I think there is life out there. Because if not, then that is a lot of wasted space. As to comeing here...doubtful because we aren't that interesting. If by some chance space aliens come, it would be to study like national geographic, not to interfer.
I enjoyed it. I was amazed in the details of L.A. and how you really had to pay attention to the facial expressions and if you could back up your accusations you had to make sure you got the right evidence. Kinda linear but still enjoyable. I played for hours when I first got it then i bought it because it was addictive.
It would be helpful you to understand the reasons for the story and to understand the game mechanics, but revelations would probably would give a summary to go over major points of the story. So in short, unnecessary but recommended to play the previous games. Kinda like watching last Lord of the Rings before the first two.