Question

9 genes opened the evolutionary door to an unprecedented number of primitive animals.
differentiation
chordate
tetrapod
homeobox
skip 8/12 complete

Explanation:

Homeobox genes are a class of related genes that control the development of the body plan of an embryo, including the number and placement of segments and appendages. They have been crucial in the evolution of a wide - variety of primitive animals. Differentiation refers to the process by which cells become specialized, chordates are a phylum of animals with a notochord, and tetrapods are four - limbed vertebrates. None of these other terms directly relate to the genes that opened the evolutionary door to a large number of primitive animals like homeobox genes do.

Answer:

Homeobox