The last common ancestor of all bilaterians is thought to have had four Hox genes. Most extant cnidarians have two Hox genes, except Nematostella (of β-catenin fame), which has 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

Question

Quizplus · Accepted Answer

The observation that most cnidarians have two Hox genes compared to the four in bilateral animals suggests that radial symmetry is a secondary trait and not an ancestral characteristic of all animals. This means that choice B is the best answer. Choice A is incorrect because radiata is not a clade, but rather a paraphyletic group. Choice C is incorrect because Hox genes have a significant role in determining an animal’s body plan. Choice D is incorrect because cnidarians are not basal bilaterians, but rather represent a separate branch in the animal kingdom. Finally, choice E is incorrect because cnidarians are not basal metazoans – that distinction belongs to sponges.

The Last Common Ancestor of All Bilaterians Is Thought to Have