows that around 1.7 million years ago Homo ergaster left Africa and colonized Eurasia. This would have led to Homo ergaster encountering new prey species and new carnivore competitors, upsetting the usual predator-host cycling for the parasite that could have led to the extinction of the parasite. However, selection pressure led to humans also becoming the intermediate host for the parasite to survive.

(An alternate hypothesis, that the early hominids were intermediate parasite hosts as regular prey to these carnivores breaks down due to the specificity of T. solium for humans.) The hares, rabbits, hyraxes and pigs need to be eaten by early humans for this association to exist, while the dog could have scavenged around human camps and preyed upon these species and so picked up the same parasite. However as the primary host, it is human activity that defines the distribution of these parasites. The intermediate hosts of T. asiatica are the domestic pig and cattle, so this species evolved under selective pressures associated with the domestication of these species.

Carnivorous ancestor

Homo ergaster displays a set of features that support the adoption of a new meat-eating or scavenging type niche as opposed to the largely vegetarian diet of the Australopithecines. It had a higher skull dome, thinner cranial bones, slim brow ridges, a light skull, and generally, a lighter build than Homo erectus. It had long, well-muscled limbs, and narrow hips, body proportions similar to modern humans living in tropical climates.
          Homo ergaster footprint foot print H. ergaster
Ancient human footprints near the town of Ileret, Kenya, made by Homo ergaster (or H. erectus) are dated at about 1.5 million years ago. Homo ergaster displays no sexual dimorphism between the two genders, has an enlarged brain (relative to forebears) (about 900 c.c.) and  its molars and premolars are smaller in size, indicating an omnivorous diet that included meat. The bulge of Broca's area found in H. habilis (2.4 and 1.5 million years ago), suggests a capability for rudimentary speech had already evolved. Stone tools in use during this period, 1.9 to 1.2 million years ago, were the Acheulean tool kit. These bi-facial tools were used to process their food. Jacobs (2000) proposes that H. erectus's and H. ergaster's ability to migrate over several continents is significant evidence of adaptation to meat eating and possible hunting.

Middle Pliocene cooling around 3.0 to 2.4 mya produced a relatively cool, dry climate in tropical Africa, altering the ecology. Many of the larger species of forest-adapted African bovids were pushed toward extinction, leading to the emergence of more cursorial, grassland habitat dwelling species. Six species of African bovids dispersed to Eurasia during the Middle Pliocene. This cooling period also led to the divergence of Australopithecus into two evolutionary lines. Paranthropus evolved robust jaws and massive teeth for a specialized coarse vegetarian diet and survived as Paranthropus boisei until about 1.2 mya. It remained sexually dimorphic, with ape-sized brains, and confined to tropical habitats.

Between 2.5 and 2.0 mya, a more omnivorous line, the earliest species of the genus Homo emerged, (Larick and Ciochonto, 1996). Oldowan core and flake tools associated with Homo habilis are found with animal bones, such as antelope lower legs (parts with little or no meat), that show tool marks (Feder, Kenneth L., and Michael Alan Park, Human Antiquity, Mayfield Publishing Company, Mountainview, CA, 1997. ). Oldowan tools were definitely used for cutting meat, possibly scavenged for marrow rather than hunted.

Genus Homo made crude bifacial tools by 2.0 mya. Typical Acheulean biface tools had evolved by 1.5 mya in the eastern Rift in Ethiopia, and in Peninj and Olduvai Gorge in Tanzania (Larick, 1996)and by 1.76 mya around Kenya's Lake Turkana (Nature, Sep 2011). Acheulian bifaces are found in Israel by 1.4 mya (Jacobs, 2000). The oldest ergaster populations made Oldowan tools, but by around 1.6 to 1.7 Mya, bifaced “hand axes” and cleavers typical of “Acheulean” industries were being made.

