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He specializes in the Precambrian era and is widely known for the theory of the Snowball Earth about phenomena that occurred in the Neoproterozoic era, co-published with Daniel P. Schrag.
In 2003, he, Frank Corsetti and David Pierce published evidence in the Proceedings of the National Academy of Sciences of fossilized microbiota in Cryogenian rocks, a revelation which has cast doubt on the notion of the near elimination of life during potential equatorial glaciation during that period (commonly referred to as Snowball Earth).