through rapidly changing technologies, harvesting of foods, and the long-distance exchange of resources. The way of life afforded by the transition from hunting-and-gathering to the production of food proved so successful that Homo sapiens was able to spread worldwide and increase in population density. Particularly over the past several centuries, these developments have led to a dramatic expansion in the human influence on global ecosystems.

The dynamic interplay between environmental changes and hominin speciation, extinction, adaptive change, and population size change has been played out on many different spatial and timescales. Three examples—from progressively older parts of the evolutionary and earth records—illustrate the way in which hominins may have interacted with the earth system and illustrate some of the enduring scientific questions that remain to be explored:

The Mayan “Collapse” Between A.D. 750 and 1150, the Classic Mayan civilization of southern Mexico and Central America underwent a dramatic transformation involving complex changes in Mayan society and an apparent collapse of population size by 70 percent or more (Turner, 1990; Rice et al., 2004). Archaeologists have long argued about the root causes of this collapse, and many explanations have been proposed for this enigmatic story. Could an understanding of the earth system context help unravel the causes and effects involved in the population collapse and the major transformations that occurred in Mayan civilization during this time? Over the past 15 years, evidence has been accumulated from sediment cores taken from lakes in the region that may help illuminate this relationship (Hodell et al., 2005). These detailed sedimentary records show that the climate history over the period of collapse consisted of a series of protracted droughts, separated by intervening moister periods. The timing of these droughts coincides with indications from geological records of dry conditions elsewhere in the tropical Americas (Haug et al., 2003). Although many scientists have argued for a linkage between this history of drought and the archaeological record of declining Mayan population size, the connection remains controversial (e.g., Diamond, 2005).

Climate and the Evolutionary Histories of Homo sapiens and Neanderthals There are continuing questions concerning the possible effect of regional climate differences on the evolution of two separate hominin species—Homo sapiens and Homo neanderthalensis. The first appearance of H. sapiens occurs in Africa, at the beginning of glacial stage MIS-6 (White et al., 2003; McDougall et al., 2005). Neanderthals arose in Europe (Klein, 2009) under the extremely cold conditions of the middle Pleistocene (Hublin, 2009), and continued to exist there through rapidly changing glacial and interglacial climatic regimes. Each species has distinctive anatomical characteristics that can be inferred to be adaptations to climatic conditions—Neanderthals were shorter with more robust limb bones and shorter forearms, comparable with cold-adapted peoples of today (e.g., the Inuit), whereas the modern human skeleton possesses longer and slenderer limb

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