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Understanding Climate’s Influence on Human Evolution Summary The hominin1 fossil record documents 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, tools, and culture. The geological record suggests that some of these evolutionary events were coincident with substantial changes in African and Eurasian climate, raising the intriguing possibility that key junctures in human evolution and behavioral development may have been affected or controlled by the environmental characteristics of the areas where hominins evolved. However, with both a sparse hominin fossil record and an incomplete understanding of past climates, the particular effect of the environment on hominin evolution remains speculative. This presents an opportunity for exciting and fundamental scientific research to improve our understanding of how climate may have helped to shape our species, and thereby to shed light on the evolutionary forces that made us distinctively human. Employing a systematic research strategy and guided by recent discoveries, the discipline is poised to make major advances concerning possible causative linkages between human evolution and Earth’s climate history. The intriguing possibilities regarding the role of climate in the evolutionary trajectories of our ancestral lineages can only be clarified—and causation established—with addi- 1 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. The term hominid includes all great apes, encompassing chimpanzees, gorillas, orangutans, and humans.
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Understanding Climate’s Influence on Human Evolution tional evidence that will require more sophisticated tools. Significant progress into the question of whether past climate changes influenced human evolution will require a coordinated, focused, and cross-disciplinary research program designed specifically to address this problem. Although we have a broad understanding of African and Eurasian climate history, this climate record generally lacks the temporal resolution and details of rainfall and temperature that potentially impacted how the hominins lived, and in particular does not adequately reflect differences in past climates between regions. Improved climate records for specific regions will be required before it is possible to evaluate how critical resources for hominins, especially water and vegetation, would have been distributed on the landscape during key intervals of hominin history. The existing records of earth system history and hominin fossil history also contain substantial temporal gaps. A general understanding of the timing of major events in human evolution exists, but our ability to interpret what has driven these events remains limited by a paucity of fossil material, particularly over the most interesting periods of rapid evolutionary change. Major breakthroughs will almost certainly have to await discoveries of additional hominin fossils and associated archaeological materials. New hominin fossil discoveries should enable the more precise understanding of the ages of various events in the hominin evolutionary record that will be needed for robust correlation of climatic and evolutionary events. Similarly, a broad understanding of earth system history, and particularly past climate history, has been gleaned from other fossils found associated with hominin fossil discoveries and from analyses of lake and ocean sediment cores. This material provides a wealth of base information that can be used with the present generation of global climate models to understand paleoclimate characteristics and the factors that controlled past climates, particularly at continental and regional scales, but these are still limited for understanding the local climates that are so important for evaluating causative factors involved with hominin evolution. This report proposes focused research initiatives that are designed, over a 10-20 year period, to dramatically improve our understanding of this research problem. These initiatives are presented in two major research themes. Theme I: Determining the Impacts of Climate Change and Climate Variability on Human Evolution and Dispersal Hypotheses linking climate change and hominin evolution are based on indications that large-scale shifts in climate or climate variability altered the landscape ecology which, in turn, presented specific adaptive or speciation pressures that led to genetic selection and innovation. However, efforts to test such hypotheses are fundamentally data-limited, constrained by gaps or poorly studied intervals in the fossil and archaeological record, coupled with the highly variable
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Understanding Climate’s Influence on Human Evolution fossil density from different time periods and regions; the inconsistent collection of all components of available fossil assemblages (e.g., invertebrates, vascular plants and algae, as well as vertebrates), which have potential to offer critical tests of the climate evolution relationship; as well as by stratigraphic and geochronologic limitations. Concerted international efforts to substantially enhance the fossil hominin, archaeological, and other faunal records of evolution are necessary to establish with statistical reliability the precise first and last appearances of species, adaptations, and behaviors within particular geographic regions and strata. Precise determinations of the timing of evolutionary events are essential for more rigorous analyses of the climate-evouliton relationship. At present there are few continuous quantitative paleoenvironmental records situated close to hominin fossil localities. By bringing the evidence of climate change and evolutionary events into close proximity, particularly by collecting high-resolution environmental records at or near the hominin fossil sites, it will be possible to test the extent to which those evolutionary events reflect responses to regional or local climate. In addition to the high-precision records of climate change that are required from long stratigraphic sequences located close to hominin sites, lake and ocean drilling records will be needed to integrate local climate records from hominin sedimentary basins with regional and global records. Understanding past climates depends on a range of data that can be used to quantitatively reconstruct the range of climatic variables—temperature, precipitation, seasonality, vegetation and land cover, paleoaltitude, etc. It is important that new tools for quantitative reconstruction of past environmental conditions continue to be developed and applied to new and existing stratigraphic records. A key requirement for each of these elements will be formalized research funding to encourage scientific exchange and strategic analysis of climate evolution hypotheses by earth scientists, paleoanthropologists, and faunal researchers. Theme II: Integrating Climate Modeling, Environmental Records, and Biotic Responses This research theme describes research strategies to define the physical and biotic mechanisms whereby past environmental changes may have produced evolutionary (and behavioral) responses in fossil hominins. The approach will require developments in climate model and paleoecologic data integration. The aim of this effort is to use the new data collected under Theme I to constrain climate model simulations to explore the physical mechanisms and regionality of past climate changes. Existing environmental records are too sparse to draw firm conclusions about particular geographic patterns of climate in Africa and Eurasia and their variability, or about climate conditions along pathways to southern Eurasia, or temporal and spatial variability of Eurasian climates. In parallel with the efforts to collect additional environmental records proposed in Theme I, there is also a need for a program to integrate regionally resolved climate models with
