Consider the Evolution of Metabolic Pathways

Consider the evolution of metabolic pathways. We believe that these pathways evolved from the

A) precursor to the end product, as enzymes became more efficient over time.
B) precursor to the end product, as cells simplified the biochemistry of the external environment.
C) end product to the precursor, as cells gradually evolved according to a predetermined plan.
D) end product to the precursor, as intermediates in the pathway were depleted in the environment.
E) end product to the precursor, as photosynthesis began to supply the energy to power the changes at each stage.