I don't think partial control of a complex system requires a system of equal complexity. Even human behavior can be influenced in the brain by something simple as a drug or virus.
Specific directives (to find a suitable place to anchor itself) require awareness of the terrain and object-recognition. Either a) the fungus has stumbled upon the signals to accomplish this singular feat or b) the fungus can read the ant's perceptions and can navigate. Finding the signal seems more likely because it is much less complex an achievement.
Or the fungus "stumbled" across an effect on the ants' nervous system that was beneficial to it, so it thrived.
Emergent properties don't require sophistication. They arise despite the lack of sophistication or the ability for specific intent. That's the definition of "emergent".
Yes, like what if there is an existing chemical signal for "climb up." the fungus might emit that signal and then just ride along, without any understanding of how that happens. Because it works, those fungus do better than ones that gave a different signal.
After a while, it gives the "bite" signal and hopefully, the ant has already climbed up. Voila, the ant has served the fungus.
So maybe some pretty simple steps result in an effect that appears carefully planned?
