The timing and source of horizontal transfer of genes that facilitate parasitism. The evolution of alveolates is shown, with approximate times (million years ago) of major branching events denoted in gray type. Ancestral apicomplexa are thought to have appeared during the transition from the Neoproterozoic to the Paleozoic era, a time when nematodes and arthropods were extant, providing a possible source for transfer of Set8, cytoadhesion protein/invasion proteins and enzymes involved in O-linked glycosylation (red arrow). The proposed acquisition of the histone modifier Ashr3 and ApiAP2 transcription factors also appear to have occurred at or near this point in evolutionary history. The acquisition of an algal endosymbiont that is thought to be a source of ApiAP2 proteins and possibly Ashr3 is more controversial (dashed green arrows), but could also have coincided with the divergence of apicomplexan parasites. The apparent convergence of these distinct events within a narrow evolutionary window suggests that this represented a key moment in the development of apicomplexan parasitism (denoted by asterisk). Adapted from: Okamato, N. 2008. The mother of all parasites. Future Microbiology 3:391–395.