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Understanding Climate’s Influence on Human Evolution paleoecologic data. These would be developed for the specific regions and specific key time periods that bear on potential connections between environmental changes and hominin evolution and dispersal. Model-record experiments will be particularly important for developing predictions in data-sparse regions, as the basis for hypotheses that can be tested by the collection of new data. Because climate models simulate spatial and temporal patterns on a regular grid and at regular time intervals, they can provide a context for integrating or synthesizing environmental and fossil records that are discontinuous in space and time, or are otherwise incomplete. The large-scale record of climate change over the past 8 million years (Ma) is reasonably well known, although much remains to be understood about the interactions and feedbacks among the various climate forcing factors. At issue are the relative influences on African and Eurasian climate from orbital monsoonal forcing, high-latitude ice volume changes, atmospheric carbon dioxide (CO2) changes, Tibetan and East African uplift, and sea-level or ocean gateway changes, all of which changed over this time period. The focus of this new research initiative is to understand how changes in these boundary conditions affected climate in the specific regions where hominins lived. For example, the possibility of a dispersal of H. erectus or H. heidelbergensis from Africa to Eurasia at approximately the time of the climatic transition to large and long glacial/interglacial cycles (1.2-0.8 Ma) raises intriguing questions concerning the effects of these major climate swings on the environments inhabited by Homo in Africa and Eurasia. Moreover, the changing amplitude and duration of the orbitally forced changes in climate, other quasi-periodic changes of shorter duration, or relatively gradual changes in forcing, may have had different effects on ecosystems than the abrupt changes that are also a characteristic of the climate record. With improved density, accuracy, and dating of environmental records, there is a major opportunity to use climate models to ask more detailed questions and to obtain more detailed information about the climate, water resources, and vegetation comprising hominin habitats. In particular, the availability of accurate estimates of atmospheric CO2 will permit simulation of the direct effects of greenhouse gases on climate, as well as the possible physiological effects on vegetation of changing levels of CO2. These models can only be accurately tested by reference to actual paleoenvironmental data from Africa and Eurasia and the surrounding oceans. With the availability of greenhouse gas records and known orbitally controlled changes in solar radiation, along with known changes in orography, volcanism, coastlines, and ocean gateways, models have proven to be remarkably accurate in simulating past climates. Implementation Strategies Implementing this research vision will require community and organizational flexibility as it embraces a more collaborative and cross-disciplinary model,
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Understanding Climate’s Influence on Human Evolution involving a transformative shift in how paleoanthropological research is conducted. Although the exploration of human origins is inherently an international activity, large field-based research projects have been mostly conducted and funded along national lines, and broader partnerships and funding efforts are still relatively rare. International collaboration and cooperation focused on understanding the extent to which the earth system was a factor in human evolution offers the potential for applying the intellectual, organizational, and funding resources of a much wider community to an important research problem. We envision a new scientific program for international climate and human evolution studies that involves both essential and supporting components. Three elements should be carefully integrated to comprise the core program of research: A major exploration initiative to locate new fossil sites and to broaden the geographic and temporal sampling of the fossil and archaeological record. This would involve systematic exploration efforts with a remote sensing component to identify new potentially fossiliferous areas and sites, coupled with a substantially enhanced program of ground exploration. A comprehensive, integrated scientific drilling program in lakes, lake bed outcrops, and ocean basins surrounding the regions where hominins evolved. Drill cores containing the continuous, fine structure of the environmental record are needed to address questions about changes in the earth system at sufficiently high resolution to describe short-duration events and processes. Each component of such a program—including truck-mounted terrestrial drilling, barge-mounted lake drilling, and ocean drilling—would provide complementary elements that could be integrated to describe the paleoclimatic, paleohydrologic, and paleovegetation history of specific regions. A major investment in climate modeling experiments for the key time intervals and regions that are critical for understanding human evolution, focused on understanding the regional climate patterns and fundamental climate forcing mechanisms, and to model at a more local scale the interactions between climate, ecosystems, and species population dynamics. Simulations at high spatial resolution will be required to resolve the relatively fine-scale details of climate, vegetation, and hydrology that are recorded in environmental records in regions of complex local topography, such as the East African Rift System. In addition, there are a number of components that will be required to complement the core research effort: A systematic analysis of existing fossil sites and collections, with application of new imaging and dating technologies, to better describe the nature and timing of species change and adaptive transitions in the hominin lineage. New techniques that should allow inexpensive sequencing of entire mam-
