Humans and other species that do not synthesize vitamin C carry a mutated and ineffective form of the enzyme L-gulonolactone oxidase, the fourth and last step in the ascorbate-producing machinery. In the anthropoids lineage, this mutation likely occurred 40 to 25 million years ago. The three surviving enzymes continue to produce the precursors to vitamin C, but the process is incomplete and the body then disassembles them.