Bacterial life appeared on this planet basically the instant asteroids stopped bombarding the planet. For all we know, life was created and destroyed several times before the Late Heavy Bombardment ceased. So it appears that simple bacterial/viral life may be commonplace throughout the cosmos. Indeed, there are tantalizing signs that Mars and Titan may harbor some form of life.
On the other hand, complex multicellular life only appeared in the last billion years, which suggests that the leap from single-cell -> multicellular life is somewhat difficult. Our sun won't be conducive to life in another billion years, so complex life "barely" made it here.
I would love to be wrong, but given the fact that planets appear to be commonplace throughout the cosmos, and we have yet to hear from anyone, it starts to shift the odds towards one or more of:
1) Complex life is relatively rare and widely separated in space and time.
2) Complex life doesn't survive long-term (nuclear war, grey goo)
3) Complex life does survive, but for some reason doesn't communicate or colonize other worlds (a "Prime Directive", or perhaps they "sublime" in the Ian Banks/Culture sense)
I actually lean a bit towards 3 myself, but humanity will eventually find out, one way or the other.