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Understanding Climate’s Influence on Human Evolution malian genomes, making it feasible to collect high-resolution, whole-genome sequences of a range of mammals. This technology could be used to sample Homo sapiens and other mammalian populations with varying ecologies from all parts of Africa, to enable comparisons of genetic changes with climate changes over the past 200,000 years. Climatic changes could also be contrasted with estimated population sizes of H. sapiens and other mammals, based on population genetics parameters, during this time period. Selected investigations of ecosystem dynamics through the collection of modern climate and calibration data will more accurately quantify the relationships between the environment and proxy records of the environment preserved in sediments and fossils. An important contribution to the understanding of evolutionary and environmental dynamics is the analysis of fauna and flora associated, geographically and temporally, with hominin fossils. An increased focus on adaptations in the faunal and floral assemblages associated with hominins—and by contrast, those that are not associated with hominins—will provide an invaluable resource for understanding the interaction between hominin evolution and past climates. Development of the informatics and data archiving tools needed to provide permanent storage for the wide array of information collected by the activities listed above, and to facilitate continued access to this information. An important corollary requirement will be speedy community access to samples and their derived data within all of the disciplinary areas encompassed by this initiative—the hominin fossils and their associated fauna and flora; as well as the ocean, lake, and terrestrial drilling samples and data. The coordination and management of a major international scientific program for International Climate and Human Evolution Research would require a science advisory structure, with members representing the broader scientific community and with a broad vision of how these research components relate to each other, to foster communication among disciplinary groups, coordinate the implementation elements, and convey the science community’s priorities to funding agencies. On the basis of community input, an advisory committee would establish and periodically update plans for exploration, drilling, and modeling, and prioritize regions to be investigated. This committee would require sufficient funding to sponsor a range of workshops and town meetings, spanning the full range of disciplines associated with this research enterprise, to obtain and distill community input. Public Outreach Opportunities The public is fascinated by media accounts and documentaries on human evolution and the long-term origin of our species. Additionally, climate change has become a focal point for public interest. The intersection of these two broad
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Understanding Climate’s Influence on Human Evolution areas of scientific research thus offers powerful opportunities for public out-reach aimed at communicating the process and value of science to the welfare of humans, all living things, and entire ecosystems. The subject matter itself, which deals with human survival and adaptation in the past, also offers avenues for inspiring the public’s curiosity about scientific findings relevant to society’s adaptation to climate change in the near and distant future. A state-of-the-art program in public education and outreach creates opportunities for diverse audiences along several avenues, which include (1) development of dynamic and up-to-date public Internet sites; (2) dissemination of findings via the Internet or using print, radio, and television media; (3) organization of seminars, lectures, and dialogues in venues that are both visible and attractive to the public; (4) interaction with national science educators, who can translate scientific findings and data into the classroom; (5) development of museum-based and less formal exhibitions, which are attractions for family and school-group explorations of and learning about science; (6) engagement of adult learners in the excitement of research and discovery and encouragement of volunteerism (docents); and (7) provision of graduate, undergraduate, and high-school training and research experiences, which offer a means of building the future generations of scientists and educators. As the items in this list illustrate, an effective program of public education and outreach requires skillful approaches to formal and informal learning in which children and adults decide whether to pursue (and for how long) any particular topic that interests them. No curriculum currently exists to inspire teachers and students to explore the relationship between past climate change, human evolution, and the long-term influence of environment on species survival, adaptation, and mitigation strategies. The following suggestions represent components of a broad effort to redress this deficiency: Develop opportunities that bring educators and scientists together, and that build new partnerships among research institutions, museums, science centers, and national scientific and education organizations, to contribute to the development of national and state science standards. Establish a National/International Educator Institute as a long-term effort that employs climate-evolution research to enhance professional educator development. Establish internships that connect students and teachers to the international scope and nature of scientific research on past climate change and human evolution. Engage adult learners who may be underserved and have ventured away from formal avenues of science education. Develop a concise and compelling education guide; curricula for teachers (available in print and online); and traveling exhibitions that introduce the
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Understanding Climate’s Influence on Human Evolution rationale, perspectives, and basic findings concerning the earth system context of human evolution. As a package, these recommendations reflect a fundamental commitment to outreach and education, working in partnership with educators and scientists nationwide and worldwide. The strategic integration of focused high-resolution modeling with new marine, lake, and terrestrial climate records proposed here will represent the most concerted research effort thus far to assess the precise influence of environmental dynamics (resolved on decadal to orbital timescales) on evolutionary history for any organism or time period in Earth’s history. The research agenda described here—although presenting a bold vision that will require substantial resources to bring it to reality—offers an opportunity to make equally bold steps toward an understanding of the role played by past climates in the evolution of our ancestral lineage.