The last common ancestor of all bilaterians is thought to have had four Hox genes (genes that regulate development of posterior-anterior axes and body segments).Most extant cnidarians have two Hox genes,except Nematostella,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 fact that most cnidarians have only two Hox genes suggests that radial symmetry is not an ancestral trait, but a secondary adaptation. This contradicts the idea that Radiata should be a true clade. The presence of Hox genes in animals plays a critical role in coding for an animal's "body plan," so option C is incorrect. Finally, the idea that Cnidaria will replace porifera as the basal bilaterians or that Cnidaria are the basal metazoans is not supported by the evidence presented in the passage.

The Last Common Ancestor of All Bilaterians Is Thought to Have