Gahd, to hear Stephen Hawking regurgitating the most vapid of pop culture[1] is extremely disappointing. I don't disagree with his point, and it's nice to hear him say it, but come on, he's arguably the greatest physicist of our time. Feed us some sort of argument on par with the Fermi paradox. Or at least something more than just 'this could happen, or that could happen.' Failing that, if you think us normals can't comprehend complexities, at least regurgitate Greg Egan.
[1] ID4 http://www.imdb.com/title/tt0116629
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Here's what I think. Despite the optimism of early sci-fi authors I see no reason to assume any alien we meet will be friendly, or indeed anything but ruthlessly self-centered. When we meet aliens we want to see them before they see us. We want to find out where they live and never let them find out where we live. This is a game of 'True names' on a cosmic scale, where our true name is our location in space. To play this game you need to ante up with the technology to spread your eggs far from the gravity well of origin. That's the bare minimum.
To think in terms of 'single-cellular life, multi-cellular life, and intelligence' does humanity's space program a huge disservice. It helps us remain fat and complacent in the sense that we're at the top of the food chain. Intelligence isn't a state to be reached; it is a continuance of evolution. And there's an unknown number of rungs for us to climb to break out of our niche, starting with basic space travel.
That's what I think; have at it. But at least it's more thought provoking than Hawking.