“If the early evolution of the human species were a machine shop, then Homo habilis would be the mixed parts bin, some of which could go into the next model, while some could go into an earlier one.”
This dilemma is at the heart of a fossil from northern Kenya that is incredibly informative, fossil number KNM-ER 64061. This partial skeleton dates back to the Koobi Fora Formation in the region of Ileret, a period of 2.02-2.06 million years ago. It is a rare assemblage of limb and shoulder bones that gives a remarkably complete picture of how H. habilis was able to use its body.
The species identity rests on the specimen’s close association with a nearly complete set of lower teeth and jaw fragments, KNM-ER 64060. Scientists have indicated that geological and chemical analyses confirm a very high probability that the teeth and the postcranial fossils belonged to the same individual, which is a rare occurrence in the Early Pleistocene fossil record. The association is important because it links the diagnostic dental anatomy to the rest of the skeletal system, which is exactly what is required for a comparison of bodies, not just faces, among early members of the genus Homo.
What immediately catches the eye is the torso. The skeleton comprises rib bones, the upper parts of the shoulder blades, and the upper and lower arm bones. This is sufficient to allow a characterization of proportions and muscularity with a degree of confidence that is rarely possible. The forearm is relatively long compared to the upper arm, a feature that can be described by a high brachial index and is characteristic of earlier hominins rather than later Homo. Even more remarkable is the fact that the arm bones have a remarkably thick cortex, a feature that betrays muscularity and is reminiscent of australopithecines, including the lineage that produced “Lucy.”
This “mosaic” body plan is not offered as a straightforward return to life in the trees. The original study warned against equating long, powerful arms with a straightforward conclusion about climbing behavior. Nevertheless, the biomechanical message is clear: this individual was better suited to bearing the weight of the upper limbs, whether for locomotion, carrying, or other repetitive activities that imprint their pattern on bone.
The estimates of body size contrast the species even further. Based on humerus length, estimates of height were around 1.6 meters (160 cm) tall. Weight estimates range from 30.7 to 32.7 kg, which is remarkably low compared to most depictions of H. erectus. These figures make the thick-walled arm bones even more fascinating, as powerful upper limbs were present in a body that was not particularly large.
The lower body is still the lingering hole. There were no leg bones found, but the sparse pelvic elements indicate lower limb dynamics more akin to the later Homo than to australopiths. This clue, combined with the primitive upper limb pattern, keeps H. habilis in an evolutionary limbo: perhaps more human-like locomotion but with an upper body that had not yet taken the same path as H. erectus.
The significance of the discovery is even more important in the context in which it was found. During the period between 2.2 and 1.8 million years ago, there coexisted in eastern Africa various species of human beings, such as Paranthropus boisei, H. habilis, Homo rudolfensis, and early H. erectus. In such a densely populated environment, the presence of a partial skeleton with teeth and substantial limb bones is important in understanding what distinguished the neighbors who may have shared the same habitat and resources. For now, KNM-ER 64061 does something deceptively simple: it makes Homo habilis harder to stereotype. The fossil fixes the species as neither a smaller version of H. erectus nor an australopith with a new name, but as a distinct morph one in which the arms retained older abilities even as the pelvis started to trend towards the later human stride.
