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 scientists have proposed that the ancestral cnidarian's were originally bilateral and, in stages, lost Hox genes from their genomes. If true, what would this mean regarding the phylogeny of animals?
A) all radially symmetric animals should be grouped together in one clade
B) the radial symmetry of extant cnidarians is secondarily derived, rather than being an ancestral trait
C) Hox genes play little actual role in coding for an animal's "body plan"
D) cnidaria may someday replace porifera as the basal bilaterians

Question

Quizplus · Accepted Answer

The presence of Hox genes, which are crucial in determining the body plan of an organism, suggests that the radial symmetry seen in extant cnidarians could be a result of evolutionary changes from an originally bilateral symmetry. This implies that radial symmetry in these organisms is a derived characteristic rather than an ancestral trait.

The Last Common Ancestor of All Bilaterians Is Thought to Have