You’ve been subjected to enough examples over the past few posts of how our anatomy differs from the extant apes, and that these differences allow us to stand upright and walk bipedally. But you deserve better.
This post is all about where and when we see the differences first appear in the hominin fossil record.
The putative hominins
These are fossils that may or may not be more closely related to humans than to chimpanzees, but still show certain adaptations which suggest they could have given rise to a lineage of bipedal apes which went on to produce modern humans.
The oldest of these fossils is called Sahelanthropus tchadensis (Sahel ape from Chad), dating to between 6 and 7 million years ago (Mya). This skull (fig. 1) has a few characteristics which suggest it is could have given rise to later hominins, including reduced prognathism (a less protruding jaw) compared to modern chimps. The foramen magnum in the base of the skull allows the spinal cord to pass from the brainstem in the skull to the vertebral column. In S. tchadensis the foramen magnum is in a relatively more forward position than in modern chimpanzees which would have helped balance the skull on the spine, making it easier for the neck muscles to keep the head facing forward when using a bipedal posture.
More direct evidence of the possibility of early bipedalism comes from fossils defined as Orrorin tugenensis (Original man from Tugen) discovered in the Tugen Hills in Kenya and dating between 5.7 and 6.6 Mya.
The near end of the thigh bone (femur) shown in fig. 2(b) shares many characteristics with later fully bipedal hominins. The head of the femur where it articulates with the pelvis is rotated forward to allow a more upright posture. The neck of the femur which holds the head is elongated to increase the balancing action of the small gluteal muscles when standing upright. It also has an oval cross section which suggests it is adapted to withstanding bending forces generated by the pull of the small gluteal muscles in balancing (see pt. ii)
Despite these features, x-rays and CT scans have shown that the internal structure of the spongy bone is more typical of a facultative (optional) biped rather than a habitual or obligate biped as modern humans are.
The most definitely described species within the fossil genus Ardipithecus (ground-ape) is Ar. ramidus (ground-ape root, as in the root of the evolutionary tree, although this fossil’s evolutionary position is highly debated). The genus is proposed to have existed from 4.3-5.8 Mya, with the fossils assigned to Ar. ramidus dating to 4.3-4.5 Mya (fig. 3).
The foramen magnum is further forward than in S. tchadensis and the bottom portion of the pelvis is rotated backward relative to the upper portion. There is also evidence that the iliac blades are rotated to allow a better balancing action for the small gluteal muscles (see pt. i), while the morphology of the talus suggests that Ar. ramidus possessed a bicondylar angle which positioned the knee close to the midline (see pt. ii and pt. iii).
Flat articular surfaces of the foot bones on the lateral margin (outside edge) of the foot suggest that this could have formed a stiff lever to aid propulsion in a bipedal gait, while the position of the articular surfaces of the phalanges suggests that the toes of Ar. ramidus bent backwards as the foot lifted, just as ours do when we walk. Together this means that the lateral and distal portions of the foot were somewhat adapted for bipedal locomotion.
Despite the adaptations I’ve just described, the skeleton of Ar. ramidus is clearly not that of an obligate biped. The hallux is clearly adducted, much like our thumb, the forearm is very long relative the lower leg and the phalanges of the hand are extremely long compared to those of modern humans, suggesting that grasping with the feet and suspending the body weight from the hands in arboreal locomotion was very important to Ar. ramidus.
So concludes our overview of the putative hominins. As ever, if you have any questions or would like to know more about the earliest hominin evolution, please leave a comment or get in touch on Twitter. Next time, it’s the turn of the archaic hominins.
Richmond, B. G., & Jungers, W. L. (2008). Orrorin tugenensis femoral morphology and the evolution of hominin bipedalism. Science, 319, 1662–5. doi:10.1126/science.1154197.
White, T. D., Asfaw, B., Beyene, Y., Haile-Selassie, Y., Lovejoy, C. O., Suwa, G., & WoldeGabriel, G. (2009). Ardipithecus ramidus and the Paleobiology of Early Hominids. Science, 326, 64–64, 75–86. doi:10.1126/science.1175802.