A non-functional pseudogene is found in other primates as well. All have collected a fair number of mutations over the years. Comparison of the human, chimp, orangutan and macaque monkey pseudogene shows a number of shared mutations as well as unshared mutations. Among the shared mutations is a deletion resulting in a frameshift which could have led to functional loss of the gene. Based on these comparisons, conversion of the active gene to a pseudogene occurred prior to the primate and old world monkey split.
Dietary habits at that time may not have maintained positive selection for the functional gene.