PerspectiveATMOSPHERE

Climate Change and Human Evolution

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Science  27 Jan 2006:
Vol. 311, Issue 5760, pp. 476-478
DOI: 10.1126/science.1116051

Climate and biological evolution have interacted throughout Earth's history, together creating many small and a few major transformations in the planet's atmosphere and biota. The role of climate in the origin and adaptations of humans relates not only to our past but also, potentially, to our future (1). A number of hypotheses propose that climate-driven environmental changes during the past 7 million years were responsible for hominin speciation, the morphological shift to bipedality, enlarged cranial capacity, behavioral adaptability, cultural innovations, and intercontinental immigration events (29). These hypotheses are based on correlations between global-scale climate shifts documented in oceanic deposits and events in hominin evolution recorded in continental fossil-bearing strata. Establishing cause-effect relationships between climate and human evolution is tantalizing but presents many challenges for paleoanthropology and the geological sciences.

The biggest challenge involves how to relate different types and scales of paleoclimatic evidence between the marine and terrestrial realms. Marine-core records show that a cooler, drier, and more variable global climate regime began about 3.0 million years ago (Ma), gradually intensifying into northern continental glacial cycles by 1.0 Ma (1012). The climate shift between ∼3.0 and 2.5 Ma thus marks the onset of Northern Hemisphere glaciation (1013), and this coincides generally with the timing of the origin of the genus Homo [reviewed in (8, 14)] (see the figure). Fluctuations in continent-derived dust and biomarkers in the marine record indicate that climate shifts recorded in the oceans affected the land as well (12, 15). However, in the continental basins that preserve hominin fossils, the record of climate change is much harder to decipher. Paleoclimatic proxy evidence includes stable isotope (8, 16, 17), pollen (18), mammal faunas (7), and lake versus land deposits (9, 19, 20). Although these signals are documented in many vertebrate fossil-bearing localities (17, 2123), each stratigraphic sequence represents only limited portions of the time-space framework of hominin evolution. In addition, the proxy records are subject to local tectonic and climatic processes that often obscure or completely overprint global-scale climate signals. Thus, we must confront the problem of relating a fossil record preserved in strata dominated by local- to regional-scale paleoenvironmental signals to a marine record dominated by continental- to global-scale signals. Long cores from deep African lakes could provide more continuous data and a stronger bridge between oceanic and continental climate records, but these are only beginning to be tapped (24).

Another challenge is deciding what constitutes a strong case for a causal link between a climate change and an evolutionary event. We can't step into a laboratory to test the impact of climate change on the human genome, but we do have the results of natural experiments—the proxy evidence for environmental changes in continental rock sequences, as well as many fossils of hominins and other organisms that were evolving on different continents during that same time period. There is a rich body of data to draw upon, but hypotheses are often structured around an assumption that “synchronous” events in the geological and paleontological record constitute evidence for cause and effect. These hypotheses, while seductive in their simple explanation of how our species came to be, do not do justice to the complexity of the climate-evolution problem (see the figure) or to the full range of evidence and scientific methodologies that now can be brought to bear on this problem.

Research into human origins, as well as other fields of science, uses probability-based evidence to test cause-effect hypotheses. Establishing a credible cause-effect relationship between events or trends in the geological record requires (i) a clear definition of what “synchronous” means, (ii) consideration of the mechanism for transmitting climatic cause to evolutionary effect, and (iii) multiple lines of proxy evidence supporting interpretations of the climatic trends or events. A recently proposed link between climate and human evolution provides an example of how these criteria can be used to assess hypotheses.

A new compilation of paleolimnological evidence concludes that lake levels were high in the East African Rift between 2.7 and 2.5 Ma (9) based on two generally synchronous lacustrine deposits in Kenya and southern Ethiopia [see the figure, land record (left)]. Radiometric dating shows that the two phases of lacustrine deposition occurred within a period of about 200,000 years (9). It is tempting to speculate [as in (9)] that there may be a cause and effect between this wet climate phase and the origin of Homo, but let us consider this in light of the three criteria above. There is debate about the precise time and place of the origin of the genus Homo (8, 14), with time estimates ranging from 2.6 to 1.7 Ma. Synchrony of the climatic signal and the evolutionary event thus remains in question. The related notion that fluctuating lake levels provided environmental stress that drove speciation does not provide a mechanism for how this could have exerted selective pressure on the immediate ancestor of Homo and resulted in the emergence of a new genus and species. Other proposals instead have linked human evolution with increasing aridity and climate variability (4, 6, 25). Finally, other paleoclimatic evidence indicates drier rather than wetter climatic conditions between 2.7 and 2.5 Ma (8, 17, 26) [see the figure, land record (center)], bringing into question the extent of a prolonged high lake phase throughout East Africa. Although the multibasin approach to establishing regional paleoclimate trends is commendable, the proposed causal link between a wet climate phase and the origin of Homo is not yet supported by sufficient evidence to establish its credibility.

Marine and land records showing paleoclimatic trends during human evolution.

Marine record: (Left) Global ice volume trend based on composite oxygen stable isotope (δ18O) data from seven different marine cores (31). (Right) Cycles of aridity in the Sahara Desert, based on percent terrigenous dust in the ODP (Ocean Drilling Program) Site 721/722 core from the Arabian Sea (32). Land records: (Left) Multibasin records of lake phases. Darkest vertical bars indicate deep lakes, lighter bars indicate shallow lakes, and lightest bars indicate land; red bars mark radiometric dating levels (9). (Center) Carbon stable isotope (δ13C) record of closed (woodland or bush) versus open (grassland) vegetation in the Turkana Basin of northern Kenya (8) (same basin as central lake phase record); (right) milestones in the fossil and archaeological record that are used as evidence for the timing of the appearance of Homo (14). There is no simple translation of the marine Plio-Pleistocene global climate shifts into the continental records, but future integration of marine and land-based evidence will allow rigorous testing of the impact of global change on the environments and evolutionary trajectories of our ancestors.

CREDIT: PETER B. DEMENOCAL/LAMONT-DOHERTY EARTH OBSERVATORY, COLUMBIA UNIVERSITY

The way forward is to carefully match the quality and scale of the data with the scale of the question. We cannot expect to link global- or continental-scale climate processes to major events and trends in human evolution without first disentangling basin- and regional-scale environmental signals in strata that contain the hominin fossil record (17, 27, 28). This complexity can work in our favor, however. A multibasin, multiproxy approach is now possible because paleoclimate data and chonostratigraphic correlations are becoming available for a large number of rock sequences (9, 29, 30). If many stratigraphic sequences and independent climate proxies show similar, synchronous environmental shifts, this would be strong evidence for climate change affecting large regions of a continent, particularly when such trends can be matched to the marine core data. On the other hand, if basins show independent patterns of environmental change, this implies that they were locally buffered against larger-scale climate forcing, or that the available evidence is not sufficient to resolve small-versus large-scale environmental processes. The hominin fossil and cultural record could be reconsidered in light of such paleoclimatic meta-data sets. The strength of this approach depends on the number of sample points (that is, different basins and regions), accurate interpretations of climatic proxies, well-resolved correlations between basins, and a healthy dose of devil's advocacy before making a leap to global-scale interpretations. It is also worth remembering that climate was only one of many factors affecting human evolution; biological processes including genetic innovation, interspecies competition, and dispersal ability also could have played defining roles (14).

Rather than a simple story of global climate drumbeat and evolutionary response, more informative and exciting revelations about the 7-million-year development of hominin morphology, behavior, and culture will likely come from detailing the prolonged tension between local ecosystems and global climate change. This is also a strikingly relevant theme for the future of our species.

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