Deep Origin of Parasitic Disease in Vertebrates

Parasitism is considered one of the most successful modes of life, as evidenced by its convergent appearance in numerous lineages and its sheer abundance among extant biodiversity. Ecotropism may be the foundation for that co-divergence. Given an ecologic niche (and hosts represent one such niche), organisms tend to advantage themselves. As environments change and organisms become extinct, organisms new to that host or to that environment find a way to utilize the new or newly available habitat.

Greater and more facile access to nutrients and greater reproductive and growth opportunity (e.g., secondary mobility) may well have been the stimulus for the transition from saprophytic to parasitic lifestyles and for climbing the phylogenetic ladder from invertebrates to vertebrates. Ease of transition [given the short generation time of parasites (relative to that of the host) allowing development of a mutation amenable to the new host] and opportunity coincided. The response of hosts to new invaders (the parasites) initiated a process (still ongoing today) wherein the parasite continues to evolve in its efforts to negate the effects of the host to neutralize or eliminate it, a co-divergent process. The discrepancy between life experiences (e.g., generation time) of parasites and that of their hosts provides ample opportunity for a parasite to explore and acclimate to changes in availability and biology [e.g., defense mechanisms developed by their current host (s)] and also to adapt to new opportunities created by availability of new hosts.