The Emergence of H ergaster

Homo habilis and H. ergaster were sympatric for approximately 400,000 years. This means that either a proto-ergaster population split away from an earlier population of H. habilis or they share a common ancestor around 2.3 Ma. The first hypothesis appears more likely, for H. ergaster does not appear in the fossil record until around 300,000 years after the earliest representatives of H. habilis: H. ergaster has a temporal range of between 2.0 and 1.5 Ma, while H. habilis has a range of between 2.3 and around 1.6 Ma (see Wood & Richmond, 2000). It was H. ergaster, however, who survived to move out of Africa, as shown by their presence in Eurasia (Georgia) by at least 1.6 Ma (though, as we will discuss later, these Georgian hominins have recently been allocated to a new species of Homo), and the Asian species H. erectus, who likely split from Eurasian populations of H. ergaster at around the same time. Whether the disappearance of Homo habilis from the fossil record was a direct or indirect result of unsuccessful competition between it and H. ergaster remains unclear.

While there is still much debate concerning the systematic status of H. habilis (some believe it represents a species of Australopithecus and call it A. habilis [see B.A. Wood & Richmond, 2000]), very few would doubt that the specimens defining H. ergaster are indeed representative of our own genus. The most famous and complete specimen of this species is the Nariokotome skeleton from Kenya, dating to around 1.5 mya (see Walker, 1993, 1994; F. Brown & McDougall, 1993). Originally this specimen was allocated to H. erectus, though we recognize H. erectus as being only an Asian species, and almost all specimens previously allocated as representatives of an African deme of H. erectus we allocate to H. ergaster (see also Andrews, 1984; B.A. Wood, 1984, 1991; B.A. Wood & Richmond, 2000), the only exception being later immigrants of H. erectus back into Africa, such as the Olduvai hominins OH 9 and OH 12 and possibly the 1-million-year-old Homo specimens from Danakil (Eritrea) and Bouri (Ethiopia).

Of the specimens allocated to H. ergaster, the best preserved and documented are the mandibular-type specimen KNM-ER 992, the mandible KNM-ER 820, the mandible and associated fragments ER 730, the skulls KNM-ER 3733 and KNM-ER 3883, the pathological skeleton ER 1808, and the juvenile skeleton KNM-WT 15000 (see B.A. Wood, 1991; Walker, 1993; B.A. Wood & Richmond, 2000) (Figure 6.2). Compared to its proposed ancestor (H. habilis), H. ergaster is differentiated by reduced

Figure 6.2 ► Homo ergaster specimen KNM-WT 15000 from Nariokotome, Kenya. This species is the first in the fossil record to show modern human-like body proportions, for even H. habilis is defined by the chimpanzee-like proportions of longer forelimbs (arms as well as trunk) but shortened hind limbs (legs).

Figure 6.2 ► Homo ergaster specimen KNM-WT 15000 from Nariokotome, Kenya. This species is the first in the fossil record to show modern human-like body proportions, for even H. habilis is defined by the chimpanzee-like proportions of longer forelimbs (arms as well as trunk) but shortened hind limbs (legs).

size of its dental complex relative to body size, increased cranial expansion associated with increasing brain size, reduced supraorbital torus and frontal sulcus development, reduced postorbital constriction (associated with increased cranial base flexure and reduced temporalis muscle development), and the emergence of frontal keeling, which it shares with H. erectus from Asia, though H. ergaster is not characterized by a number of more specialized features observed in H. erectus (Cameron et al., in press; see also, partly, Walker & Leakey, 1993a). As indicated by WT

15000, the postcranial skeleton is very much derived in the modern human condition, different from the primitive australopithecine-like condition of H. habilis (Walker & Leakey, 1993b; B.A. Wood & Richmond, 2000). Its rib cage is not funnel shaped, its pelvis is narrower, and in body proportions, especially in its limbs, it is very much like modern humans, with longer lower limbs, indicating a full striding gait. Overall, H. ergaster had a locomotor pattern that was very similar to later humans; long-range full terrestrial bipedality (Ruff & Walker, 1993; Walker & Ruff, 1993; B.A. Wood & Collard, 1999; B.A. Wood & Richmond, 2000).

Not only is the Nariokotome specimen of international significance because of its excellent preservation and the number of parts preserved, but the analysis of this skeleton has truly changed our views of early hominin evolution. This specimen is an adolescent boy, between 9 and 11 years old, who obviously would have kept growing in height and stature, because his epiphyses have yet to fuse, clearly indicating that growth would have continued if the youth had not died (Walker & Leakey, 1993b). Estimates of its adult height by Ruff and Walker (1993) place it at around 185 cm (6'1"), with a body weight of around 68 kg (150 lbs), which is truly surprising, not just because of the relatively small stature of earlier hominids and hominins, including H. habilis; this adolescent would also be taller than most modern human adults. Indeed, Walker (1993) places this and other H. ergaster specimens within the top 17% of modern human populations in terms of height. All evidence suggests that this was no anomaly, but a relatively normal growth pattern for the species (see Ruff & Walker, 1993; Tattersall & Schwartz, 2000).

