A review of anaerobic metabolism (end-products, energy-substrates and enzymes) of the invertebrate and vertebrate phyla reveals three main pathway-types. The lactate and opine pathways yield, respectively, high and medium rates of energy production and are used for maintaining or increasing metabolic activity. The glucose-succinate pathway is a high (relatively) efficiency pathway used for anoxia survival.

The distribution of the pathways amongst the phyla is consistent with their phylogeny. A relationship between the Mollusca and Sipuncula is possibly indicated. A hypothetical scheme of the evolution of the pathways indicates early presences for amino acid based pathways, the opine pathways and a fourth pathway-type, the aspartate-succinate pathway.

A consideration of chemical, fossil and functional approaches to the reconstruction of the beginning of life suggests an early role for the opine pathways in providing energy for the burrowing of infaunal worms of the Precambrian era. Biological rather than chemical factors are concluded to have been the main pressures for the evolution of anaerobic metabolism.

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