Figure 1 The phylogenetic relationships of animals. (A) monophyletic scheme; (B) polyphyletic scheme. (Reproduced with permission from Miller DA and Ratcliffe NA (1989). The evolution of blood cells: facts and enigmas. In: Endeavour 13: 72 77).

scheme. In this latter system, the animal kingdom bifurcates at the coelenterate level into the proto-stomes and deuterostomes with a separate evolution of the immune system in each of the groups.

In both of these evolutionary schemes, it is most probable that the ancestors of vertebrate immunity arose in the echinoderms, hemichordates and proto-chordates since these groups eventually led to the vertebrates. Similar structures in animals in these groups would be homologous, while any similarities with the protostomes would be purely analogous, having arisen by convergent evolution. The echinoderms and tunicates (urochordates) have been particularly studied (see below) in attempts to find the precursors of vertebrate immunity. A major barrier in tracing the evolutionary origin of the vertebrate immune system is that not only is there disagreement as to the exact origin of the vertebrates but the immediate ancestors of vertebrates are extinct. Thus, any conclusions drawn will be speculative and assume that comparisons with living species are valid.

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