"On the Origin of Species by means of Natural Selection"
by Charles Darwin, 1859.
"The Selfish Gene" by Richard Dawkins, 1974 is a good appendum there as well but once mastering "Origin" one really has the basics one needs.
What precludes an "intelligently designed" being from being the first non-terrestrial life-form we encounter?
For all the talk we have here about the Singularity, I would assume that the first signs of life from Earth that any other civilization would meet would probably be something engineered by man. (Assuming we're meeting them somewhere other than our solar system.)