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 cnidarian's were originally bilateral and, in stages, lost Hox genes from their genomes. If true, this would mean that ________.
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 observation that most extant cnidarians have only two Hox genes while the last common ancestor of all bilaterians had four Hox genes suggests that the ancestral cnidarians were originally bilateral and lost Hox genes over time, leading to the secondary development of radial symmetry in those lineages. Therefore, option B is the most appropriate choice. Options A, C, and D do not accurately reflect the information provided in the passage.

The Last Common Ancestor of All Bilaterians Is Thought to Have