Sunday, August 7, 2011

The Savanna: Reloaded

Cerling et al report in Nature on the results of a survey of modern soil carbon isotope profiles from East Africa. They compare the modern results to a very large sample of fossil soil (paleosol) samples from the Awash valley and the Omo Turkana basins.  These two valleys in Ethiopia collectively sample approximately the last 6.5 million years of human evolution.  Their results indicate that open environments (not forests) have dominated these two valleys over much of the course of the last 6.5 million years.  This study is also part of the ongoing debate over the paleoenvironment of Aramis, the site from which Ardipithicus ramidus is best known.   Tim White and the Ardi crew argue that the Aramis site was heavily wooded and, further, that Ardi puts the coffin nail in the savanna hypothesis. Cerling et al. replied, arguing that the Aramis isotope evidence indicated more open environs than White and colleagues claimed.  This most recent paper provides evidence consistent with the interpretation that the Aramis site was pretty open, even more open than later A. afarensis sites. 

Relationship between δ13C values and fraction of woody cover
The most interesting part of this study for me is their "paleo-shade proxy", which is essentially a continuous variable based on the relationship that they demonstrate between soil δ13C values and the percentage of the ground that is covered by woody cover. This sounds really promising as a way to quantify aspects of habitat openness versus closedness, moving beyond  the frustrating and rather vague habitat classification schemes (open woodland, shrubland, grassland etc.) used in many other studies.

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