The Last Common Ancestor of All Bilaterians Is Thought to Have

question 42

Multiple Choice

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

---

Definitions:

Motor Skills

These are abilities that allow for movement and coordination of the muscles and the body, key for performing tasks from simple to complex.

Sensorimotor Intelligence

A stage of Jean Piaget's theory of cognitive development where infants learn about their world through their senses and motor activities.

Social Referencing

The process by which individuals look to others in their environment to obtain cues on how to behave or react to a certain situation or stimulus.

Object Permanence

The understanding that objects continue to exist even when they cannot be seen, heard, or touched, typically developed in infancy.