Figure 1.

The interrelationships among the vertebrate lineages (jawless hagfishes and lampreys, as well as jawed vertebrates) and the nonvertebrate chordates (cephalochordates and urochordates) inferred from molecular data. Most evidence supports two rounds of genome duplication (1R, 2R) prior to the divergence of the agnathan and gnathostome lineages (but see the text), potentially permitting the origin of the many key innovations that characterize the vertebrates; adaptive immunity, however, likely arose independently in the agnathan and gnathostome lineages. Abbreviation: HP, hypothalamic–pituitary.