Even with these many categories it is still possible for interactions between organisms to be surprising in their complexity. Parasites are defined by the fact that they cause damage to the host, but organisms evolved in the context of parasitic infection and removing the parasite from the host can lead to unexpected consequences. One example of this is the link proposed between the lack of parasitic infection in the developed world and the prevalence of allergy. In areas of the world where parasitic infections are still the norm it is almost unheard of to find people suffering from anaphylactic reactions to allergens. However where people lack parasitic infections there is a prevalence of allergic reactions to everything from nuts to dust. The immune system evolved alongside parasites and these parasites provided a vital set of brakes on the immune system, modulating it or educating it by their presence. In their absence, therefore, the immune system is not functioning in the context in which it evolved. It has been shown in mice that experimental infection with tiny worms called schistosomes can prevent the development of diabetes. In fact Koichiro Fujita, an environmental parasitologist from Tokyo, has been hosting worms in his gut voluntarily and he claims it has kept him slim and has cleared up his hayfever.
So these close, long-term interactions have potentially led to the development of sex, to the very structure of the cells which make up all eukaryotic organisms and have produced an environment in which apparently detrimental characteristics have been selected. It is clear that the evolution of one species cannot be understood in isolation. Why doesn’t sickle cell anaemia get eradicated? Why does the peacock have a showy tail? Why is allergy on the rise? Answers come from an examination of these amazing symbiotic relationships. It is crucial not only to consider the abiotic conditions in which organisms evolved but also to remember that truly no man is an island.
Written by Ed Roberts