During their long history on Earth (about 500 million years), gastropods have evolved various ways to feed. Flat snails (e.g., Maclurites) likely lived like clams, sitting in one place and eating small food suspended in the water. They were common in the Paleozoic, but are now extremely rare. Most gastropods actively find detritus or algae to eat, scraping food off rocks or other surfaces using a special mouthpart (the radula). Other gastropods are carnivores: shells with holes in them are evidence of gastropods using their radula to drill a hole and eat the animal inside. The earliest gastropods were detritivores, but carnivory has evolved independently several times (convergent evolution). During the shift from herbivory to carnivory, the teeth on gastropod's radulas were lost or modified for use as predatory tools. Early gastropods lived in the sea, but by the Carboniferous (360 - 299 million years ago) some freshwater gastropods invaded land. During the transition to land, gastropod shells either remained rather thick (in dry climates) or were reduced or even lost altogether (in humid climates).

Mollusks have inhabited the Earth for at least 540 million years, dating back to the early Cambrian period. The fossil record of mollusks consists mostly of shells. Rarely, it includes mouthparts (radulas) or the trapdoors (opercula) that cover shell openings. The rest of the body is soft and usually does not fossilize. Before the Cambrian, taxa such as Kimberella had many traits expected of early molluscs, but no shells or spines. Cambrian, worm-like animals with spiny scales, such as Wiwaxia, had mollusk-like mouthparts. The earliest agreed-upon mollusk looked like a snail with a single, cap-like shell that curved at the tip. More than 90,000 living species of mollusk have been described, plus another 70,000 species known only from fossils. Because many mollusks live in remote places such as in the bottom sediments of deep ocean, and new species are being discovered at a rapid rate, scientists think that many more species remain to be described.