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Evolution and relationships

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia or Atelocerata. In the first decade of the 21st century, however, this was called into question, and it appears the hexapods' closest relatives may be the crustaceans.

The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha, or as several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than to the Collembola (springtails).

A 2002 molecular analysis suggests that the hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period 440 million years ago, coinciding with the appearance of vascular plants on land.

Since then remipedians have been revealed as closest living relative of hexapods. Several hypotheses about their internal relationships have been suggested over the years, with proturans as the sister group to the other hexapods and collembolans and diplurans belonging together in Antennomusculata as the latest suggestion:

The following cladogram is given by Kjer et al. (2016):

An incomplete possible insect fossil, Strudiella devonica, has been recovered from the Devonian period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago, although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect. In 2023, a hexapod-like arthropod fossil from the Ordovician marine fossil site Castle Bank was reported, although further study is needed.