Something interesting to note about Plaga varieties is that they are equally comfort in water (del lago), land, and air. This hints at an incredibly ancient lineage. This means that they could possibly date back to the earliest periods of the Cambrian explosion, possibly even before, as their use of microscopic spores might reflect direct ancestry going back almost to the beginning of life ( with few to no breaks from extinction events). Las Plagas exhibit characteristics of arthropods, annelids, and onychophorae, indicating a very ancient lineage. The fossils seen in RE4 look somewhat developed but have archaic features as well.
Las Plagas fossils found below Salazar's estate:
Burgess Shale fossils, well over five hundred million years old:
They have a hierarchy, but as far as we know, each member reproduces on its own; in essence, they seem to have evolved the characteristics of multiple types of organisms over a long period of time, with individuality coupled with a hive mind. Their intelligence and flexibility suggests that they lived in an incredibly competitive environment, and that they might have lived in the area around modern-day Spain in an equilibrium with similar organisms semi-isolated from outside influences.
The region seen in RE4 looks pretty cold and miserable, and this change in temperature might have forced the ecosystem to shrink and close in on itself. It's possible that, under extreme pressure, the various species might have all melded into a super-species that was entirely self-sufficient and relied on cooperation not competition, hence the strong community bonds described by the leaders of Los Illuminados, as well as the fierce hostility to all outsiders. They had the ability absorb other creatures because they had to in order to survive. When humans encountered them, the earliest peoples shown in the rock carvings might have formed a symbiotic bond with the parasites and kept the organisms in check through sacrifice and ritual warfare. Past the early Middle Ages, however, these practices were probably banned, but :as Plagas were now used to interacting with humans and had to be shut away by force by local rulers. Once they were given access to modern weapons, however, the only advantage humans had over them was the partial lack of autonomy among the drones; overall, plaga are superior lifeforms to humans and are not unstable like the lab-created BOW's.