The short: parasite in cat, cat poops, rats get infected, parasite enters rats brain, rats start to like cats, cats eat stupid rats, cat poops…
Berdoy M, Webster J, Macdonald D (2000). Fatal Attraction in Rats Infected with Toxoplasma gondii. Proceedings of the Royal Society of London, B267:1591-1594. PMID 11007336
We tested the hypothesis that the parasite Toxoplasma gondii manipulates the behaviour of its intermediate rat host in order to increase its chance of being predated by cats, its feline definitive host, thereby ensuring the completion of its life cycle. Here we report that, although rats have evolved anti-predator avoidance of areas with signs of cat presence, T. gondii’s manipulation appears to alter the rat’s perception of cat predation risk, in some cases turning their innate aversion into an imprudent attraction. The selectivity of such behavioural changes suggests that this ubiquitous parasite subtly alters the brain of its intermediate host to enhance predation rate whilst leaving other behavioural categories and general health intact
Some points to consider. Other animals are the prey of cats such as Lions. Tigers, Leopards. Humans have been known to be eaten by cats. If our ancestors became less afraid of these vicious cats, then they were more likely to be eaten.
Could the domestication of cats have resulted because of the parasite?