Acheulian hand tool
      Acheulean biface tool


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Meat eating allowed H. ergaster to migrate across vast distances and into new ecological niches with different plants. This led to H. ergaster's dispersal at a time of climate change a process probably enabled by meat scavenging. Fossil finds of H. ergaster from Dmanisi, Georgia and dated at 1.7 million years old are the oldest evidence of this species early dispersal out of Africa. The two skulls were found with fossil bones (a dozen kinds of animals including giraffe, gazelle, rhinoceros, and ostrich (Archaeology Online News)) and over 1000 tools of the simpler "pebble-chopper" (Oldowan) type that preceded the Acheulean tools in Africa, appearing about 2.4 million years ago. The site itself is older than any known Acheulean tools (Gabunia, et al 2000).  It is clearly a less developed hominid than Homo erectus previously thought to be the first hominid to move out of Africa to populate Asia and Europe. The associated animal bones show no signs of butchery and there is no evidence that the Georgian hominids had mastered fire.


Ancestral tree and timeline: Homo erectus or Homo ergaster?

So what is Homo ergaster's position on the human evolutionary tree (Alles, 2001)? H. ergaster arose in Africa and appears to have been the only possible candidate as our ancestor from 1 to 1.5 million years ago. The type specimen for ergaster is KNM-ER 992. Its immediate ancestor was either Homo habilis (2.0 – 1.6 mya) (OH 7) or Homo rudolfensis (2.4 – 1.6 mya) (KNM-ER 1470). A little older (2.5 Mya) than these species is Australopithecus garhi, believed to have evolved from A. afarensis (Afshaw et al, 1999). In some schemesHomo rudolfensis is classified as Kenyapithecus rudolfensis, as a decendant of K. platyops, removing it as a possible human ancestor. Homo ergaster and H. erectus (1.9 mya – 27,000 years BP) are often considered geographic (and temporal (Bilsborough, 2000) = chronospecies?) variants and not separate species. The immediate ancestor to humanity and the Neanderthals is considered to be Homo heidelbergensis (800,000 – 100,000 years BP). Between H. heidelbergensis and H. ergaster is possibly (but not necessarily) H. antecessor as a common ancestor. (H. antecessorH. heidelbergensis.) As more discoveries are made, H. ergaster may simply turn out to be early H. erectus from Africa. 


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As Nowaczewska (2000) noted, discrepancies between morphological and DNA analysis means that morphologically based phylogenesis of our species may never show the full truth. Morphologically, the chimpanzee appears to be more closely related to the gorilla than to man. Analysis of genetic material has shown that the DNA structure of genus Pan is more similar to that of Homo sapiens than to that of gorilla. Also,  many "good" living (extant) species are osteologically and dentally indistinguishable (e.g. Cercopithecus species), so the hard tissue-bound fossil record will always underestimate the number of species (Wood & Richmond, 2000). Homo ergaster is however an important hominid on the path to humanity.

Bilsborough (2000) found a relative homogeneity of H. erectus world-wide. He found that the neurocranial diversity in the species was remarkably limited over wide temporal and spatial spans. Some diversity is expected and attributed to population sizes being smaller in the Lower/ Middle Pleistocene and to forces driven by contrasting environmental pressures, founder effects, inter-generational sampling perturbations and genetic drift. Also, the H. erectus samples span a long interval resulting in "chronological separation" between H. erectus groups. Temporal variation is likely to contribute significantly to the observed H. erectus diversity.

Bilsborough concluded that overall the results confirmed the general similarity in form of Homo erectus crania to the extent that he did not support making the early East African fossils a new species as as did Wood (1991, 1992a,b) for H. ergaster. He agrees with other researchers (Kramer,1993), (Rightmire, 1998) whose studies both confirm the general similarity of H. erectus, finding "extensive overlap" between African and Asian specimens "in the very characters claimed to differentiate them". A recent fossil find of a one-million-year-old Homo erectus skull, unearthed in Ethiopia, perpetuates the controversy. Some scientists say the find proves H. ergaster is a misnomer (Asfaw, 2002), as it links Asian and African forms of H. erectus.  Wood, who first proposed H. ergaster as a distinct group, believes the new find bears too little resemblance to H. ergaster to rule them out as a separate group.

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by Laurence Evans 1998 - 2008

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