Repetition Without Repetition: A Comparison of the Laetoli G1, Ileret, Namibian Holocene and Modern Human Footprints Using Pedobarographic Statistical Parametric Mapping

Abstract

It is traditionally held that early hominins of the genus Australopithecus had a foot transitional in function between that of the other great apes and our own but that the appearance of genus Homo was marked by evolution of an essentially biomechanically modern foot, as well as modern body proportions. Here, we report the application of whole foot, pixel-wise topological statistical analysis, to compare four populations of footprints from across evolutionary time: Australopithecus at Laetoli (3.66 Ma, Tanzania), early African Homo from Ileret (1.5 Ma, Kenya) and recent modern (presumptively habitually barefoot) pastoralist Homo sapiens from Namibia (Holocene), with footprints from modern Western humans. Contrary to some previous analyses, we find that only limited areas of the footprints show any statistically significant difference in footprint depth (used here as an analogy for plantar pressure). A need for this comparison was highlighted by recent studies using the same statistical approach, to examine variability in the distribution of foot pressure in modern Western humans. This study revealed very high intra-variability (mean square error) step-to-step in over 500 steps. This result exemplifies the fundamental movement characteristic of dynamic biological systems, whereby regardless of the repetition in motor patterns for stepping, and even when constrained by experimental conditions, each step is unique or non-repetitive; hence, repetition without repetition. Thus, the small sample sizes predominant in the fossil and ichnofossil record do not reveal the fundamental neurobiological driver of locomotion (variability), essentially limiting our ability to make reliable interpretations which might be extrapolated to interpret hominin foot function at a population level. However, our need for conservatism in our conclusions does not equate with a conclusion that there has been functional stasis in the evolution of the hominin foot.

StW 573 <b><i>Australopithecus prometheus</i></b>: Its Significance for an Australopith Bauplan

“…This skeleton is the first discovered for which limb lengths can be directly measured, not estimated, and is roughly contemporaneous with the Laetoli footprint trail, so will be highly informative on gait of early australopiths such as the trail makers [reviewed in e.g. McClymont and Crompton, 2020]. This specimen, StW 573, is an adult Australopithecus prometheus, most likely female [Clarke, 2019a], and includes a partially deformed cranium, full dentition, hands and a partial left foot, almost complete upper and lower limbs, clavicles, scapulae, pelvis, ribs and vertebrae.…”

Intra-subject sample size effects in plantar pressure analyses

“…Pressure records are used to diagnose and evaluate abnormalities in the feet and lower limbs of humans (e.g., Frykberg et al, 1998;Pham et al, 2000;Boulton, Kirsner & Vileikyte, 2004) and companion animals (e.g., Stadig, Lascelles & Bergh, 2016;Romans et al, 2004), to identify therapeutic interventions (e.g., Arts & Bus, 2011;Bus et al, 2008;Paton et al, 2011;Melia et al 2021) and in furthering our understanding of fall avoidance in the elderly (Xi et al, 2020). They provide key insights into the modulation of foot function (e.g., Simpson et al, 1993;Maluf & Mueller, 2003;Stepháne 2008;Caravaggi, Leardini & Giacomozzi, 2016;Taş & Çetin, 2019), in addition to delivering fundamental insights into the evolution of hominid foot morphology and function (e.g., Vereecke et al, 2003;Crompton et al, 2012;Bates et al, 2013a;Bates et al, 2013b;DeSilva & Gill, 2013;McClymont & Crompton, 2021). Recent work using large intra-subject human datasets (>500 steps per subject) identified high levels of both inter-(i.e., between individuals) and intra-subject (i.e., within subjects step-to-step) variance in peak plantar pressure in the midfoot (Bates et al, 2013a), and across the whole plantar surface in healthy adults (McClymont et al, 2016).…”

“…Where larger sample sizes (n = 200 steps) have been considered, only one pressure metric (maximum peak pressure) was evaluated for sensitivity to the number of steps used to quantify the central tendency of individual participants (Melvin et al, 2014). Concerns about sample size have been highlighted by McClymont & Crompton (2021) in the study of fossil footprints (a parallel field to pressure analysis; e.g., Crompton et al, 2012;Bates et al, 2013b), in particular in cases where locomotor behaviour and function are interpreted from a very small sample size of sequential fossil footprints (e.g., Hatala et al, 2016), as usually is the case in this field. Therefore, it is currently unknown how the observed variance in different plantar pressure metrics reported from small sample sizes of steps (n < 50), compares to that reported from large sample sizes (n > 500) using different pressure metrics in barefoot treadmill walking.…”