The speed and displacement of the Laetoli Site G track-maker hominins

Abstract

Since their discovery in 1978, the hominin fossil footprints from Laetoli have been the focus, of extensive research on the locomotion, speed, body size, and behavior of the responsible track-makers (nominally Australopithecus afarensis). In this work, we show that careful examination of walking speed and displacement yields valuable information regarding the interaction between individuals, as well as group behavior of the track-maker hominins. First, we calculate statures for the Site G and S hominins using appropriate footprint length/stature ratios, obtaining statures ranging from 1.2 to 1.6 − 1.7 m. This range is somewhat taller than that proposed for Au. afarensis from skeletal remains (roughly 1.1 to 1.6 m), thus suggesting that comparatively large individuals are possibly represented in the Laetoli footprints. We then calculate speeds for the Site G track-makers from a stride length-speed scaling for humans. Stride and step lengths are measured from a digital model using the deepest point on the heel area of the G1 and G3 footprints as reference, and the deepest point on the hallux area for the overprinted G2 footprints. The hominins responsible for the Site G trackways all walked at a very similar mean speed, around 0.8 m s−1, which is faster than the speeds proposed by some previous studies. Detailed step-to-step speed calculations for G1 and G3 hominins show that the walking speed varied substantially (up to a factor of 1.5) over distances of just a few meters, perhaps due to surface conditions or behavioral reasons. The centers of mass of the G1 and G3 hominins traced very similar trajectories along the trackway portion examined. Thus, all Site G track-makers appear to have been moving together: the G1 and G3 hominins walked over the Laetoli ashes together in parallel, following behind the G2 hominin.