*** Edit *** Jan 30, 2011 ***Mostly cleaning up the bulk of the theory. Just a couple sentences added here and there. More added to my thoughts on the Hive Mind in the last paragraph. *** Please Enjoy!
Recently, before I read this post, I was coming up with my own theories as well (as I am sure all nid players have). Im not so sure that the Tyranids were capable of evolving on a single planet. A lot of nature seems to take a lot of time for creatures to evolve. In that time nature will usually find a balance to it. Ofcourse, it was mentioned in a much earlier post about an "event" or "trigger" that got the evolution of a particular species ramped up higher than nature intended or could "balance."
The theory I have been becoming ever more fond of is an idea similar to that "trigger-effect" but caused by a virus of an explorer team in another galaxy. A team lands on a life bearing planet (be this the races first planet they landed on or just another routine investigation in a young space dwelling empire). This virus, relatively harmless to the carrier race through vaccinations or their immune systems, just wreaks havoc across this new world.
The RNA binds with the hosts DNA/RNA in random cells for reproduction then releases violently inside species that have never encountered it before. The combination of this particular virus' reproduction and the chances of genetic variations from the host species alters the virus into several variations as it is dispersed in the planets population. A viral parasite is born, perhaps, that is capable of compelling (not controlling) its host. Attacking or fusing with the hosts neurons inside the brain or equivalent and even onto nerve endings. The parasites in the brain begin bridging gaps or destroying bridges and acting as a transmitter themselves, begin driving the host insane or killing it (the later most likely). The parasite becomes the leading cause of death amongst the fauna of this region, but to those that survived are changed/compelled by the virus to just feed. As the parasite has destroyed all but the primitive parts of the brain: to feed, protect, reproduce etc.
The now single celled parasite spreads to insect equivalent species, perhaps one that uses a type of weak psychic link that was once necessary for those insects to survive. Now with the parasite latching onto the nerves or specialized cells that allow the link to be possible further changes the the parasite (whom still breeds like that of a virus), through genetic variation, into a collection of cells that possess a link to other similar cells of this strand. Through the insects network, compels them to gather nearer each other, unknowingly to the insects, to force the spreading of this new super virus.
In time this region, be it a continent or island or some other form or land that is naturally secluded is now being over run and beginning to see the signs of stabilizing. The "Collective-Virus" has become the dominant virus as it is now stable enough in the insects. As their brain or equivalent is already "primitive" they suffer little physical damage from the virus. Also the survivability of the insects working as a collective is far greater than the self destructive versions of the other divergent viruses in the larger creatures that are hunted/attacked or just not capable of hosting the virus that ends up killing it before too long. The virus is passed through to a couple of generations of the insects. Imagine that as the insect develops, its brain or equivalent is hindered/fed upon as the virus takes its usual place. These new generation insects, though still linked with their brethren, have only ever really felt the compulsion of the virus. These generations have become far more aggressive in the defense of its territories killing off other competing insects or smaller animals and also attacks larger organisms to feed a hive that is quickly breeding and dominating. The "Collective-Virus" has allowed the insects to hunt with far more effectiveness because of both the aggression and synchronicity available to these newer generation insects.
The natural barriers are no longer capable of holding back the spreading swarms as they spill over into other territories. Perhaps then this virus runs into a new type of creature. One similar to the krootox of our galaxy in the respects that it can intentionally choose the path of its evolution by modifying themselves using prey animals to help them down this path. Though the "Collective-Virus" is not as deadly as its other divergences, it still infects and spreads with frightening speed, while leaving the bulk of the natural order of things vaguely intact. Though it has spread to other and new creatures, none have really changed the virus until this Krootox like species. Though the "Collective-Link" is still present nearly all those infected are still capable of operating independently with the exception of those whom evolved a similar link naturally as the insects did. The Krootox like creatures were still just that, creatures. Maybe they used primitive tools maybe not? Later generations were hindered and affected much the same way as the insect hives were that allowed them to become so dominant. The later generations also felt the compulsion of the "Collective-Virus" but as there was larger brain or equivalent capacity the collective effect was now able to be felt amongst these later generations further increasing the compelling nature of the virus.
