> It’s been suggested that humans lack an adept sense of smell because they could rely on dogs’ sense of smell

I don't think this is correct. We have a larger olfactory bulb than Neanderthals did.

There are plenty of examples of coevolution. One particularly interesting one is the relationship between leaf cutter ants and fungi. Leaf cutter ants don't actually eat the leaves they harvest; they bring their harvest back to their nests and use it as substrate to cultivate fungi, which they then feed to their larvae.

Of course, parasitism and predation can also drive coevolution. The relationship between two species doesn't have to be mutually beneficial to drive selection in both.