Animals have a broad diversity of diets, including carnivores that pursue dangerous prey, insect herbivores that specialize on a few plants, and marine invertebrates that passively filter feed on tiny fare. What an animal eats is a defining aspect of lifestyle.
Choanoflagellates are a group of free-living, single-celled, marine eukaryotes. They have a whip-like tail that lets them swim and stir bacteria and other foodstuffs into a collar that surrounds their rear. Choanoflagellates consume the catch from their collar.
The first animal, over 800 million years ago, was likely similar to a choanoflagellate. Animals first evolved eating seafood. “The ancient creature that is most closely related to all animals living today might have eaten bacteria and other protists rather than plants,” American evolutionary biologist John Wiens speculates.
Carnivory remains the most common animal diet. 63% of animal species eat meat.
Insects were the first animals to evolve on land. They were in verdant lands flush with vegetation. Unsurprisingly, many insects took to herbivory. 32% of animal species are herbivorous; most of them insects. There are an estimated 6–10 million insect species, comprising over 90% of all animal life forms. “Herbivory may go hand in hand with new species appearing in certain taxa, but it clearly is not a universal driver of new species,” said John Wiens.
Omnivores are relatively rare. Only 3% of animal species have wide-ranging diets.
The efficiency of specialization is a more common evolutionary outcome than generalization. “There is a big difference between eating leaves all the time and eating fruits every now and then. The specializations required to be an efficient herbivore or carnivore might explain why the two diets have been so conserved over hundreds of millions of years,” noted John Wiens.
Generalists typically evolve when their habitat is in considerable flux, and so dietary specialization would be a risky strategy. Humans descended during variable climatic conditions, with often unreliable food sources.
“You can be better at doing what you do if that is all you do. In terrestrial vertebrates, for example, eating a diet of leaves often requires highly modified teeth and a highly modified gut. The same goes for carnivory. Nature generally seems to avoid the dilemma of being a jack-of-all-trades and master of none, at least for diets.” ~ John Wiens
Sources:
Daniel Stolte, “The Paleozoic diet: why animals eat what they eat,” University of Arizona (21 August 2019).
Cristian Román-Palacios et al, “Evolution of diet across the animal tree of life,” Evolution Letters (9 July 2019).