[user104658]

Originally Posted by James

One of the reasons I have some doubts (now) about intelligent life existing elsewhere in the universe is that on Earth simple single-cell life started quite quickly, after conditions became right, but it took about 2 billion years for eukaryotic cells to come about which led to complex living organisms, and scientists think that this only happened because of a single chance encounter.

There's this Brian Cox video where he explains it - http://www.bbc.co.uk/programmes/p029n23h

Even then, though, it's when you take scale into account. Earth is a speck of dust even on the scale of our own galaxy. Our galaxy isn't even a particularly big one - and there are hundreds of billions of galaxies just in the theoretical observable universe. There may well be countless more. And then... There's a good chance that if you "zoom out further" it's the case that our universe is just one of an unknown number. The scale is mind-boggling, to the extent that I'd say if something is POSSIBLE then it's highly improbable that it has or will only happen once. It will have happened, out there, somewhere.

What I suppose it does mean though is potentially only a tiny number of "habitable zone" planets have more than single cell life, and intelligent (to the point of technology) species might be much rarer than is technically possible based on the number of planets that COULD support it. Maybe entire galaxies with only a couple or none at all.

Which would still probably explain "no contact". Even if travelling between stars becomes theoretically quick-ish, travelling between galaxies would be something far beyond that.