Since 1964 more material has been discovered, not only at Olduvai but at other African localities as well. One intriguing specimen is OH 24. This cranium is more complete than others from Olduvai. Because some of the bones are crushed and distorted, however, the face and braincase are warped. OH 24 may differ from Australopithecus in brain size and dental characteristics, but it resembles the australopiths of southern Africa in other features, such as the shape of the face. Complete agreement concerning its significance has not been reached, partly because the fossil is damaged.

Several mandibles resembling that of OH 7 have been recovered from the Koobi Fora area, and teeth that may belong to H. habilis have been found farther to the north, in the Omo River valley of Ethiopia. Some additional material, including a badly broken cranium, are known from the cave at Swartkrans in South Africa. At Swartkrans the fossils are mixed with many other bones of robust australopiths. An early species of Homo may also be present at Sterkfontein, not far from Swartkrans. Here again the remains are fragmentary and not particularly informative.

A more valuable discovery was reported from Olduvai Gorge in 1986. A jaw with teeth and skull fragments as well as pieces of a right arm and both legs were found. The bones seem to represent one individual, called OH 62. Although the skull is shattered, enough of the face is preserved to suggest similarities to early Homo. The find is especially important because of the limbs, which show that OH 62 was a very small hominin. The arm is long relative to the leg, resulting in body proportions that differ dramatically from those of more modern hominins.

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.

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 (Oldowan industry) 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 (Acheulean industry) 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.

The general interpretation of the fossil evidence is that H. habilis is not only substantially different from Australopithecus but that it represents the beginning of the trends characterizing human evolutionary history, particularly expansion of the brain. Some specimens clearly have a larger cranial capacity than that of Australopithecus, and the capacity increases progressively afterward with H. erectus (Homo erectus), archaic H. sapiens (Homo sapiens), and modern humans. H. habilis is also thought to exhibit the origins of other trends such as smaller teeth and changes in facial structure, especially the nasal region.

The theory that H. habilis is intermediate between relatively primitive Australopithecus and more advanced Homo appears to be generally accurate, but several aspects of this view can be challenged. Although there are not many H. habilis fossils, it is becoming clear that there are anatomic differences among the East African assemblages. Some of the newer discoveries have confirmed the expectation that early Homo craniums should be relatively large, with the rear of the skull being rounded and its base shortened. Other fossils have proved less easy to assign to H. habilis, and there has been controversy over their interpretation. Some braincases are considerably smaller, and it is frequently suggested that this variation is one of the differences between males and females (sexual dimorphism), the larger skulls being ascribed to males. But there are differences in shape as well as size, and several of the smaller skulls depart from the morphology of large-brained H. habilis in ways that are not obviously related to sex. There are also facial similarities between some specimens and Australopithecus. Thus, there is the possibility that two different species rather than one sexually dimorphic group are actually represented by the fossils.

Several approaches have been used to date H. habilis fossils from Olduvai, and a reasonably accurate timescale for Olduvai has been developed. The oldest remains, including OH 24, are from about 1.85 mya. Others such as OH 7 and OH 62 are not quite so ancient. The youngest Olduvai skull that is representative of early Homo is OH 13. No radiometric date for it is available, but other dating methods estimate it to be about 1.5 million years old.

In the Koobi Fora region a number of important fossils have been located near a level of volcanic ash that also contains stone tools. This ash bed was dated to about 2.6 mya. When ER 1470 was discovered several metres below this layer in 1972, it was thought that the newfound cranium must document Homo from a time well before the Olduvai deposits had accumulated. This assumption was soon questioned on the basis of other evidence, however, and before 1980 it was clear that the age had been overestimated. A series of radiometric determinations done subsequently has yielded a date of 1.88 mya. The ER 1470 skull and other H. habilis specimens recovered below this ash layer, therefore, must be close to two million years old. Evidence from East Africa thus suggests that H. habilis lived for a half-million years or so before giving way to later Homo species.

Classifying the various specimens separately means that each must be fitted into a scheme of hominin descent, or phylogeny. One interpretation assigns specimens with smaller craniums to a gracile species of Australopithecus. According to this scenario, only the larger skulls represent early Homo evolution. Others, questioning the notion that all species of Homo proceed along a simple linear progression, believe that early human populations were more diverse than has been recognized; although these researchers recognize two separate species, they prefer to lump both in the genus Homo. In this view, two species may have lived contemporaneously 2.0–1.5 mya. Only one could be the direct ancestor of H. erectus, and so perhaps it was the large-brained form that evolved further, while the smaller hominin became extinct.