Figure 1.

Evolutionary trajectory of vertebrates and their adaptive immune systems.

(a) Cladogram depicting the evolutionary split between and within jawless and jawed vertebrates. About 500 million years ago, the hypothetical common vertebrate ancestor gave rise to both jawless and jawed vertebrate ancestors, and subsequently to their diversified descendants. In jawless vertebrates, extant species are restricted to the clades of hagfishes and lampreys, whereas jawed vertebrates comprise a more diverse group, here exemplified by cartilaginous fishes and mammals. (b) Cladogram representing the emergence of B-like and T-like cells during vertebrate evolution. Following the evolutionary timeline of (a), the two arms of the adaptive immune system originated from the hypothetical pan-lymphocyte, a primordial lymphoid cell type present in the common vertebrate ancestor. B-like and T-like cell lineages were specified at a later stage, however most likely before the emergence of the distinct ancestors of jawless and jawed vertebrates. This evolutionary scenario explains why both jawless and jawed vertebrates possess B-like and T-like cell types.