Fig. 7. Sequential ordering of animal steroid metabolic pathways.

(A) Two theoretical models about metabolic pathway evolution: anabolic pathways evolving backward, with the more downstream reactions appearing first (47), and catabolic pathways evolving forward, with the more upstream reactions appearing first (50). (B) Sequential view of pathway appearance during successive periods in animal steroidogenesis. Colors indicate successive periods corresponding to the appearance of new types of enzymatic activities. Final compounds appearing at each period are indicated. All compounds appear in fig. S5. My B.P., million years before the present.