A tree diagram showing the suggested relationships between Homo ergaster and later human species.
At Olduvai Gorge, habilis or one of its variants may have persisted until 1.6 million years ago, but thereafter ergaster survived alone. Its history after 1 million years ago is debatable, because few relevant fossils are known, but on present evidence it may have persisted largely unchanged until about 600,000 years ago, when brain size increased rapidly and new, more advanced human species arose.
Sometimes it seems as if controversies over species assignments and ancestor-descendant relationships dominate paleoanthropology, but in fact paleoanthropologists know that their first priority must be to understand how ancient people looked and behaved. Alan Walker and Richard Leakey, who co-directed the excavation of the Turkana Boy's skeleton with Meave Leakey, realized that it provided a unique opportunity to explore the biology of a primitive human species. They thus invited anatomist colleagues to study it with them, and the result was a comprehensive, stimulating monographic description that fleshes out our knowledge of ergaster.
On average, brain volume in ergaster was only about 900 cc, large enough to invent the new kinds of stone tools with which it is associated, but also small enough to explain why the tools then changed little over the next million years or so. Based mainly on dental development, the Turkana Boy was probably about 11 years old at time of death, but his stature compared more closely with that of a modern 15-year-old and his brain with that of a modern 1-year-old. The sum has led Walker to conclude that "While he may have been smart by ape standards, relative to [living] humans the Turkana Boy was tall, strong, and stupid." The same statement might apply equally well to everyone who lived between 1.8 million and 600,000 to 500,000 years ago, before a spurt in brain volume brought it much closer to the modern average.
Body form and size tell a different story, and in this regard ergaster was as human as anyone alive today. The shortening of its arms relative to its legs signals the final abandonment of any ape-like reliance on trees for feeding or refuge. A greater commitment to life on the ground meant an even greater emphasis on bipedalism, and this could explain the narrowing of the hips (pelvis) and the concomitant development of a barrel-like chest. The narrowed pelvis increased the efficiency of muscles that operate the legs during bipedal movement, and it would have forced the lower part of the rib cage to narrow correspondingly. To maintain chest volume and lung function, the upper part of the rib cage would have had to expand, and the modern barrel shape would follow. The narrowing of the pelvis also constricted the birth canal, and this must have forced a reduction in the proportion of brain growth that occurred before birth. Infant dependency must then have been prolonged, foreshadowing the uniquely long dependency period that marks living humans.
Pelvic narrowing must also have reduced the volume of the digestive tract, but this could have occurred only if food quality improved simultaneously. Direct archeological evidence for new foods is lacking or ambiguous, but the choices are larger quantities of meat and marrow, greater numbers of nutritious tubers, bulbs, and other underground storage organs, or both. Cooking might also be implied, since it would render both meat and tubers much more digestible, but so far, persuasive fireplaces or hearths are unknown before 250,000 years ago, by which time ergaster had been replaced by more advanced species.
Archeology shows that ergaster was the first human species to colonize hot, truly arid, highly seasonal environments in Africa, and this may partly explain why the Turkana Boy was built like a modern equatorial east African, with a lanky body and long limbs. As the trunk thins, body volume decreases more rapidly than skin area, and greater skin area promotes heat dissipation. Long limbs provide the same benefit. In people like the Inuit or Eskimo who must conserve heat, we see the reverse—stocky bodies and short limbs that reduce heat loss. Adaptation to hot, dry conditions can also explain why ergaster was the first human species to have a forwardly projecting, external nose.
In living humans, the external nose is usually cooler than the central body, and it thus tends to condense moisture that would otherwise be exhaled during periods of heightened activity. Finally, given that ergaster was shaped for a hot, dry climate, we can speculate that it was also the first human species to possess a nearly hairless, naked skin. If it had an ape-like covering of body hair it could not have sweated efficiently, and sweating is the primary means by which humans prevent their bodies—and their brains—from overheating.
When the Turkana Boy's skeleton is considered with isolated limb bones from other individuals, it becomes clear that ergaster was not only taller and heavier than earlier humans, but also that the sexes differed no more in size than they do in living people. This stands in sharp contrast to the australopiths and perhaps habilis, in which males were much larger than females. In ape species that exhibit a similar degree of sexual size difference, males compete intensely for sexually receptive females and male-female relationships tend to be transitory and non-cooperative. The reduced size difference in ergaster may signal the onset of a more typically human pattern in which male-male competition was reduced and male-female relationships were more lasting and mutually supportive.
A small brain surely means that ergaster was less intelligent than living people, and if brain size were all we had to go by, we might wonder if it differed cognitively from habilis (or habilis/rudolfensis). But we also have artifacts, and these show that it did. The tools also help us to understand how ergaster was able to colonize the more arid, seasonal environments to which it was physiologically adapted and also how it became the first human species to expand out of Africa.
The first tool makers, the Oldowan people, mastered the mechanics of stone flaking, and they were very good at producing sharp-edged flakes that could slice through hides or strip flesh from bone. At the same time, they made little or no effort to shape the core forms from which they struck flakes, and to the extent that they used core forms, it was perhaps mainly to crack bones for marrow. For this purpose, core shape didn't matter very much. Ergaster, however, initiated a tradition in which core forms were often deliberately, even meticulously, shaped, and shape obviously mattered a lot.
The characteristic artifact of the new tradition was the hand axe or biface—a flat cobble or large flake that was more or less completely flaked over both surfaces (hence the term biface) to produce a sharp edge around the entire periphery (Figure 4.4). Many hand axes resemble large teardrops, as they narrow from a broad base or butt at one end to a rounded point at the other. Ovals, triangles, and other forms are also common, and in some places, hand axe makers produced pieces with a straight, sharp, guillotine-like edge opposite the blunt butt (Figure 4.5). Archeologists often call such pieces cleavers to distinguish them from hand axes, on which one end tends to be more pointed.
John Frere, the great-great-grandfather of Mary Leakey, is sometimes credited as the first person to recognize the human origin and great antiquity of hand axes. In 1797, he sent a letter to the Society of Antiquaries in London describing two carefully crafted hand axes he had recovered from ancient lake deposits at Hoxne in Suffolk, England. Bones of extinct animals occurred nearby, and Frere concluded that the hand axes had been "used by a people who had not the use of metals"
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