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Understanding Climate’s Influence on Human Evolution 1 Introduction The intersection of human evolution and Earth’s environmental history brings together two areas of scientific study with exceptionally high public visibility and broad societal interest—human evolution and climate change. The origin of our species has long been a compelling focus of human curiosity, and the record of past climate change and its impacts on hominin evolution provide an ideal context for considering potential intersections between future climate change and the responses of our species to such environmental changes. Of all the records of fossil organisms, the one offered by paleoanthropology is unique for its rich evidence of behavioral and ecological interactions derived from hominin (Box 1.1) and other fossil remains, as well as the unique aspect of hominin material culture in the later part of the record. This fossil record contains a history of critical evolutionary events that have ultimately shaped and defined what it means to be human, including the origins of bipedalism; the emergence of our genus Homo; the first use of stone tools; increases in brain size; and the emergence of Homo sapiens, more advanced tools, and culture. Some of these events appear to have coincided with major changes in African climate, raising the intriguing possibility that key junctures in human evolution and behavioral development may have been climatically mediated. This report recommends high-priority areas of scientific enquiry that should be pursued to investigate this possibility.
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Understanding Climate’s Influence on Human Evolution BOX 1.1 Hominins and Hominids Throughout this report, the term hominin is used for any member of the evolutionary group of bipedal species most closely related to Homo sapiens that evolved following the split between humans and chimpanzees—it is a convenient way of referring to the evolutionary group that includes humans and our bipedal ancestors and evolutionary cousins. The term hominid includes all great apes, encompassing chimpanzees, gorillas, orangutans, and humans. HUMAN INTERACTIONS WITH ECOSYSTEMS All living things interact with the earth system—the combination of land, atmosphere, and oceans—that make up our environment. As the earth system changes over time, individual species respond to these changes. In some cases, species disperse to new locations that match their preferred habitats. They may also adapt to the environmental changes, which sometimes leads to the formation of new species. And in some cases, species become extinct. A simple example in today’s world is the change in the range and population size of the polar bear. As Arctic climate has rapidly warmed over the past ~50 years it has become increasingly difficult for polar bears to feed, as their means of hunting—stalking seals from sea ice—has become more precarious as the Arctic ice pack retreats. Eventually, with a near total loss of summer ice cover in the Arctic Ocean, polar bears may become extinct. Through the processes of evolutionary change, dispersal, and extinction, organisms also modify the earth system, often in profound ways. On the largest scale, the evolution of oxygen-producing microorganisms permitted subsequent multicellular organisms to evolve. Or at a very local scale, large animals in Africa, such as elephants, substantially modify their physical environment by altering vegetation patterns and thereby affect the remainder of their ecological community. Study of the relationship between environment and evolution thus depends on understanding the basic interactions between biological and earth processes. Humans are part of the global ecosystem and have an evolutionary history that has almost certainly been affected by—and in turn has affected—the earth system. The study of human evolution shows that, like other organisms, humans have evolved over a long period of time in the face of environmental challenges and opportunities. These challenges affected how early humans secured food, found shelter, escaped predators, and developed social interactions that favored survival. The capacity to make tools, share hunted-and-gathered food, control the use of fire, build shelters, and create complex societies based on symbolic communication set the stage for new ways in which humans interacted with their surroundings. More recently, humans have interacted with their surroundings
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Understanding Climate’s Influence on Human Evolution 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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Understanding Climate’s Influence on Human Evolution 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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Understanding Climate’s Influence on Human Evolution used to convincingly demonstrate that organism interactions with the earth system have contributed to the evolution of life on Earth over the past several billion years. One dramatic example is the evolution of the earliest photosynthesizing unicellular organisms, which radically altered the early earth system by adding free oxygen to the atmosphere and thereby eventually providing the conditions for animals to survive and diversify. The fossil record also demonstrates that the causative linkages and feedbacks do not always occur in simple or immediate ways—careful and creative investigations are usually required to demonstrate cause-and-effect relationships. A chemist can replicate an experiment many times to demonstrate a cause-and-effect relationship and can thereby reject a hypothesis when it is not supported by the replicated results. However, for historical sciences, our “experiment” has been run and it cannot be precisely replicated. In addition, there often are multiple causative factors as well as complicated feedbacks that controlled events recorded in the fossil and archaeological records. Accordingly, the task of historical scientists studying evolution is to test hypotheses through other means (e.g., Frodeman, 1995): By looking