Ichnotaxonomy of the Laetoli trackways: The earliest hominin footprints

Abstract

At 3.6 Ma, the Laetoli Pliocene hominin trackways are the earliest direct evidence of hominin bipedalism. Three decades since their discovery, not only is the question of their attribution still discussed, but marked differences in interpretation concerning the footprints’ qualitative features and the inferred nature of the early hominin foot morphology remain. Here, we establish a novel ichnotaxon, Praehominipes laetoliensis, for these tracks and clarify the distinctions of these footprints from those of later hominins, especially modern humans. We also contrast hominin, human, and ape footprints to establish morphological features of these footprints correlated with a midtarsal break versus a stiff longitudinal arch. Original photos, including stereo photographs, and casts of footprints from the 1978 Laetoli excavation, confirm midtarsal flexibility, and repeatedly indicate an associated midfoot pressure ridge. In contrast, the modern human footprint reflects the derived arched-foot architecture, combined with a stiff-legged striding gait. Fossilized footprints of unshod modern human pedestrians in Hawaii and Nicaragua unambiguously illustrate these contrasts. Some points of comparisons with ape footprints are complicated by a variable hallucal position and the distinct manner of ape facultative bipedalism. In contrast to the comparatively rigid platform of the modern human foot, midtarsal flexibility is present in the chimpanzee foot. In ape locomotion, flexion at the transverse tarsal joint, referred to as the “midtarsal break,” uncouples the respective functions of the prehensile forefoot and the propulsive hindfoot during grasp-climbing. At some point after the transition to habitual bipedalism, these grasp-climb adaptations, presumed to be present in the last common ancestor of apes and humans, were initially compromised by the loss of divergence of the hallux. An analogous trajectory is evident along an array of increasingly terrestrial extant ape species. However, a flexible midfoot was retained, presumably to spare lateral toes from bending stresses by concentrating push-off from the forefoot, beneath the metatarsals. Only later did the evolution of the longitudinal arch permit increased mechanical advantage of the plantarflexors for speed and improved economy of endurance distance walking and running.