bones indicating adaptations for warm environments (Trinkaus, 1981; Churchill, 2006). Eventually, H. sapiens expanded across the globe whereas Neanderthals became extinct at ~28 ka. Although the case for a climatic role in creating and/or regulating adaptive differences between these two species has received support (Finlayson, 2004, 2008), any causal relationships between climatic events and species anatomy remain to be determined.


Bipedality and Vegetation Changes There has been a long-standing assumption that hominins became bipedal as a consequence of the climatically controlled expansion of grasslands in Africa (e.g., Darwin, 1871). However, this assumption has been challenged as additional hominin fossils, recovered over the past 15 years, were found together with fauna that did not indicate grasslands (Reed, 1997; White et al., 2006). The expansion of grasslands in Africa over the past 3 Ma has been used to suggest causation for many events in human evolution, including not only the origin of bipedalism (and thus the earliest hominins), but also the development of megadont molars (Teaford and Ungar, 2000), the origin of Homo erectus (Stanley, 1992), and the origination of two separate hominin lineages (Vrba, 1988; Stanley, 1992). These latter authors suggested that vegetation became more open with fewer trees during the appearance of Homo and Paranthropus, induced by cooler and drier climatic regimes over Africa, and that these grassland habitats were factors in the further speciation events for both lineages. Grasslands expanded and contracted across Africa in the past 5 Ma (Cerling, 1992), and the extent to which these expansions and contractions impacted human evolution remains to be determined.

There is a common thread in these three examples of interactions between our human ancestors and the earth system—in each case, scientists face major limitations in resolving fascinating questions about our origins and history. A transformation in our understanding of the human story requires an improved understanding of the timing of critical evolutionary and climatic events, an improved sampling of the fossil and archaeological evidence for critical intervals in human prehistory, and—perhaps most importantly—a dramatic change in the way in which earth scientists, climate scientists, and anthropologists work together to interpret this story.

DEMONSTRATING CAUSALITY FOR HUMAN-ENVIRONMENTAL INTERACTIONS

Although scientists frequently seek to demonstrate temporal and causative correlations between environmental and evolutionary events, the processes that underlie the connections between the two are poorly known. These processes play out over extended periods of time, rather than in the “instant” of time often invoked in other scientific disciplines to demonstrate correlation. Nevertheless, a combination of the fossil record and the geological record of past climates can be



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