Body structure

Olduvai and Koobi Fora fossils have allowed researchers to make some determinations about the anatomy of early humans. It is clear that the braincase of H. habilis is larger than that of Australopithecus. The original finds from Olduvai Gorge include two sizable bones from the skull of OH 7. An incomplete brain cast was molded by putting the bones together to form a partial cranium. This cast has been used to estimate a total brain volume of about 680 cc. A brain cast from ER 1470, which has a more-complete cranium, can be measured directly; its volume is about 775 cc. One or two additional fragmentary skulls appear to be about the same size as that of ER 1470. Others are much smaller—such as ER 1813, which has a cranial capacity of only about 510 cc (31.1 cubic inches). Thus, brain sizes ranging from slightly more than 500 to nearly 800 cc (30.5 to nearly 48.8 cubic inches) seem to characterize H. habilis.

The skulls by and large have thin walls and are rounded, rather than low and flattened; they do not have the heavy crests and projecting browridges characteristic of later H. erectus. The underside of the cranium is shortened from the back of the palate to the rear of the skull, as in all later Homo species. This is an important contrast to the so-called gracile australopiths, in which the cranial base is relatively narrow and elongated.

The facial bones of several specimens are at least partly preserved, and facial proportions vary considerably. One of the Olduvai hominins, OH 24, seems anatomically similar to Australopithecus in having prominent cheekbones and a flat nasal region. This gives the central region of the face a depressed, or “dished,” appearance, and the upper part of the nasal profile is obscured by the cheek when the specimen is viewed from the side. Such hollowing of the face is characteristic of some South African australopiths but is not seen in later Homo. The facial skeleton of ER 1470 is large relative to the braincase, and it shows flattening below the nose—Australopithecus-like features. The walls of the nasal opening, however, are slightly everted, and there is at least an indication that the nose stands out in more relief than would be expected in australopiths. The face of ER 1813 is even more modern.

The front teeth of H. habilis are not much different in size from those of Australopithecus, but the premolar and molar crowns—particularly in the lower jaw—are narrower. The jaw itself may be quite heavily constructed like that of gracile australopiths. This is the case for OH 7 and also for at least one specimen from Koobi Fora. Other jaws are smaller but still robust in the sense of being thick relative to height. For example, the mandible of OH 13 is similar in many respects to that of H. erectus, and this individual might have been called H. erectus if its jaw had not been found along with small, thin vault bones.

Only a few other skeletal parts have been discovered. Some limb bones from Olduvai and Koobi Fora have been grouped tentatively with H. habilis on the basis of general anatomic similarity to later humans. These fossils, however, are not associated with any teeth or skulls, and it is probably not appropriate to use them as the basis for describing early Homo. One individual for which body parts are more fully represented is OH 62. Arm and leg bones of OH 62 are fragmentary, but the arm is relatively long. The skeleton may be similar in its proportions to small australopiths. OH 62 probably walked on two legs as efficiently as other early hominins, but this diminutive individual was unlike later humans in many respects.

Another important specimen is the immature hand of OH 7. These bones, found with skull bones, are still apelike in some aspects, but it is almost certain that the individual from which they came had dexterous hands. Stone artifacts and early Homo fossils have been found at Olduvai and other sites. These tools are called the Oldowan industry, and, though they are crude, they indicate that H. habilis could shape stone.

Tool use and tool construction, however, may not have originated with H. habilis or even with Homo itself. The oldest known tools, which were found near Kenya’s Lake Turkana, predate the emergence of Homo by almost 400,000 years. Barring the discovery of an as-yet-undiscovered species of Homo, the tools were likely constructed by members of Australopithecus or Kenyanthropus, who inhabited the region at that time.

Behavioral inferences

The stone tools and unused waste materials (mainly crude chopping tools and sharp flakes) left by H. habilis provide important clues about the behaviour of these early humans. Olduvai Gorge has been a rich source of Oldowan tools, and the tools are often found with animal fossils. Originally, the occurrence of artifacts with bones was interpreted to mean that H. habilis hunted animals and brought the carcasses to where it lived for butchering, but it is now known that the situation was more complicated. Assemblages such as those found at Olduvai can be created through various means, not all of which are related to hominin activities. Olduvai H. habilis certainly used animal products, however. With the aid of a scanning electron microscope, it has been shown that cut marks on some of the bones must have been made by stone tools, but this does not prove that animals were hunted. Analysis of Olduvai animal fossils also shows that some marks were made by either rodent or carnivore teeth, the indication being that at least some of the animals were killed by nonhominin predators. In all likelihood, the hominins at Olduvai could obtain larger carcasses only after the animals had been killed and partially eaten by other predators. H. habilis may have hunted small prey, such as antelope, but definitely was a scavenger.

It is debatable whether or not the Olduvai sites were home bases. Nothing recovered indicates that people resided where the animal bones accumulated. Such areas were presumably dangerous since they undoubtedly attracted numerous predators. These sites may have been caches of stone tools and raw materials that were established in areas convenient for rapid processing of animal parts. Therefore, where the hominins lived or whether their social structure was prototypical of later hunter-gatherers remains unknown, although H. habilis must have engaged in cultural activities.

Whether or not early Homo had acquired language is another fundamental question, and the indirect evidence on this issue has been variously interpreted. It is the belief of some anatomists that endocranial casts of H. habilis fossils indicate that the regions associated with speech in modern humans are enlarged. Others disagree with this assessment, particularly since the number of braincases preserved well enough to make detailed casts is small. Anthropologists have also based their interpretations on the archaeological record. According to some, the crude Oldowan artifacts indicate the ability to use language. Critics of this view assert that the Oldowan industry represents only opportunistic stonework. They argue that, because the later Acheulean tools of H. erectus are more carefully formed and are often highly symmetrical, this later hominin was the first to use symbols and language. One of the problems with this theory is that no clear link between technological and linguistic behaviour has been established—even the more-sophisticated tools could have been made by nonspeaking hominins. Thus, it is not certain when Homo developed the linguistic skills that characterize modern humans.