Kim Shepard wrote...
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What I'd like to know is more detail about why the Reapers need to use other species for reproduction, and why only one species was considered good enough when their evolution was guided by the Reapers.
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All I want to point out here is that we don't actually
know what the reapers had planned. We don't know that they only choose
one species. Perhaps, in light of Shepard's success against them, they decided that humans should be the first to fall. As simple as taking out the most threatening target first.
I can back this up too. Remember the Rachni Queen talking about her song being soured? Most people believe this to be Reaper meddling, and for the sake of my argument lets say it is. So here we have the reapers using the Rachni as Vanguards, now who was it that rofl-stomped the rachni? Oh, yes,
The Krogan! My bet is that,at the time the reapers were planning on making a krogan reaper, but then the Krogan were hit with the genophage, and their numbers dwindle. Solution? Saren.
He cures the genophage. In ME1 we all thought he was crazy for wanting a Krogan population boom, we assumed that he was going to use indocturnation to control them. And, maybe that's what Saren himself thought, but we all know that indocturnation is sloppy, and potentionally lethal, its more likely that the boom was intended to get the krogan numbers back up to reaperifying levels.
Once again their plans are thwarted, but this time by the new comer race, Humans. So they investigate, kill Shepard and take his/her body for research, discover if s/he is an isolated threat, or if the whole of humanity is as cunning, and determined. It's here that they match wits with TIM, and find that, no Shepard is not the only human threat.
Now a new plan is born, create a human reaper and eliminate the human threat, and from there hit the other races. Then, Shepard puts an end to this new found plan, and the reapers must "find another way". This time screw making another reaper, just out right genocide.
Thats all just speculation of course, it's also possible that the reapers have a human fetish, but to me, that just seems a bit boring, and unimaginative.