Eating plants turned out to be harder to evolve than eating meat
Switching from insects to meat took early four-legged animals little effort. Living on tough, fibrous plants demanded a whole kit of new teeth, jaws and gut microbes. Fossils show a lag of 50 to 100 million years between plants growing new organs and animals learning to eat them.
A herbivore is an animal built by evolution to live chiefly on plants, particularly leaves, fruit and seeds, and loosely also on mosses, algae and lichens, though not on rotting matter or mushrooms. Plant eaters need jaws or mouthparts that crush tough material and enzymes such as amylase and cellulase for complex sugars. Grazers like horses and cattle carry broad, flat grinding teeth, and many chew the cud or re-eat soft droppings to squeeze out more nutrition. Most rely on gut bacteria and protozoa to break cellulose, whose tightly linked fibres resist digestion far more than meat does.
The word goes back to herbivora, a modern Latin coinage cited in Charles Lyell's Principles of Geology of 1830, and Richard Owen used the English form in 1854. Plant-eating was once thought to have started in the age of dinosaurs, but arthropods were nibbling land plants within 20 million years of their appearance, eating spores of Devonian plants and piercing tissues as the Rhynie chert shows. Evidence for feeding on roots and seeds shows up only around 330.3 million years ago, a gap perhaps caused by low oxygen slowing evolution.
Among four-limbed vertebrates, herbivory arose in the Late Carboniferous, roughly 307 to 299 million years ago, with Desmatodon and Tyrannoroter among the oldest known examples. Several groups, from captorhinids to the sail-backed edaphosaurs and caseids, took up plants independently, and much later the entire dinosaur order Ornithischia ate vegetation. Teeth that meet precisely between upper and lower jaws evolved separately in several lineages and greatly boosted how much plant food could be processed.
Ecologists class herbivores as primary consumers linking plants to carnivores. A mammal counts as a grazer if at least 90% of its food is grass and a browser, like the moose, if at least 90% is leaves and twigs, with mixed feeders between. Small species pick the richest food while big ones are less choosy, consistent with Kleiber's law that metabolic rate scales with body mass to the three-quarters power, so larger animals need less food per kilogram.
Source: Herbivore
