This concept means that whereas the particulars of historical or evolutionary phenomena are not replicable in the sense of laboratory science, evolutionary and climate records do exhibit a surprising number of iterations—for example, the repeated emergence of certain food processing adaptations, recurrent geographic expansions, or the concurrent rise or loss of species diversity across diverse animal groups, all of which can be examined in relation to repeated periods of climate warming, cooling, heightened variability, or stability, along with the repetitive expansion and contraction of habitats. These iterations mean that there are numerous cases over time and space where the detailed relationships between climate and evolutionary response in humans can be defined, examined, and compared against the responses in the contemporaneous biota. Common patterns or regularities have the potential to yield new understandings of evolutionary processes and the influences of climate.
Common to all hypotheses linking climate change and faunal evolution is the notion that large-scale shifts in climate or climate variability altered the landscape ecology, which, in turn, presented specific adaptive or speciation pressures leading to genetic selection and innovation. This view holds that the most significant evolutionary junctures—those evolutionary and behavioral transitions that were fundamental to shaping who and what we are today—were linked to some aspect of paleoenvironmental change. As noted earlier, the possibility that environmental change had negligible impact on evolutionary change also needs to be considered, because factors such as genetic mutation, resource competition, and social interaction were in effect under all environmental conditions and thus may have impinged on evolution independent of specific environmental transitions. Even if this were so, evolutionary success or extinction depends on the increase or reduction of a species and its particular way of life, which are inevitably influenced by large-scale or abrupt changes in environmental conditions. On the basis of well-tested ideas in evolutionary biology, therefore, environmental variables are critical in shaping the adaptations and geographic distributions of all organisms, and are expected to have made a difference in shaping the course of human evolution, the success and demise of earlier hominin species, and, ultimately, the existence and influence of our own lineage.
Hypotheses of how climate change affected evolution generally start by correlating patterns of evolution recorded in continental basins with marine or lake climate records that are located hundreds or thousands of kilometers away. Identification of such broad correlations has stimulated productive research, but these largely independent efforts have yielded fossil samples and climate records that are distant from one another, unable to be analyzed quantitatively, or are otherwise inadequate to address critical questions about evolutionary processes (see, e.g., Barnosky, 2001). Future progress depends on how well scientists move