With more than a million species that have already been described, the hexapods (insects and allies) constitute the largest animal group. Still their origin and phylogenetic affinities are matter of intense debate. Although previous morphological work generally considered the millipedes as sister taxon of the hexapods, molecular phylogenetic analyses agree that hexapods are actually closely related to crustaceans. Recent studies have provided evidence that the Remipedia, enigmatic crustaceans that have been discovered only 30 years ago in anchialine cave systems, may be the closest living relatives of hexapods. Support for this hypothesis comes from similar brain architecture, presence of an insect-type respiratory haemocyanin in remipedes and phylogenomic studies. Thus hexapods may have evolved from a Remipedia-like marine crustacean. These data evokes doubt on the generally described hypotheses in textbooks that might present an outdated picture of arthropod phylogeny.

Key Concepts:

• Hexapods are the most successful animal group, but their relationship to other arthropods and evolutionary origins are matter of debate for more than a century.
  
• Molecular phylogenetic studies have demonstrated that crustaceans are the closest living relatives of hexapods.
  
• Crustaceans are most likely paraphyletic in terms of hexapods, thus one crustacean taxon is more closely related to the hexapods than the other crustaceans.
  
• Brain morphology, haemocyanin structure and evolution, and phylogenomic studies suggest that the crustacean class Remipedia are the closest living relatives of hexapods.
  
• Remipedia live in anchialine caves, which connect the inland ground water body with the salt water from the ocean.
  
• Remipedia harbour a mixture of ancestral and derived morphological characters.
  
• First hexapods may have evolved from marine Remipedia.
  
• Remipedia thus occupy a key position for understanding hexapod evolution.