The last common ancestor of all bilaterians is thought to have had four Hox genes. Most extant cnidarians have two Hox genes, though some have three Hox genes. On the basis of these observations, some have proposed that the ancestral cnidarians were originally bilateral and, in stages, lost Hox genes from their genomes. If true, this would mean that .

 it was known that, in the last common ancestor bilaterians used to have 4 Hox genes. Most extant cnidarians are having 2 Hox genes, though some of them have 3 Hox genes.

On the basis of these observations, some researchers have proposed that the ancestral cnidarians were originally bilateral and, in stages, lost Hox genes from their genomes. If true, this would mean that

the radial symmetry of extant cnidarians are actually the secondary derived, but not an ancestral trait .