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Further experiments using single-cell analysis on amphioxus larvae will be needed to help determine how the complex architecture of the vertebrate brain arose.

All homeobox genes identified in the genomes of the two hemichordates and amphioxus are listed in the Supplementary Table for Extended Data Fig.

Pairwise distances to amphioxus are shown for each species above their respective branches.

The lowest group of Vertebrata, including the amphioxus, in which no skull exists.

Holland notes that a lot more research—on acorn worms, amphioxus and other extant invertebrates—is needed.