You assume it was very hard, yet it happened so quickly on earth, why is that?
Well it isn't really all that quickly, 2 billion years is still an immense amount of time. I'm not assuming it was really hard, it was really hard. All of evolution is entirely to chance, so the chance that the proper changes are made to make multicellular organisms that start developing individual organs are infinitesimal. My point is exactly that it's so unbelievable, intelligent life is incredibly difficult to make and it's mind blowing that it could have happened in 2 billion years here. It's absolutely incredible. Against-all-odds mind-blowingly incredible. That's exactly the reason that it's so unlikely to exist elsewhere. The chances of it happening twice in one universe is more than improbable, hence why it's a major proponent of the multiverse theory. If an infinite number of universes exist, that increases the odds of intelligent life forming in one of them by a whole lot.
Here's a Stephen Hawking quote, from
http://www.hawking.org.uk/life-in-the-universe.html:
One possibility is that the formation of something like DNA, which could reproduce itself, is extremely unlikely. However, in a universe with a very large, or infinite, number of stars, one would expect it to occur in a few stellar systems, but they would be very widely separated. The fact that life happened to occur on Earth, is not however surprising or unlikely. It is just an application of the Weak Anthropic Principle: if life had appeared instead on another planet, we would be asking why it had occurred there.
Actually, wow. A lot of things on that page are exactly what I was saying. You should read it.
Here's another good quote.
Maybe the probability of life spontaneously appearing is so low, that Earth is the only planet in the galaxy, or in the observable universe, in which it happened. Another possibility is that there was a reasonable probability of forming self reproducing systems, like cells, but that most of these forms of life did not evolve intelligence. We are used to thinking of intelligent life, as an inevitable consequence of evolution. But the Anthropic Principle should warn us to be wary of such arguments. It is more likely that evolution is a random process, with intelligence as only one of a large number of possible outcomes. It is not clear that intelligence has any long-term survival value. Bacteria, and other single cell organisms, will live on, if all other life on Earth is wiped out by our actions. There is support for the view that intelligence, was an unlikely development for life on Earth, from the chronology of evolution. It took a very long time, two and a half billion years, to go from single cells to multi-cell beings, which are a necessary precursor to intelligence. This is a good fraction of the total time available, before the Sun blows up. So it would be consistent with the hypothesis, that the probability for life to develop intelligence, is low. In this case, we might expect to find many other life forms in the galaxy, but we are unlikely to find intelligent life.
