I don't know if they exist in our galaxy, but I find it hard to believe that life did not evolve independently at several other places in the universe, if not millions.
The universe is huge. More vast than we can ever imagine.This Mandelbrot zoom will give you an idea of how big it is...
http://www.youtube.com/watch?v=ATWrMlIKRBk (ignore the annoying noise, or just turn your speakers off)
So if life could evolve on a speck of dust rotating around an average sized star, and not only human life, but abundant life, everywhere... I find it highly improbable that similar or even identical conditions do not exist elsewhere in the universe, and have not formed life.
Earth could be the only planet with life. Maybe in some fluke, in the ENTIRE FUCKING UNIVERSE, our almost infinitely microscopic speck of rotating dust was somehow the ONLY one in which DNA formed, and replicated, and continued to replicate over millions of years, evolving into our plants and animals, forming a perfect ecosystem.
But I don't think so. I think if the right conditions are met, life just happens. It's a natural phenomenon. I expect it to have happened elsewhere in the universe, or to be happening right now somewhere else.