Ever wonder how spawning fish can return to the precise stream from which they emigrated years before? It's an astonishing feat made possible by an extraordinarily complex olfactory system that allows the fish to detect stream-specific chemicals in the water.
Somehow, pelagic species of fish like the sockeye salmon detect these chemicals in the ocean and follow them back to the stream in which they hatched.
This video uses computer animation to explain a little bit about the microscopic olfactory system that enables salmon to accomplish their amazing journey.
Of course, salmon aren't the only creatures with such a highly sophisticated sense of smell. Many insects are equally as gifted.
Whether in fish or in insects there are really just two live options for explaining how such systems arose: Either they're the product of thousands (millions?) of lucky and highly improbable genetic mutations over eons of time, or these receptor systems and neuronal circuits were somehow intentionally engineered.
Of course, we have no evidence that blind, purposeless mechanistic processes can produce systems like this by chance, but we do have lots of evidence right in front of us (our computers) and all around us (the wiring in our houses) that intelligent agents can do it.
As the great skeptic David Hume argued, we should always base our beliefs on our experience. If we have a uniform experience of a phenomenon occurring through natural mechanisms then we should be extremely reluctant to attribute the occurrence of such a phenomenon to non-natural processes.
Hume was arguing here against belief in miracles, but if his principle is sound it surely has broader application. We have a uniform experience of systems and circuits similar to those illustrated in this video being the work of minds but no indubitable, non-question-begging experience of such contrivances occurring through undirected processes.
Thus, an intelligent agent, an electrical engineer of sorts, would seem to be the most probable and thus the most reasonable explanation for the salmon's wonderful olfactory capacities.
For the biologically-minded there's more on the research into the salmon's abilities here.