Maxillopods have been around since the mid-Cambrian, about 510 million years ago. Their shortened bodies and juvenile external features make them look like larvae. One hypothesis is that the common ancestors of all maxillopods were organisms that became reproductively mature before their bodies changed into externally adult forms (neoteny). The oldest known fossil maxillopod is a barnacle (Priscansermarinus barnetti). Unlike the barnacles of today, this species did not have an external protective shell. Later maxillopods were shelled, but their fossil record is patchy, in many cases consisting of just shell fragments. Because maxillopods often live in turbulent environments, like rocky shorelines, their remains get broken up. Beginning during the Early Devonian (about 415 million years ago), common maxillopod fossils are casts of borings where barnacles drilled into shells of other animals. Sediments infilled the boreholes and fossilized, creating trace fossils in the shape of the boreholes. Copepods are another group of tiny maxillopods that are now abundant and diverse, but are nearly absent from the fossil record.

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.