Most ancient species of Homo found in East Africa between 2.3 and 1.6 million years ago.
Based on anatomical similarities between the African apes and humans, Charles Darwin and Thomas Henry Huxley forecast that the oldest fossil evidence for human ancestors would be found in Africa (Huxley 1863). In 1924 Raymond Dart recognized a child’s skull from South Africa as a potential ancestral human and named it Australopithecus africanus (“southern African ape”). Sometimes referred to as “ape-men,” these early hominins were found in abundance at a number of sites in South Africa and were assigned to several distinct species.
After Mary Leakey’s discovery of a 1.8 million-year-old cranium at Olduvai Gorge in 1959, field research began to focus more extensively on East Africa and the Great Rift Valley. The cranium, Olduvai Hominid 5 (OH 5), popularly known as “Nutcracker Man,” due to its massive molars, was ultimately placed in its own species, Australopithecus boisei.
Oddly enough by 1959 much more was known about the anatomy, adaptations, and diversity of the genus Australopithecus than of the human genus Homo (Latin for man) to which modern humans, Homo sapiens (sapient, or wise man), belong. Most scholars were searching outside of Africa for the earliest members of the human genus, and many focused on Europe as the finishing school for modern humans.
Discovery and Anatomy
In 1960 the discovery at Olduvai Gorge of OH 7, a partial mandible containing 13 teeth, much smaller than those of A. boisei, was thought to represent a very different kind of early human ancestor from OH 5 (Leakey et al. 1964). Further excavation recovered other fossil bones presumably belonging to the same individual as the lower jaw. These included hand, wrist, and finger bones, left and right parietal bones (sides of the braincase) of the cranium, an upper molar, and a partial foot. The foot, mandible, and cranial bones exhibited carnivore tooth marks perhaps inflicted by a hyena responsible for the death of this individual.
This fragmentary, juvenile skeleton, also dated to 1.8 million years, indicated the presence of a second kind of hominin at Olduvai Gorge. Discovered by the Leakey’s eldest son, Jonathan, the find was dubbed “Johnny’s Child.” In 1964 Louis Leakey, Phillip Tobias, and John Napier published a new species, Homo habilis, in the journal Nature (Leakey et al. 1964). The species name meaning “handy man” was suggested by Raymond Dart, making reference to Oldowan stone tools found nearby likely made by this early human.
Oldowan tools include scrapers, choppers, spheroids, and proto-hand axes made by simple stone on stone percussion. The raw material, predominately quartz, was obtained from a source area several kilometers away from the lakeside where they were utilized to butcher meat that was scavenged from carnivore kills. The oldest Oldowan tools date back to 2.6 mya (million years ago), but there are no associated hominin remains.
Features such as the reconstructed cranial capacity for H. habilis of approximately 674 cc (roughly three cups of coffee) as well as the slender anatomy of the mandible and the narrow, elongated molars all pointed in the direction of Homo and not Australopithecus. Compared with A. africanus from South Africa, the cranial capacity for “Johnny’s Child” was some 50% greater justifying its placement in the genus Homo.
Controversy and Further Finds
This new species was widely contested by other paleoanthropologists, but further remains of H. habilis fossils from Olduvai Gorge added legitimacy to this new taxon as a valid species. These included OH 24, an adult, crushed, female cranium, referred to as “Twiggy,” OH 13, a female upper and lower jaw associated with cranial fragments, nicknamed “Cinderella,” and OH 16, a fairly complete male cranium, lacking a face, was dubbed “George.” H. habilis was an upright walking, early member of our genus, Homo, with a cranial capacity greater than Australopithecus and with hands capable of stone tool making, bearing jaws and teeth reduced in size and gracility compared to Australopithecus. Fossils assigned to this species have been found in Kenya, Ethiopia, and perhaps South Africa, but there is continued debate as to which of these finds are identical to H. habilis (Tobias 1991) as defined at Olduvai or constitute or another species of Homo (Wood 2014).
