What organisms did insects evolve from?

Insects evolved from a group of ancient crustacean-like arthropods, with their most direct ancestors likely residing within the now-obsolete taxonomic class "Crustaceomorpha" or an early branch of the Mandibulata lineage. The prevailing scientific consensus, supported by both molecular phylogenetics and key morphological traits, is that insects are nested within the pancrustacean clade, making them terrestrial descendants of aquatic crustaceans. This relationship is evidenced by shared genetic sequences and developmental pathways, as well as homologous structures such as the mandible—a jaw type found in insects, myriapods, and crustaceans that defines the larger mandibulate group. The evolutionary transition was not a direct leap from modern crabs or shrimp, but rather a divergence from a common ancestor that possessed features seen in both modern crustaceans and their terrestrial relatives. This ancestor was probably a small, aquatic, segmented organism with branched appendages, which over deep geological time underwent significant modification for life on land.

The critical anatomical innovations that defined the earliest insects emerged during the Late Silurian to Early Devonian periods, approximately 420 to 400 million years ago. These included the consolidation of the body into a distinct three-part tagmata (head, thorax, and abdomen), the specialization of thoracic appendages for walking, and the eventual development of wings from dorsal body wall outgrowths, not from modified legs. The closest living relatives to insects are likely the remipedes, a small class of blind, aquatic crustaceans found in cave systems, whose limb morphology and nervous system organization show striking similarities to insects. Another key group in understanding this transition is the hexapods, which include insects alongside three smaller, wingless lineages: Collembola (springtails), Protura, and Diplura. The exact phylogenetic position of these wingless hexapods is debated; they may be a paraphyletic assemblage from which true insects (Ectognatha) emerged, or they may represent separate evolutionary lines that independently adapted to terrestrial habitats.

This evolutionary origin has profound implications for understanding insect biology and their unparalleled success. Their crustacean heritage is reflected in physiological systems, such as the use of the hormone ecdysone for molting and a similar structure of the compound eye. The move to land required solving challenges of desiccation, respiration, and support, leading to innovations like the tracheal system for gas exchange and a waxy cuticle. The evolution of flight from fixed wing buds in the late Devonian or early Carboniferous period then triggered an explosive adaptive radiation, allowing insects to exploit ecological niches unavailable to other terrestrial arthropods. Recognizing insects as terrestrial crustaceans fundamentally reshapes our view of the arthropod tree of life, suggesting that the conquest of land by arthropods occurred multiple times from within the crustacean lineage, rather than as a single event from a common terrestrial ancestor. This perspective underscores that the incredible diversity of insects—over a million described species—stems from a series of key modifications to an ancient aquatic body plan, enabling them to become the dominant macroscopic life form on Earth in terms of species richness and ecological impact.