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Energetic characteristics and functional roles define two main types of anaerobic pathways in the animal kingdom: high efficiency/low rates of energy production pathways geared to anoxia survival (aspartate-succinate and glucose-succinate pathways), and low efficiency/high rates of energy production pathways geared to maintaining or increasing metabolic activity (multiple opine pathways and lactate pathway). The aspartate-succinate and opine pathways require both amino acids and carbohydrate as substrates, whereas the glucose-succinate and lactate pathways are dependent on carbohydrate only. Phylogenetic, functional and chemical considerations indicate an evolutionary progression from amino acid-linked to carbohydrate-based anaerobic pathways. The tauropine and strombine pathways are possibly the most ancient opine pathways so far discovered, and the octopine pathway the most advanced. The roles of the aspartate-succinate and opine pathways may originally have been not too dissimilar. A hierarchy of "rates of energy production pathways" of phosphagen > lactate > octopine > other opine pathways is proposed, which defines much of their phylogenetic selection and how they are used. The different properties of phosphocreatine compared to other phosphagens is indicated to have been a key factor in the emergence of vertebrates.