As suggested by Walker and Ruff (1993), the analysis of body stature and height indicates that H. ergaster was the first hominin to adopt a more modern human-like body plan; an adaptation that can be associated with the tropical conditions and most likely a loss of body hair, resulting in increased sweating, which would help keep the body cool (Walker, 1993). The increased body height would enlarge the surface area, which would help further regulate body cooling. The pigment of the skin was probably also dark, to help prevent the formation of skin cancers due to a lack of body hair. Why this adaptation occurred is problematic, because H. habilis, with its more primitive australopithecine-like condition, was occupying the same region in time. A major climatic shock occurred around 1.8 Ma, resulting in a further reduction in forests. But the earliest H. ergaster specimens predate this by at least 200,000 years, and the spe-ciation event is probably at least 200,000 years earlier than that. Climate and habitat deterioration, however, must have its beginnings in an earlier time, and this phase of climate change can be traced back to at least 2.5 Ma (Potts, 1996).

The development of the derived morphological pattern of H. ergaster may be associated with its more efficient type of bipedal locomotion. Carrier (1984) suggests that this type of body plan would result in increased running speeds and endurance, which would force prey to avoid them in an inefficient manner, making H. ergaster an effective predator who could run down prey without succumbing to overheating (see also Walker, 1993), though this itself does not explain the adaptive pressures associated with the evolution of its more efficient bipedalism. One must be very careful not to fall into circular and ad hoc argument when trying to explain aspects of physical and cultural evolution. Regardless of the reasons for the change, there was a clear demarcation in body plan with the emergence of H. ergaster.

That is not to say that in all aspects H. ergasters reflect the modern human condition. Given its mean age of 10 years, we would expect, using modern-day human analogies, for the second and third molars in the Nariokotome youth not to have erupted at this stage. In modern human children, the second molar tends to erupt around age 12 and the third molar around age 18. While the third molar had yet to erupt, the second molar in the Nariokotome boy had erupted, suggesting that it matured earlier than modern humans (H.B. Smith, 1993; see also B. Brown & Walker, 1993; Walker, 1993), so he should be given an age of 12-plus according to the dental evidence. But the overwhelming postcranial evidence (patterns of epiphyseal development — closure of the end plates of the long bones onto the shaft) strongly indicates that the age is between 9 and 10 years. Either way there is a discrepancy between dental and skeletal development in H. ergaster that is unique, quite different from the pattern observed in later hominins (see H.B. Smith, 1993; Walker, 1993).

MacLarnon (1993) studied in detail the vertebral canal of the Nariokotome skeleton and noticed that this youth has a relatively narrow thoracic canal (Figure 6.3), suggesting that the thoracic spinal nerves were reduced compared to modern humans. This, according to MacLarnon, gives rise to two possible functional explanations. The first is less muscular movement or control of the trunk, which involves both the intercostal and abdominal muscles, implying a slightly less efficient mode of bipedalism than in modern humans. The second explanation is reduced muscular control associated with breathing, which may reflect poorer control of vocalization — speech. MacLarnon (1993) concluded that the spinal cord of the Nariokotome youth, beginning

Figure 6.3 ► Vertebra from H. ergaster specimen KNM-WT 15000 showing the reduced size of the vertebral canal (striped area), indicating that their spinal cord was less developed than in modern humans. Adapted from MacLarnon (1993).

high up in his neck, displayed fewer nerve fibers than in modern humans; fewer fibers available for innervation of the skeleton and soft tissues means less ability to control muscles associated with vocalization and breathing and less innervation of the involuntary-voluntary action of the diaphragm muscle, which is used by modern humans in the production of speech (see also Walker, 1993). If this is so, it may go some way to refuting the idea that H. ergaster could "run down" prey (see earlier), given its reduced control of the diaphragm muscles.

In summary, the emergence of H. ergaster in Africa is associated with a major shift in both anatomy and behavior. Its body plan is much closer to that of later hominins than is that of H. habilis and earlier hominins/hominids. Though it has its own unique growth pattern, this nonetheless approaches the modern human condition. There is also a shift to increased meat eating and a reliance on technology, including not only an increase in stone tool manufacture, but most likely also "soft tool" manufacture (e.g., water-/food-carrying items made from wood, skins, vegetative material), as suggested by the long-term survival of ER 1808. The story of H. ergaster, however, does not stop here; around 1.8 Ma, populations of this species were the first to migrate out of Africa into Europe.

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