for robust correspondences of events in time and in the predicted cause-before-effect order. This requires an accurate and precise understanding of the ages of events. By testing whether the predicted cause-and-effect outcome took place multiple times, either under similar situations at different geologic times or, in the case of evolution and ecology, across multiple taxa (different organisms) for a given event. For example, multiple groups of animals with similar characteristics can be analyzed to determine whether their fossil records responded in similar ways to a proposed causative event (e.g., Vrba, 1988, 1992, 1995; Potts, 1996a, 1998). By “rerunning” this historical experiment multiple times with computer models, to test and understand the underlying dynamics of the possible cause-and-effect relationship as informed by a combination of hypothesized causal factors (climate forcing functions), initial environmental conditions, and findings from the fossil record. An important consideration in any discussion of causality is the possibility that hominin evolution was largely unaffected by climate change—the evolution–environment “null hypothesis.” Ecological factors such as predation, competition, and disease among organisms operate in all environments, and these interactions have an important influence on their evolutionary history. Such interactions can be—but are not necessarily—strongly shaped by climatic conditions with their resultant habitat characteristics, and thus detailed climate studies can provide a critical context for understanding evolution. For example, animals preying upon other animals
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Understanding Climate’s Influence on Human Evolution BOX 1.2 Statement of Task Earth scientists, paleoanthropologists, and archaeologists who study human evolution have long recognized the likelihood that environmental parameters, particularly paleoclimate, significantly impacted the evolution of our species. Nevertheless, many of the details of the paleoenvironmental context for the more than 7 million years of hominin evolution are poorly constrained, making inferences concerning the nature and extent of such impacts problematic. To address this shortcoming, an NRC committee will assess the present understanding of the earth system context for hominin evolution during the past 8 million years; describe high priority research directions for an enhanced understanding of the paleoenvironmental context for hominin evolution; and describe optimum strategies for achieving the priority research objectives, with particular emphasis on interdisciplinary initiatives. In addition, the committee will suggest strategies for broad scientific dissemination of credible information concerning the earth system context for hominin evolution. in grasslands have different capture techniques than predators inhabiting rain forests. Such ecological behaviors, which can be identified in the fossil record, serve as important ties that can help test the potential effects of climate on the evolution of organisms (Kappelman et al., 1997; DeGusta and Vrba, 2005). Although genetic mutations operate independently of climate change, the spread of beneficial mutations is central to the process of evolution. These mutations become widespread because natural selection relies on the concept that environment plays the vital role in the difference between evolutionary success and extinction. An improved understanding of environmental change—that is, the earth system context as a dynamic force in evolutionary success and extinction—will substantially advance the scientific understanding of life on our planet, including human evolution. COMMITTEE CHARGE AND SCOPE OF THIS STUDY The National Science Foundation, with responsibility for supporting basic research activities in the United States, commissioned the National Research Council (NRC) to identify focused research initiatives that would, over a 10- to 20-year period, transform our understanding of the origin of human adaptations to environmental change. The study committee was also charged to present advice on research implementation and public outreach strategies (Box 1.2). To address the charge, the NRC assembled a committee of 13 experts with disciplinary expertise spanning paleoanthropology, earth system science, climate
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Understanding Climate’s Influence on Human Evolution modeling, and genomics. Committee biographic information is presented in Appendix A. The committee held four meetings between September 2007 and October 2008, convening in Washington, DC; Irvine, California; Tucson, Arizona; and Woods Hole, Massachusetts (see Appendix B). The major focal point for community input to the committee was a 2-day open workshop held in February 2008, where concurrent breakout sessions interspersed with plenary addresses enabled the committee to gain a thorough understanding of community perspectives regarding research priorities. Additional briefings by sponsors and keynote addresses from other speakers were presented at the initial meeting of the committee. This report is organized following the task statement—Chapter 2 contains a description of the existing scientific understanding of the approximately 8-million-year record of hominin evolution and climate change in the regions where hominins lived, as well as the interaction between other relevant organisms and climate. Chapter 3 contains a description of two overarching research themes, encompassing a range of individual research initiatives, which have the greatest potential to provide major advances in the understanding of potential interactions between the earth system and hominin evolution, and Chapter 4 describes the implementation strategies that will be needed to address these themes and initiatives. Recognizing deficiencies in the understanding of evolutionary principles and processes in the wider nonscientific community, Chapter 4 also contains the committee’s response to the specific charge to suggest dissemination strategies. Chapter 5 briefly summarizes the conclusions and recommendations arising from the earlier chapters.