Sexual dimorphism in Homo erectus inferred from 1.5\u2009Ma footprints near Ileret, Kenya

Brian Villmoare,William Jungers,Kevin G Hatala

doi:10.1038/s41598-019-44060-2

Abstract

Sexual dimorphism can be one of the most important indicators of social behavior in fossil species, but the effects of time averaging, geographic variation, and differential preservation can complicate attempts to determine this measure from preserved skeletal anatomy. Here we present an alternative, using footprints from near Ileret, Kenya, to assess the sexual dimorphism of presumptive African Homo erectus at 1.5 Ma. Footprint sites have several unique advantages not typically available to fossils: a single surface can sample a population over a very brief time (in this case likely not more than a single day), and the data are geographically constrained. Further, in many cases, the samples can be much larger than those from skeletal fossil assemblages. Our results indicate that East African Homo erectus was more dimorphic than modern Homo sapiens, although less so than highly dimorphic apes, suggesting that the Ileret footprints offer a unique window into an important transitional period in hominin social behavior.

Highlights

One of the most important proxies for social behavior in a fossil species is its sexual dimorphism, which has the potential to provide a unique window into the social structure of an extinct population[1,2,3,4,5]
Our results find that the pattern of size dimorphism in the Ileret footprints is more consistent with the pattern seen in modern humans than with the much higher level of dimorphism seen in gorillas
Social behaviors of fossil taxa are often inferred from their apparent levels of sexual dimorphism, as this characteristic often relates to social structure

Summary

Introduction

One of the most important proxies for social behavior in a fossil species is its sexual dimorphism, which has the potential to provide a unique window into the social structure of an extinct population[1,2,3,4,5]. Prior to the discovery of KNM-ER 4270032, and recent work on postcranial fossils[33] (and see below), most analyses of variation in Homo erectus had identified only a relatively modest degree of dimorphism, similar to the pattern observed in modern humans[3,4,34,35]. This low degree of variation stood in sharp contrast to the higher degree of dimorphism seen in Australopithecus and Paranthropus (see above). A recent analysis by Grabowski et al.[33] has made the picture of sexual dimorphism in H. erectus even fuzzier, as they found a level of variation in postcranial measures, across both African and Asian samples that suggested a level of body size dimorphism that exceeded that observed in modern humans

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Conclusion