7 Of rats and men
The two most successful species of omnivore - humans and the brown (or Norwegian) rat - both arose within mammalian groups that are not particularly omnivorous. Most members of our own family, the primates, are exclusively or predominantly herbivorous. However, chimpanzees do occasionally hunt and eat monkeys and the human species is far from being exclusively vegetarian. Brown rats are rodents and almost all rodents use their continuously growing incisors to feed on seeds, nuts, grass, stems, roots, etc. Nevertheless, DA provides an amazing list of items apparently acceptable as food by brown rats: all parts of any plant, slugs, fish, insects, meat, bone, hair, hide, guts, toenails, beeswax, soap, cardboard, lead pipes, concrete and the plastic covering of electric cables [p. 179]!
Can it be coincidental that these two species (rats and humans) - so successful in terms of both population size and geographical range - have avoided becoming encumbered with restrictive adaptations to particular diets in their evolutionary past? Are there omnivore characteristics that have been brought together in these two mammals that may have contributed more generally to their current levels of success as species?