Dramatic change in brain chemistry may be the ultimate basis of human evolution

Researchers at Kent State University find evidence that an early shift in brain chemistry may have given our earliest ancestors an edge- 
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Also covered in Science Magazine

PNAS articles: A neurochemical hypothesis for the origin of hominids by Raghanti et al.
                        Early hominids may have been weed species by Meindl, Chaney, and Lovejoy