This is the part of the story where the "Collective-Virus" in combination of the shaping abilities of the new creature began to actually blur the virus out and actually create a creature whom "naturally" has a psychic link and the ability to change its progeny form through consumed prey and still maintain the "Collective-Link" effect. From here as the race that originally brought the virus returns to the planet, these new creatures overwhelm and consume as many as possible. The ship(s) that brought the settlers begin to flee but like genestealers of now do, some creatures got aboard and actually spread to other planets creating more or less the first Tyranid attacks against other worlds. In some planets it was successful that the creatures actually were capable of over running its dominant inhabitants others not so much. The collective link, or now early brood telepathy, keeping these creatures working and hunting in concert, soon taking their genetic divergences capable of better hunting their new prey.
The Tyranids were not more than a menace floating around on space hulks. Certain collectives had experienced situations in which survivors attempted to "space" Tyranids. With their links, those who were not spaced were capable of conceiving ideas to resist the cold hard vacuum of space (but could not be immune). The idea of space travel was still nothing that crossed the "minds" of these early Tyranids. In space however strange things can happen. Strange creatures can evolve and prosper, in our galaxy there is the C'Tan, star vampires. Beings composed of energy who actually feed off stars energies and were later "trapped"/"summoned" into Necrodermis bodies. There were creatures, few in number most likely, whom were born in space. A space hulk may have hit or even been attacked by one. The early Tyranids either in defense or just looking for a way to board this new space hulk and spread, lashed out against it. It was during this attack that space travel to the Tyranids was opened up to them. Before the Tyranids died in the vacuum of space new eggs were laid with yet another leap of Tyranid evolution. The new genetic material allowed for the Tyranid to be far more resistant to the void than any previous Tyranid before it. The new Tyranids consumed the beast from within but only a handful of new Tyranids were produced. Their size however was greater than the others - maybe the size of a car. The first Tyranid ships were born and also the true meaning of hunger. The manipulation of its later generations were having fewer and fewer limitations as it no longer required a particular prey, just the right components. These new ships slowly ferried Tyranids across space engaging in space board actions and even running across another of the space beasts or 2. The later generation Hive ships were becoming larger in size. Now, with this small hive fleet, were Tyranids actually evolving multiple types of creatures to better deal with boarding actions. This large collective link didn't house "memories" ...but experiences. Only in time did this Hive Fleet begin to create a "Synaptic Web" to better store and share experiences and to allow a mass, almost communication with one another very much like the those first insects. Going beyond a brood telepathy but creating the Hive Mind. Experience taught the Hive Mind to drop creatures onto a planet and keep a strong enough link with the creatures below so that they would return with food and nutrients. It was no longer just animals that was hunted as prey but plants as well. With enough of genetic material on boards these ships they found creatures capable of absorbing nutrients out of the very air. Even atmospheres were no longer safe.
....thats my thoughts, and this is just following a select path of evolution. At each divergence it created a new species who would not be capable of recognizing the others who had similar collective effect. It was always more of a Brood Telepathy than the all encompassing Hive Mind. As the first true rudimentary hive fleet began to consume the galaxy, previous and other divergences were consumed as well. There was no "Oh hey what's up long lost cousin" any and all that did not spawn from the Hive Fleet was consumed as though it was prey. And of course through experience the Hive Mind only ever became stronger, being capable of what it is currently. Do the Tyrants and Norn Queens etc possess intelligence? Sure! But I think of it as they just possess and retain a far greater amount of experience from the Synapse web allowing them to make better decisions on the battlefield when fighting/hunting prey as it collects experience from the rest of the army and can relay back instructions in real time. Though hive fleets seem to posses a communication that spans the galaxy or even greater. I think its the hive just sensing itself, if a part of the Hive begins to grow/proliferate then more fleets are attracted to the area. If it senses the hive diminishing maybe they will tend to stray away. Scouting will help determine such areas without sacrificing a whole fleet. As Hive Fleets get closer to each other I believe that all the collected experiences can be shared amongst themselves, perhaps new creatures bred in each fleet to contain and keep the new experiences. This is why I think the lictors and genestealers are vital as a hive fleet draws closer. It would collect from them their experiences and spawn or use creatures appropriate to the encountered prey. Who knows what that effective range is. This would allow nearly all hive fleets to develop uniquely in their area of the galaxy and as they come in range of another fleet, either entering another fleets "hunting grounds" or visa verse, the sharing of experiences will better prepare each fleet for that particular area they are in.