[rybosome]

I think we're nearly at confirmation of basic alien life. IIRC there are talks of missions to Europa as early as the 2020's, and I would honestly be surprised if no microbial life was found there. Liquid water, chemical processes that could support chemosynthesis, a likely abundance of organic compounds and protection from radiation and cosmic rays are all critical factors. That may even extend to larger fish-like life...there's a chance that the moon's icy shell releases oxygen into its interior liquid ocean in large enough quantities to promote the freakishly-large growth we enjoy. If we don't discover larger multi-cellular life within our solar system (perhaps also on Titan, though that seems less likely), then it'll be many, many years before we discover it. Interstellar travel is a massive hurdle. =)

Intelligent life, though...that's tricky. Given the size of the universe, it is almost certain that there is other intelligent life but who knows whether or not it survives long enough to come into contact? Discovering another civilization would be one of the most incredible experiences I can imagine, so I certainly hope it happens (peacefully) within my lifetime.

[lumpypua]

Can't speak to likelihood of discovering life on Europa but it would be fucking awesome.

To be clear, microbial life in no way entails higher life. It's only by the insanely chance event of the eurkaryotic cell emerging that sophisticated multicellular life became possible on earth. Even if life is common in the universe, sophisticated multicellular life is extremely rare.

The selective pressures on bacterial life favors small size and rapid reproduction, jettisoning any unnecessary genetic material.

Mitochondrial ancestors relieved those pressures in a couple ways. Once the eukaryotic cell developed, multicellularity has evolved several times. All the cool things (multicellularity, eyes, flight) have evolved independently a bunch of times, and the eukaryote has happened only fucking once. Here's a deeper explanation of why it's so rare: http://rstb.royalsocietypublishing.org/content/368/1622/2012...

Even if life is common in the universe, multicellular life is extremely rare because the evolution of eukaryote-esque organisms is extraordinarily rare and chance.

Edit: That is to say, there's a strong case that step 4 is the Great Filter: http://en.wikipedia.org/wiki/Great_Filter#The_Great_Filter

[rybosome]

> Can't speak to likelihood of discovering life on Europa

Honestly, me neither. I'm just a software engineer who is excited about space and hopes for the best. =)

> but it would be fucking awesome.

Wouldn't it? Holy hell, that would be wonderful. I hope it happens soon. Discovering life on Europa would be our generation's moon landing.

Thanks for the clarification. My knowledge of biology is limited, so I imagined each step (crude self-replicating molecules -> sophisticated DNA -> the cell -> etc.) being roughly as unlikely as the next. I had no idea eukaryotic cells were such a massive jump.

Earlier today I was watching a documentary about the Kepler telescope's search for exoplanets[0] that estimated the number of planets in our galaxy at 10^19. So many of those planets are unsuitable for any life, microbial or otherwise, but such a large number fills me with hope that the unlikely miracle of eukaryotic evolution could happen more than once. Then again, there are very plausible interpretations of the Drake equation[1] that pin the number of intelligent civilizations in our galaxy to less than 5...maybe even 1...so who knows?

[0]: https://www.youtube.com/watch?v=DD6QHP9ouuU [1]: http://en.wikipedia.org/wiki/Drake_equation

[lumpypua]

Yah there's really good explanation of why eukaryotes are jump posted up above!

http://ronbarak.tumblr.com/post/25996121029/life-is-it-inevi...