Todays issue of Science contains a report on the identification of bipedal hominin footprints found at Ileret in Kenya. Of course older hominid footprints, probably of Australopithecus arafensis, were found some years ago at Laetoli and have become iconic images.
The new discovery is of fossil footprints which are morphologically distinct from the much older Laetoli footprints, and are thought to have been left by Homo ergaster or Homo erectus. These footprints were found on two distinct strata at the site. The authors have analysed the prints in detail using laser scanning. Stride analysis of the tracks suggests the originators were about 1.75 to 1.76m tall (though there are some sequences of prints from a smaller individual), considerably larger than A.afarensis. Some of the footprints (see panel B in the figure below) show well-preserved imprints of individual toes.
In normal walking of humans, the weight-bearing foot contacts the ground first via the heel, followed by the outside edge of the foot, the metatarsal heads, then finally the weight transfers to the ball of the foot, making the deepest impression on the substrate. As the walker moves to make the next step, weight is transferred to the big toe (the hallux) – the deepest impressions left in the footprint are often made under the first and second metatarsal heads, along with the big toe impression. This is distinct from that of great apes such as the chimp (which knuckle walks rather than walking properly bipedally) which have different foot morphology. In the case of the chimpanzee, the heel and lateral mid-foot make contact with the ground first, followed by contact with the lateral toes and the big toe – though the imprints left by chimps are much less stereotyped than are those of humans.
The Ileret prints are far closer in the details of their impressions to modern humans than to the older Laetoli prints – most notably this is shown by the angle of the hallux – this is the angle between the big toe and the rest of the toes. For footprints made by modern humans, this angle is around 8 degrees, while the Laetoli prints show an angle of 27 degrees. The Ileret prints have an abduction angle of 14 degrees. Most significant is the conclusion that the prints appear to have been made by individuals with a long stride and extended lower limbs.
The attribution of the prints to either Homo ergaster or Homo erectus is based upon estimates of body stature (height and weight) – data derived from post-cranial remains of Paranthropus boisei and Homo habilis indicate these were smaller than the animals responsible for the Ilerat tracks. While there remains uncertainty about this assignment, the tracks do represent the earliest evidence of a relatively modern human–like foot, adding to evidence of a hominin transition to forms with longer stride length, conferring advantages of a lower energetic cost of locomotion and correlating with archaeological indications of activities in a variety of ecological settings and the transport of resources.
M. R. Bennett, J. W.K. Harris, B. G. Richmond, D. R. Braun, E. Mbua, P. Kiura, D. Olago, M. Kibunjia, C. Omuombo, A. K. Behrensmeyer, D. Huddart, S. Gonzalez (2009). Early Hominin Foot Morphology Based on 1.5-Million-Year-Old Footprints from Ileret, Kenya Science, 323 (5918), 1197-1201 DOI: 10.1126/science.1168132