Linking African Lake Variability and Theories of Human Evolution

We suggest that ephemeral lakes, expanding and contracting on precessional timescales, would have evoked wide-spread, regional-scale, rapid, and extreme environmental variability. However, the difficulty in invoking orbital forced changes in local hydrology arises not out of the question of scale, but of timing: what part of these climate variations may have influenced the speciation and extinction events? Figure 13.3 presents three different models of the lake response to local orbital forcing.

The first model suggests that there is a relatively smooth, gradual transition between periods with deep lakes and periods without lakes. If this 'smooth' model is correct, then there may have been prolonged periods of wet and arid conditions, which may invoke the Red Queen or the Turnover Pulse Hypothesis (TPH) as possible causes of evolution (Van Valen, 1973; Vrba, 1993). Alternatively, there may have been non-linear dynamic changes related to the complex interaction of precipitation, temperature, and seasonality patterns that produced threshold changes in the local vegetation which may have influenced evolution (Maslin, 2004).

The second model is a 'threshold' model, so, instead of a smooth gradual transition from wet and drier condition, the ephemeral lakes expanded and contracted extremely rapidly, producing a wide-spread, regional-scale, rapid, and extreme environmental variability, required by the Variability Selection Hypothesis of human evolution (Potts, 1998). Model three is a more extreme example of the threshold model, in which there is 'extreme climate variability' during the rapid transition from deep-lake to no-lake phases. This would provide extreme short-term variability that could influence speciation and extinction events, especially if this climate change occurred over a large geographic region.

There is, of course, a fourth possibility, namely that all three models contain prolonged extreme wet and dry periods, which would have provided prolonged periods of either extremely abundant or scarce water and food resources. The extreme dry periods would support a model such as the TPH. In contrast, the extreme wet periods, with very deep freshwater lakes, are rare events in the paleoclimatic history of East Africa. As such, speciation events may have occurred in the high energy/high competition environments provided by the wet periods. This would conform to the Red Queen Hypothesis.

At present, the preliminary data from Lake Baringo (Kingston et al., 2007) suggest the diatomites are typically bracketed by 20-30 cm of fine sand and silt horizons containing fish fossils. These grade into high-energy terrestrial facies, indicating relatively rapid cycling between deep lake and fully subaerial conditions. This suggests that for this region, at least, model three - extreme climate variability - is the most likely. What is now required are high resolution paleoclimate data with which to test the different models outlined above. There are also other methods for testing which of the three theoretical models is closest to reality. First, oxygen isotopes of the diatoms in the lake sediment can be analyzed, as these provide a measure of the evaporation-precipitation balance of the whole lake. This, in turn, provides an estimate of how quickly the lake was expanding and contracting. Second, one of us (MT) has already sampled the Late Glacial - Early Holocene paleo-lake in the Suguta Valley of northern Kenya, and its appearance and disappearance has been dated using radiocarbon (Garcin et al., 2009). This will yield an accurate estimate of how quickly recent lakes can vary providing an analog for the older material.

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