I was fascinated to learn why cats are much more sensitive than other species to drugs like panadeine and aspirin. Cats lack the liver enzyme that metabolises these drugs. They have a defect in the gene coding for this enzyme.
A recent paper explained when this genetic variation occurred in evolutionary history. Researchers concluded that it occurred when Felidae split from others in the suborder Feliformia that includes hyenas, civets, binturongs, and mongoose. The split occurred approximately 37 million years ago.
They think that this genetic variation arose as a consequence of the cat's diet, which is primarily animal-based rather than plant-based. The gene encoding the missing enzyme is necessary for successful utilization of plant material. Interestingly it also codes for the taste receptors for sweets of plant origin like fruits and berries.
Because cats have a mainly animal-based protein diet the genes for the missing enzyme were not selected for and so they evolved without it.