We present a series of QM/MM calculations aimed at understanding the mechanism of the biological dehydration of glycerol. Strikingly and unusually, this process is catalyzed by two different radical enzymes, one of which is a coenzyme-B-12 dependent enzyme and the other which is a coenzyme-B(12-)independent enzyme. We show that glycerol dehydration in the presence of the coenzyme-B-12-dependent enzyme proceeds via a 1,2-OH shift, which benefits from a significant catalytic reduction in the barrier. In contrast, the same reaction in the presence of the coenzyme-B-12-independent enzyme is unlikely to involve the 1,2-OH shift; instead, a strong preference for direct loss of water from a radical intermediate is indicated. We show that this preference, and ultimately the evolution of such enzymes, is strongly linked with the reactivities of the species responsible for abstracting a hydrogen atom from the substrate. It appears that the hydrogen-reabstraction step involving the product-related radical is fundamental to the mechanistic preference. The unconventional 1,2-OH shift seems to be required to generate a product-related radical of sufficient reactivity to cleave the relatively inactive C-H bond arising from the B-12 cofactor. In the absence of B-12, it is the relatively weak S-H bond of a cysteine residue that must be homolyzed. Such a transformation is much less demanding, and its inclusion apparently enables a simpler overall dehydration mechanism.