Abstract
Human footprint fossils have provided essential evidence about the evolution of human bipedalism as well as the social dynamics of the footprint makers, including estimates of speed, sex and group composition. Generally such estimates are made by comparing footprint evidence with modern controls; however, previous studies have not accounted for the variation in footprint dimensions coming from load bearing activities. It is likely that a portion of the hominins who created these fossil footprints were carrying a significant load, such as offspring or foraging loads, which caused variation in the footprint which could extend to variation in any estimations concerning the footprint’s maker. To identify significant variation in footprints due to load-bearing tasks, we had participants (N = 30, 15 males and 15 females) walk at a series of speeds carrying a 20kg pack on their back, side and front. Paint was applied to the bare feet of each participant to create footprints that were compared in terms of foot length, foot width and foot area. Female foot length and width increased during multiple loaded conditions. An appreciation of footprint variability associated with carrying loads adds an additional layer to our understanding of the behavior and morphology of extinct hominin populations.
Highlights
In the process of reconstructing the hominin lineage, every available clue has the potential to provide key insights into the timeline and pathway by which the hominin lineage has evolved
Since many studies pool the sexes even in unloaded conditions, we investigated how much of the variation between the sexes in the unloaded condition alone was due to size, and how much might be due to sex-specific differences in foot morphology
These results indicate that both sex and load can significantly and substantially influence features of footprint shape and can possibly confound evidence as to the size of the footprint maker
Summary
In the process of reconstructing the hominin lineage, every available clue has the potential to provide key insights into the timeline and pathway by which the hominin lineage has evolved. Skeletal elements have been the basis for the identification of newly discovered hominin species [e.g. Ardipithecus ramidus [1], Australopithecus sediba [2]] and provide insights into what specific and unique adaptations each species may have had, the intrapopulation diversity [3], and when particular human-like traits (e.g. increased encephalization and bipedalism) first appear in the fossil record. Skeletal remains provide valuable insight into morphology, behavior and niche. The stature of extinct hominins has been estimated based upon the reconstruction of the skeleton [4, 5], femur/stature ratio [6], and regression formulae [7, 8] with each method independently producing comparable results. Hominin body mass can PLOS ONE | DOI:10.1371/journal.pone.0118619. Hominin body mass can PLOS ONE | DOI:10.1371/journal.pone.0118619 March 4, 2015
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