For the first four million years before the evolution of Homo habilis, the fossil record is characterized by primitive anatomical features, such as reduced molar size and an elongate postcranial shape. Examples of such species include the ape-human forms Sahelanthropus, Orrorin, Ardipithecus, Australopithecus, and Paranthropus. Around 2.5 million years ago, as the global climate changed at the end of the Pliocene, a parallel shift in the human fossil record can be observed. As the environment cooled, anatomical and behavioral changes in the fossil record became apparent, marking the origin of the genus Homo, to which modern humans belong.
The Discovery of Human Fossils
In 1964, Louis Leakey, Philip Tobias, and John Napier addressed the question of where the Australopithecus lineage ends and Homo begins (Leakey et al., 1964). The discovery of skull fragments, jaw fragments, and wrist and limb bones from Olduvai Gorge, dating to about 1.75 million years ago (Susman, 2008), further piqued the authors' curiosity about these human fossils. Until now, no Homo specimens have been recorded that show more primitive features than Homo erectus. Subsequent discoveries of early Homo fossils in East Africa have been largely unsuccessful (Leakey, 1961, 1971, 1973a; b; Day, 1971; Day & Leakey, 1973; Day et al., 1976; Johanson et al., 1987). These findings contributed to the widespread scientific acceptance of Homo habilis, though it was established only after 15 years of resistance (Tobias, 2009).
The most well-known species of the earliest members of the genus Homo include Homo habilis, Homo rudolfensis, and Homo erectus (Table 1). Compared to Homo erectus, Homo habilis and Homo rudolfensis exhibit more primitive evolutionary traits. Much of the research on how these species differed anatomically and behaviorally from each other, and from earlier Australopithecus species, remains limited. What is clear, however, is that these three hominids represent a significant leap forward in human evolution, particularly in terms of brain and body size, as well as the use and manipulation of increasingly complex tools.
It is widely accepted that Homo habilis, which dates back 2.3 to 1.4 million years, is a descendant of the Australopithecus species. If Homo habilis evolved in East Africa, it would have evolved from the ancestors Australopithecus afarensis and Australopithecus gahi, while if it evolved in South Africa, Australopithecus africanus and Australopithecus sediba would be candidates. The answer to this question will depend on future research, but based on current studies of Homo habilis, it is generally accepted that it was a common ancestor of all later species of the genus Homo, including Homo sapiens.
Leakey, Tobias, and Napier continued to argue that the fossils they found did not have the brain or body size of Homo erectus and were not as primitive and ape-like as Australopithecus. They introduced several basic criteria for specimens to belong to the genus Homo. First, the brain size must be greater than 600 cubic centimeters, which is larger than the cranial capacity of all Australopithecus specimens. Additionally, the bones of the skull must be smooth and rounded, with minimal or absent hair compared to Australopithecus.
Beyond cranial morphology, they argued that a second criterion for belonging to the genus Homo was the ability to make stone tools, which would indicate an increase in behavioral and cognitive complexity associated with a larger brain. Although no Homo fossil skeleton has yet been found in a context with stone tools, archaeologists conclude that they were likely the creators of nearby stone tools.
Leakey and colleagues further suggested that the face and jaw of Homo were smaller than those of Australopithecus, and that the postcranial skeleton was similar to that of modern Homo sapiens. Despite the regular upright locomotion of Homo habilis, it still retained some primitive features (such as a slightly divergent big toe). Unlike earlier Australopithecines, however, the feet were fully adapted for walking on two feet. The most striking features of the Homo fossil record include complete bipedalism and a large cranial capacity. Additionally, there was a smaller tooth arrangement, with relatively smaller molars and premolars compared to the incisors, thicker enamel, and a parabolic dental arch. That is, the teeth formed a horseshoe shape, rather than the V- or U-shaped arrangement seen in earlier hominids and non-human apes.