Investigation of HOX genes indicates that the animal family tree" can be simplified to only how many different types of animals?

three evolutionary lines

Explanation:

The Hox genes are master developmental genes found in animals, which are involved in the regulation of development by specifying the body plan of an embryo along the head-tail axis. Recently, several research groups studied the assortment of the Hox genes in three different marine organisms: 1- a priapulid worm (an unsegmented marine worm), 2- a brachiopod (phylum Brachiopoda), and 3- a polychaete worm (related to earthworms). Then, these research groups combined their research efforts to compare the assortment of the Hox genes observed in these species to those observed in previously studied animals including, among others, mice, fruit flies and sea urchins. From this combined study, researchers concluded that the animal kingdom can be divided into three primary evolutionary lineages.