Christopher Packham writes:

It seems so obvious that someone should have thought of it decades ago: Since parasites have plagued eukaryotic life for millions of years, their prevalence likely affected evolution. Psychologist Marco Del Giudice of the University of New Mexico is not the first researcher to suggest that the evolution of the human brain could have been influenced by parasites that manipulate host behavior. But tired of waiting for neurologists to pick up the ball and run with it, he has published a paper in the Quarterly Review of Biology that suggests four categories of adaptive host countermeasures against brain-manipulating parasites and the likely evolutionary responses of the parasites themselves. The idea has implications across a host of fields, and may explain human psychology, functional brain network structure, and the frustratingly variable effects of psychopharmaceuticals.

Detailed and gruesomely readable, the paper is a work of theory intended to provide a roadmap for deeper study that is likely to be agonizingly complex, and which will eventually require the involvement of neurologists, evolutionary biologists, psychologists, parasitologists and many others.

Many parasites manipulate host behavior in order to increase reproductive success and to spread across wider areas. Dr. Del Giudice cites such examples as Toxoplasma gondii, which hitches a ride in a rat and induces epigenetic changes in the rodent’s amygdala. These changes diminish its predator aversion around cats, the protozoan’s intended destination, and the only animal in which it can reproduce. (As a side effect, it can infect humans—people are a reproductive dead end for T. gondii, but it is also believed to alter human behavior.)

Del Giudice also cites rabies, which increases production of infectious saliva and induces the host’s aversion to water, which further concentrates the saliva, and then engenders violent aggression to increase the likelihood of biting, a transmission route. And many sexually transmitted pathogens are known to manipulate host sexual behavior.

The point is that parasites are really bad for hosts, and it therefore stands to reason that the evolution of modern humans includes protective countermeasures that were selected for success and likely shaped the stupefyingly complex central nervous system

The paper is organized by four countermeasures hosts have evolved against manipulative parasites: restricting access to the brain; increasing the costs of manipulation; increasing the complexity of signaling; and increasing robustness. Within each category, Del Giudice suggests evolutionary responses by parasites to these countermeasures. [Continue reading…]