Early hominins evolved within non-analog ecosystems

Abstract

Present-day African ecosystems serve as referential models for conceptualizing the environmental context of early hominin evolution, but the degree to which modern ecosystems are representative of those in the past is unclear. A growing body of evidence from eastern Africa's rich and well-dated late Cenozoic fossil record documents communities of large-bodied mammalian herbivores with ecological structures differing dramatically from those of the present day, implying that modern communities may not be suitable analogs for the ancient ecosystems of hominin evolution. To determine when and why the ecological structure of eastern Africa's herbivore faunas came to resemble those of the present, here we analyze functional trait changes in a comprehensive dataset of 305 modern and fossil herbivore communities spanning the last ∼7 Myr. We show that nearly all communities prior to ∼700 ka were functionally non-analog, largely due to a greater richness of non-ruminants and megaherbivores (species >1,000 kg). The emergence of functionally modern communities precedes that of taxonomically modern communities by 100,000s of years, and can be attributed to the combined influence of Plio-Pleistocene C₄ grassland expansion and pulses of aridity after ∼1 Ma. Given the disproportionate ecological impacts of large-bodied herbivores on factors such as vegetation structure, hydrology, and fire regimes, it follows that the vast majority of early hominin evolution transpired in the context of ecosystems that functioned unlike any today. Identifying how past ecosystems differed compositionally and functionally from those today is key to conceptualizing ancient African environments and testing ecological hypotheses of hominin evolution.

Keywords: functional traits; megaherbivore; non-analog faunas; paleoanthropology; paleoecology.

Fig. 1.

The geographic distribution of the modern (continental map) and fossil (Inset) larger herbivore communities.

Fig. 2.

Temporal trends in the functional trait composition of eastern African large herbivore communities over the last 7 Myr. Purple shading represents the modern range of variation for each trait. White diamonds indicate fossil communities that fall within the modern range of variation for all 8 functional traits. Orange circles indicate fossil communities that are non-analog; they fall outside the modern range of variation for one or more of the functional traits illustrated here. Dark gray lines represent LOESS regressions with a smoothing factor of 0.75; 95% confidence limits in light gray (see SI Appendix for further information).

Fig. 3.

Comparison of the functional composition of modern and fossil large herbivore communities. (A) The first 2 axes of a PCA of richness-corrected functional trait residuals for modern and fossil communities; the light gray hull encloses the modern range of community variation, and the dark gray hull encloses the fossil range of variation. (B) PC1 and PC2 loadings for all functional traits. (C) Stacked barplot of the number of non-analog versus analog fossil herbivore communities grouped by their geological epoch. Fossil communities are considered non-analog if the residual for any given trait falls outside the modern range of variation (Fig. 2).

Fig. 4.

Temporal trends in the functional and taxonomic evolution of eastern Africa’s large herbivore communities relative to climatic and environmental proxies. (A) The percentage of C₄ vegetation inferred from soil carbonate δ¹³C of eastern African paleosol carbonates; %C₄ data from Faith et al. (26) using δ¹³C data compiled by Levin (16). (B) Dust flux proxy for aridity from the Mediterranean (Ocean Drilling Program [ODP] Site 967; red line) (69) and the Arabian Sea (ODP 721/722; yellow line) (2, 70). (C) Fossil PC1 scores through time (Fig. 3). (D) Fossil PC2 scores through time (Fig. 3). (E) The evolution of taxonomic modernity as reflected by the highest pairwise Simpson similarity index value observed between a fossil assemblage and each modern community (0 = no taxonomic overlap; 1 = complete taxonomic overlap). Orange circles in C–E indicate non-analog fossil communities, and white diamonds or triangles indicate analog communities or those lacking extinct taxa, respectively. Dark gray lines in C–E indicate LOESS regression (smoothing factor = 0.75); 95% confidence limits in light gray. (F) Changes in large carnivoran richness across 0.5-Myr bins. As a control for differential sampling of the fossil record, values represent residuals derived from an ordinary least-squares regression modeling the relationship between the number of fossil sites and the number of taxa in a given bin (see SI Appendix for full details and SI Appendix, Table S3 for raw data).