When food is browned, peroxides have been produced. These are mutagens and can react to form free radicals. Heating of oils forms peroxides. Natural plants make carcinogenic chemicals, most likely for defense from predators, animals who eat plants. Total food intake is related to cancer risk.
Humans have a lower rate of metabolism.
Possible explanation is oxygen. Oxygen oxidizes our food so we get the maximum possible energy out. The OH radical is an example. This extra electron pairs with the unpaired electron of the OH radical so the OH ion doesn't have any unpaired electrons. The OH ion does have a negative charge because of the extra electron.