The Wikipedia article on the evolution of cephalopods indicates that even though there are gaps in the fossil record and much controversy about the development of these remarkable creatures, it is pretty clear they evolved right here on earth and share common molluscan and pre-molluscan ancestors.

I'm no biologist either, but I find it hard to accept a naturally evolved alien virus could hijack terrestrial DNA and create its own offshoot of creatures. I just find it difficult to accept a foreign life form could successfully invade organisms that already had billions of years of evolution behind them. I doubt a foreign bug would be able to do anything with domestic DNA, not even eat it, much less take over its evolution. Now if we were talking about a deliberately engineered alien virus, an artifact, specifically designed to do that, then maybe that might be a possibility.

Still, something very strange happened in the Cambrian Explosion, and it happened very quickly and with no warning or preparation. As I have alluded before, I feel it is a viable speculation that the "kick" came from outside, and that perhaps the invading microbes were not necessarily the cause of that event, but that our own local biota underwent the explosive evolution to metazoan as a defense strategy to overcome the alien invaders. All of the contemporary multicellular phyla, even the vertebrates, seemed to have arisen in the Cambrian explosion. Although several have since gone extinct, no new ones have appeared since.

This dovetails nicely with my recent suspicions that perhaps extraterrestrial intelligence could populate the galaxy by scattering engineered DNA on the stellar winds. FTL travel may be impossible, and subluminal interstellar craft impractical, but we know if time is no object then microbes could cross the galaxy in several hundred million years, using natural dispersal mechanisms. And if more than one civilization is doing this, we may experience pulses and waves of colonization constantly going on. Some may even be secondary expeditions from previously colonized worlds.

Perhaps even advanced intelligence itself is the result of some foreign traveler subtly guiding our DNA towards sentience. It may not be as much fun as little green men coming out of spacecraft, but it sort of fits in more convincingly with just how fucking strange our universe really is.