The Current Cast of Characters The Earliest Hominins
Modern apes and humans differ greatly, but the earliest hominins contrasted in subtle ways from living apes primarily in their increasing reliance on walking upright known as bipedalism. The skeletal indicators of bipedalism include a forward placement of the hole at base of skull where the spinal column enters (the foramen magnum), and a shortening and broadening of the pelvis to make it "bowl-shaped". These changes were accompanied by shifts in muscle groups particularly the gluteal and hamstring muscles, a lengthening of the lower limbs particularly the femur in the genus Homo, and changes in the feet to become weight-bearing structures (Poirier & McKee, 1999).
The earliest possible hominin to date is Sahelanthropus tchadensis from Chad in the Sahel region of sub-Saharan Africa, which has tentatively been dated to between 6 and 7 million years old (Brunet, 2002). The fossils include an almost complete cranium with a mosaic of ape and hominin features but no post-cranial skeletal material that could confirm whether Sahelanthropus tchadensis was bipedal.
The earliest hominin bipeds may have appeared between 7 to 5 m.y.a.. An example of these early bipeds may be the fossils of Orrorin tugenensis found in the Tugen Hills of Kenya and dated to 6 m.y.a. (Senut, et al., 2001). Thick-enameled although relatively small molars and a human-like femur link it with later hominins. It also exhibits muscle attachments on the humerus and curved finger bones that are consistent with arboreal activity linking it to the apes. Based on associated plant and faunal remains, Orrorin tugenensis probably preferred open woodlands near forests.
In addition to Orrorin tugenensis there are 11 specimens representing at least 5 individuals of Ardipithecus kadabba from the Middle Awash area of Ethiopia dating to 5.8-5.2 m.y.a. (Haile-Selassie, 2001). Another 50 partial individuals, representing a separate species, or possibly subspecies, Ardipithecus ramidus, were recovered at the 4.4 m.y. old site near Aramis, Ethiopia (Klein, 1999). The oldest specimens exhibit derived dental features that are only shared with later hominins (Haile-Selassie, 2001). The fossils from the Aramis site exhibit a forwardly placed foramen magnum and apparently free upper arms, traits consistent with bipedalism (Klein, 1999). However, the association with high altitude, closed canopy woodland habitat and thin enamel on the molar crowns are characteristics not found in later hominins. This suggests a species at or close to the shared ancestor of humans and modern chimpanzees.
Cranium of Sahelanthropus tchadensis (Photograph courtesy of the journal Science)
- Upper Femur of Orrorin tugenensis
(Photograph by Marc Deville courtesy of the journal Science)
Fossil hominin remains from the Late Miocene Middle Awash deposits assigned to Ardipithecus ramidus (Haile-Selassie, 2001).
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