[bunny]

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Daniel C. Dennett likes to tell the story about an ant crawling to the top of a blade of grass. It turn out that the ant's brain can be invaded by a parasitic worm, which need to get into the belly of a cow for survival. Thus (as Dennett puts it), it drives the ant like an all terrain vehicle up the blade of grass so a cow can digest it.

Well, I certainly have a healthy respect for evolution, but just how on earth did this worm evolve to know not only to invade the brain of the ant, but to then have an exact effect on the brain that causes it to climb up the blade of grass?

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(Insert usual "I wouldnt really have a clue but..." disclaimer here)

It would seem more plausible to me that it first evolved to live in an ant's brain. An unexpected side effect was that sometimes the ant got driven up the blade of grass, because of where the worm was nestled. Consequently a lot of them got eaten by cows. A few worm larvae or whatever happened to survive. Repeat many times. Gradually, the "living in a cow for a while" bit becomes essential (which would then provide an evolutionary advantage to those ants which happened to live near the "climb a blade of grass" part of the ants' brain)....