One of the big revelations people seem to have missed in this game (and really, in the AC games for now), is that after Altair got the apple from Al Mualim, he spent a lot of time using it and finding the knowledge hidden within and through time (as seems to be the case).
Because of all the info he got from the apple, he was able to develop the hidden gun, as seen in one of the final Altair episodes in ACR (before anyone else was either 1- able to develop a gun or 2- able to miniaturize it as much, like Leonardo says in AC2 when you bring him the codex). He was also able to understand his purpose in the saga of the Assassins (and basically Ezio's) : they are prophets, put on this "pilgrimage" of sorts to send a message to Desmond to save the earth in 2012.
So Altair knew that he had to preserve his apple, so that Ezio could find it and thus Desmond could get the message in it (AC2 apple had a message in it from Minerva, which Ezio activated for Desmond. Temple at end of ACB had message from Juno which Desmond activated from Ezio's apple when he found it).
But I can hear people say : why couldn't Desmond hear the message in Altair's apple from Altair directly? He spent a lot of time with it! Well you have to remember : Animus only allows people to see the memories of their ancestors. Remember the small Altair sequence in AC2, when you run after Maria and finally, at the top of the tower, the two basically have sex, then Altair jumps down, but you stay with Maria? That's because Desmond's line is from that child conceived. As such, anything Altair did after conceiving this child is not present in Desmond's DNA. Ezio, having had no kids until late in his life (his first child, and thus Desmond's line, seem to be with Sofia), Ezio was needed as a prophet for Desmond to be able to hear Altair's Apple's message. Also, there is speculation (or fact, if I read the Extra section of the official guide to ACR correctly) that Altair's apple was destroyed, thus Desmond can't find it in 2012, hence why it was needed for Ezio to discover it so that Desmond could get the message in it.
It does look like such convolutions and/or time travel shenanigans, but remember what Minerva, Juno and especially Tinia (or by his other names, Jupiter and Zeus ) : the first civilization had 6 senses, one of them being knowledge. And like Tinia said at the end of ACR, Altair's library was a nexus of time (or the nexus ain't the library itself, but rather the fact that Ezio/Tinia/Desmond are all at the same place at the same time, relatively, the nexus). And Tinia wasn't used to these Time calculations. So the first civilization, with their sense of knowledge, would definitely be able to foresee the catastrophe of 2012, hence their knowing about Desmond's role in saving the earth in 2012, and the need for his line to discover the knowledge the first civilization had so that they can "send" it to Desmond so that he can save the earth.
As for those wondering how Desmond can use the first civilization's knowledge to save the earth when they couldn't, remember that Minerva, Juno and Tinia keep saying, in their messages, that the war between the first civilization and their slaves, the humans, prevented them from seeing the catastrophe coming before they could prevent it. If they hadn't been distracted from the war, I'm sure they would have been able to easily prevent the damage the solar flare caused.
Phew, that took a while to write!
by: alphasyndrome
http://www.ps3trophies.org/forum/assassins...spoilers-5.html