Sporadic sampling, not climatic forcing, drives observed early hominin diversity

Abstract

The role of climate change in the origin and diversification of early hominins is hotly debated. Most accounts of early hominin evolution link observed fluctuations in species diversity to directional shifts in climate or periods of intense climatic instability. None of these hypotheses, however, have tested whether observed diversity patterns are distorted by variation in the quality of the hominin fossil record. Here, we present a detailed examination of early hominin diversity dynamics, including both taxic and phylogenetically corrected diversity estimates. Unlike past studies, we compare these estimates to sampling metrics for rock availability (hominin-, primate-, and mammal-bearing formations) and collection effort, to assess the geological and anthropogenic controls on the sampling of the early hominin fossil record. Taxic diversity, primate-bearing formations, and collection effort show strong positive correlations, demonstrating that observed patterns of early hominin taxic diversity can be explained by temporal heterogeneity in fossil sampling rather than genuine evolutionary processes. Peak taxic diversity at 1.9 million years ago (Ma) is a sampling artifact, reflecting merely maximal rock availability and collection effort. In contrast, phylogenetic diversity estimates imply peak diversity at 2.4 Ma and show little relation to sampling metrics. We find that apparent relationships between early hominin diversity and indicators of climatic instability are, in fact, driven largely by variation in suitable rock exposure and collection effort. Our results suggest that significant improvements in the quality of the fossil record are required before the role of climate in hominin evolution can be reliably determined.

Keywords: Africa; climate; early hominin diversity; fossil record quality; sampling bias.

Fig. 1.

Early hominin diversity estimates, sampling metrics, and terrigenous dust flux through geological time. (A) Taxic diversity estimate (TDE). (B) Phylogenetic diversity estimate (PDE) based on the median of the four estimates shown in SI Appendix, Fig. S1. The blue envelopes represent upper and lower 95% CIs based on the median of 1,000 time-scaling replicates. (C) Hominin-bearing collections (HBCs). (D) Primate-bearing formations (PBFs). (E) Terrigenous dust flux curve (5). Original data appear in orange; interpolated means are shown in white circles with black outline. Data points are plotted at the midpoint age of each time bin.

Fig. 2.

Scatter plots showing the relationship between early hominin taxic diversity and possible explanatory variables. ∆ indicates that the time series has undergone generalized differencing before statistical testing. (A) Taxic diversity estimate (TDE) against primate-bearing formations (PBFs). (B) TDE against aridity (5). See SI Appendix, Table S2 for correlation coefficients and P values.