A part of me wonders if it would literally take an alien discovery to unite the world. An idea explored forever, I see it as a viable outcome. Suddenly, the human tendency to create an "other" would rise to the occasion such that with a point of reference so different (the alien life), we would recognize more acutely the humanity in one another.
This, of course, is a purely speculative claim. Humans have a funny way of defying logic, so maybe this would just cause some countries to try to broker an advantageous trade deal. Who knows?