I don't claim to be an expert on this subject as I've only ever mildly entertained the idea that shrooms came from outer space but from Terence McKenna's lectures I believe the theory goes a little like this (doing this from memory so someone jump in if I left anything out):
Terence postulates that if we use our own species as an example (which we have to having no other reference point), then we are just now at the age in our development where we have mapped the human genome and have begun toying with the genetics of the very thing that makes us human which is our DNA. A highly advanced civilization we can presuppose would come to the same conclusions about their own genetic make-up and we can imagine a scenario where a very advanced society would be able to control their very genetics.
If such an advanced civilization decided it wanted to go out and explore the cosmos but didn't know where to start or how to look would want two things just based on logic: 1.) an affordable and energy efficient solution that can cross the vastness of interstellar space and 2.) a way in which to branch out and explore as much of the universe as possible. We as humans may very well have the wrong approach with spending billions of dollars making high tech tin cans to carry our fleshy mammalian bodies through the vastness of space, a truly intelligent civilization would find a much more efficient means of exploration.
When one examines the chemical makeup of the mushroom spore, it seems like, in theory, the spore could survive the harshness of intergalactic travel. In fact, if your trying to store mushroom spores, you actually WANT to create an environment that is as close to a vacuum as you can to preserve them. The casing of the spore is one of the most electron dense organic materials we have found in nature, a denseness that actually approaches the consistency of a metal and apparently has an ability to reflect certain levels of radiation.
One thing about the mushroom that he talks about (and as far as I know he's the originator of this theory) is that they are a strange combination of artifact and entity. Mushrooms are the lowest of the low on the foodchain, absorbing their energy from decomposing matter. In his scenario the mushroom is like an ultra buddhist vegetarian, able to adopt a karma-free lifestyle of feeding only on the junk nothing else will feed on. From a scientific standpoint we can view this as mushrooms needs a food source that nothing else wants in order to grow which gives them their own place on the food chain of any ecosystem they might encounter.
When the conditions are right, mushrooms can form vast colonies and networks of underground mycelium which he postulates is like a neural network of information relaying circuits, similar to our human brain. It's a curious observation that when one ingests a psychedelic mushroom an 'entity' like voice appears to communicate strange and universal concepts telepathically to you.
It's sort of a long mental leap to make the claim that mushrooms came from space just based upon the loose evidence given. However, it's an interesting theory and we can imagine a scenario in our own development where we might adopt this approach. For instance, if today we figured out how to manipulate our DNA to a point where we could condense the entire human intellect into the size of a spore (imagine some great nanotechnological breakthrough that could make this possible) and then coded into that spore a program that would allow it to replicate itself at regular intervals, what you would have would be humanity branching out of it's home planet in a cloud of human intellect-spores. Over billions of years, these spores would land on planetary bodies in interstellar space, and on some of these planets the conditions might allow the spores to begin their growth process. Our spore nano machines would be programmed to begin developing a neural network of human intellect underground to avoid harmful radiation and predation, and at a certain phase a program would execute the development of recievers or antenna possibly to communicate back to us that they had found a suitable environment to carry out its programs. If we can imagine this much, it's not too far of a stretch to think if the planet our human spores had begun to colonize on had an indigenous life form, and if this life form found our mycelium human neural network and decided to ingest our 'recievers' (the mushroom itself, which sort of looks like an antenna), then it is entirely possible that would be able to open up a direct communication line in the neural network of the creature that found our organic space probe. Indeed if we had the forethought to make sure we had setup our nano spore machines with a direct two way communication, I can see a scenario where this would be a highly effective means of communicating across the vast distance of space.
