Back in 1758, a Swedish naturalist who wrote under the Latin name Carolus Linneaus, devised a method of classifying all living things. Under this classification system, which is still in use today, humans are assigned to the genus Homo. The primary anatomical characteristics of this species include bipedalism (that is, walking upright on two legs) and a relatively large brain. While there was originally only one species assigned to the genus Homo—Homo sapiens—fossil finds during the twentieth century expanded the genus to include a number of ancient humans.
Nearly two million years ago a new human species evolved in Africa. Named Homo ergaster, this species had a brain that averaged 870 cc (two-thirds the size of modern human.) It had a rounded cranium and a prominent brow ridge. Unlike modern humans, the face jutted out in front of the braincase.
While the teeth are significantly larger than those of modern humans, they are smaller than the earlier australopithecines.
From the neck down, Homo ergaster was beginning to resemble modern humans. Its legs were relatively long and the toes tended to be short. There were arches on the feet. It appears to have been well adapted for running
Homo ergaster had narrow hips. With the somewhat larger braincase, the narrow hips may have created problems in childbirth and, like modern humans, babies may have been born premature and required a period of parental care after birth. Nicholas Wade, in his book Before the Dawn: Recovering the Lost History of Our Ancestors, writes:
“Its pelvis was narrower than habilis’s, and the smaller birth canal meant that much of the infants’ extra brain size had to be acquired by growth after birth. This in turn means that infants would have to be carried, making the women more vulnerable.”
In their book
From Lucy to Language, Donald Johanson and Blake Edgar write:
“The implication is that Homo ergaster also gave birth to infants that required long-term constant care after birth, and perhaps in this strange twist on primate gestation lie the seeds of the elaborate socialization and prolonged learning that came to characterize the species.”
Turkana Boy, a nearly complete
Homo ergaster skeleton, was only 8 years old when he died but appears to be nearing maturity. This suggests that maturation was much faster than among modern humans.
Homo ergaster also shows a reduction in sexual dimorphism. Sexual dimorphism is a primate trait in which males are significantly larger than females. In the earlier Australopithecus afarensis, for example, males were about 40% heavier than females. Sexual dimorphism appears to be correlated to mating practices as well as to competition versus cooperation among males. Donald Johanson and Blake Edgar write:
“As a rough generalization, primates that exhibit little or reduced dimorphism tend to be monogamous.”
Nicholas Wade writes:
“Ergaster is the first species along the human lineage to show a sharp reduction in male size compared with female, although the females are still smaller. This is a hint of some important change in social structure, very possibly a switch from the separate male and female hierarchies of chimp communities to the male-female bond that characterizes human societies.”
There are also changes in the arms and hands. In
Smithsonian Intimate Guide to Human Origins, Carl Zimmer reports:
“Its forearms were short and its fingers were no longer hooked. Its shoulders were low and wide.”
Homo ergaster is often associated with a stone tool tradition known as Achulean which is also associated with Homo erectus. These stone tools were fashioned consciously and symmetrically on both sides, to a deliberate shape. In Paleofantasy: What Evolution Really Tells Us About Sex, Diet, and How We Live, Marlene Zuk writes:
“To obtain and process game animals, Homo ergaster made more sophisticated tools than earlier humans had made, including hand axes with nifty double-faced blades and sharp narrow points. Interestingly, Homo ergaster also shared a doubtless unwelcome advancement with modern humans: intestinal parasites.”
Donald Johanson and Blake Edgar write:
“The heavy-duty tools of ergaster suggest that this hominid was a butcher par excellence of hunted or scavenged game meat.”
With regard to language, Ian Tattersall, in his chapter in
The Epic of Evolution: Science and Religion in Dialogue, reports:
“There are incipient signs of the flexion of the cranial base that signals the presence of a vocal tract capable of producing the sounds associated with articulate speech, but speech and hence language are belied by the narrowness of the thoracic vertebral canal that carries the innervation of the thoracic musculature.”
Homo ergaster appears to lack the fine breathing control needed for modern spoken language. In his book
Humans: From the Beginning, Christopher Seddon explains it this way:
“The vertebral canal is the space in the vertebral column through which the nerve-bearing spinal cord passes, and the thoracic vertebrae are those located between the neck and the base of the rib cage. The nerves passing through this region control breathing, and he might therefore have lacked the fine breathing control necessary for modern speech.”
There is disagreement among paleoanthropologists as to whether or not
Homo ergaster should be considered a distinct species, or if it should be viewed as an African variation of Homo erectus. Those who favor the idea of
Homo ergaster as a distinct species point out that there are several features that distinguish
Homo ergaster from
Homo erectus, including thinner bones of the skull and the lack of an obvious sulcus, or depression, just behind the brow ridge. Without DNA, however, it is difficult to definitively resolve this disagreement.