Crabs, lobsters, shrimp, pillbugs, and similar arthropods belong to the Class Malacostraca. The earliest fossil malacostracans are from the Cambrian Period (about 510 million years ago). While this group did not have the full body covering (exoskeleton) of a modern Malacostracan, they did have a hard carapace on their backs. Many malacostracan limbs were leaf-like in shape (phyllopodus), for which they were named phyllopodans. Today, only one group of phyllopodans remain that includes inconspicuous, tiny, shrimp-like species, such as the tadpole shrimp which inhabit vernal pools. Larger, familiar organisms such as crabs and lobsters belong to a different group of malacostracans that diversified later. For example, the ancestors of isopods, such as pillbugs and sowbugs, first appeared about 305 million years ago in shallow seas, but later invaded the land, evolving to eat dead material in soil. The oldest crab fossils are from the Jurassic Period, beginning about 200 million years ago, followed about 50 million years later by lobsters.

Arthropods have been on Earth for more than 540 million years, and were diverse almost from the beginning. Different lineages of arthropods, such as crustaceans, diverged as early as 525 million years ago. The evolution of an external body covering (the exoskeleton), and the presence of body segments and paired appendages (mouthparts, legs, claws, antennae) signaled the transition from early worm-like precursors to arthropods. While modern arthropods live in nearly every habitat, the earliest arthropods were probably tiny, bottom-dwellers scavenging detritus at the bottom of warm seas. The enormous success of arthropods is at least partly due to their appendages. Located on all body regions, their appendages became specialized especially for feeding through the mouthparts, but also for getting oxygen through respiration (gills), reproducing (elaborate external genitalia), and moving around including walking, swimming, and/or flying. The gradual adoption of a modular body plan with multifunctional appendages has allowed arthropods to thrive in an impressive variety of environments.