Figure 2.

Evolutionary tree for the different classes of gnathostomes (jawed vertebrates) in relation to cyclostomes (lampreys and hagfishes), urochordates (tunicates) and cephalochordates (lancelets). It is still unclear when the two genome-wide duplications (2R) took place in relation to the split of the cyclostomes from the gnathostomes. It is also still unclear whether lampreys and hagfishes are monophyletic or paraphyletic, which is why their deep branch is dashed in the figure. A third independent genome duplication (3R) took place in the teleost lineage.