While the definition of H. habilis rests on cranio-dental features, little was known about the postcranial anatomy (below the neck) until the 1986 discovery by the Institute of Human Origins team from Arizona State University, Tempe, Arizona, of a partial skeleton, OH 62, of this species (Johanson et al. 1987). This specimen eroded from 1.8 million-year-old strata where some 302 fragments were recovered.
Parts of a right arm, humerus (upper arm), two forearm bones, radius and ulna, a fragmentary left femur (thigh bone), and a scrap of right tibia (shin bone) were recovered as well as associated splintered fragments of teeth, small shards of cranium, and fortunately a partial maxilla, the upper jaw. Reconstructed from 32 fragments, the maxilla held the key to the species identification of the skeleton. Not only did the maxilla resemble other H. habilis finds at Olduvai Gorge in anatomy and size but it also lacked the megadont teeth so characteristic of A. boisei.
The recovery of OH 62 constitutes the first time upper and lower limb bones of H. habilis were securely associated. This presumably female individual was relatively elderly as inferred from the heavily worn adult teeth. Estimated to be approximately a meter tall (39 inches), OH 62 is the smallest ancient hominin ever recovered, suggesting that the larger H. habilis at Olduvai Gorge such as OH 7 were probably males, indicating a large degree of size difference between males and females, a condition referred to as sexual dimorphism.
Although fragmentary, the humerus and femur afforded estimated lengths used in calculating a ratio of upper arm and lower thigh bones. In chimpanzees, quadrupeds, the humerus is 100% the length of the femur indicating that fore and hind limbs are used in locomotion. In modern humans, the humerus is only about 70% of the length of the thigh, a biomechanical arrangement reflecting our dependence on bipedal locomotion. Surprisingly in OH 62 the humerus is 95 per cent the length of the femur, similar to the condition seen in A. afarensis, the ‘Lucy’ skeleton. The implication is that evolutionary change toward modernity first centered on the cranio-facial region and only later on the postcranial skeleton.
Reconsideration of Homo habilis
In 2015 a state-of-the-art computer reconstruction of the original OH 7 mandible and cranium has thrown new light on the anatomical nature of this type specimen for H. habilis (Spoor et al. 2015). The mandible was severely distorted as a result of lateral forces during fossilization resulting in a significant distortion of the shape of the dental arcade. After mirror-imaging the undistorted left half of the mandibular dental arcade and correcting for a damaged area along the midline, the correct shape of the dental arcade could be reconstructed. Surprisingly the dental arch was not rounded, as is the case in modern humans, but exhibited relatively long, parallel tooth rows, reminiscent of Australopithecus.
A computer visualization of the cranial bones of OH 7 also revealed something surprising in that the cranial capacity for this specimen was significantly underestimated. Previous cranial capacity calculations centered around 674 cubic centimeters, but the new estimates suggested a much higher value around 729–824 cubic centimeters, falling within the range of Homo erectus, known from Java and elsewhere.
The mixture of more advanced features like an enlarged brain and more ancient features such as a long, parallel sided dental arch in OH 7 has significant implications for the placement of H. habilis on the human family tree. The more rounded, human-like dental of a 2.3 million-year-old maxilla, upper jaw, from the site of Hadar, Ethiopia, would not have articulated with the reconstructed OH 7 mandible. This suggests that prior to 2.3 mya, the Homo lineage had already split into two lineages implying that H. habilis from Olduvai Gorge is not necessarily the progenitor to later species of Homo as previously proposed.
The recovery of a 2.8 million-year-old partial mandible at Ledi-Geraru, Ethiopia, exhibits a mandibular anatomy and a dentition that places it comfortably within Homo (Villmoare et al. 2015). Other anatomical features of the jaw suggest evolutionary affinities with older Australopithecus afarensis mandibles. This suggests that Lucy’s species, A. afarensis, that disappears around 3 mya, should be considered the ancestor to the Homo lineage (Johanson 2017).