Image taken from http://www.seti.org
The origin and evolution of life is a very interesting topic, be it for debates between the followers of Creation vs those of Evolution or for some research scientists who try to search for another life in the universe. Bible told us, life was created by God on the third day of "Earth Creation". Human were created on the seventh day (opsst..hang on, I need to check my Bible first...I was wrong it should be the sixth day). While, scientists said life began in the form of the simplest micro-organisms such bacterias and protozoans and it started 3.5 - 4 billions years ago.
Recent theory by Prof. Paul Davies even proposed that life might have started in Mars and somehow "transfered" to the Earth in the rock blasts off the red planet due to comet impacts. Paul Davies, Professor of Natural Philosophy in the Australian Centre for Astrobiology in one of the scientists whose research interests are searching for the nature and origin of life. He is even going a bit further, by thinking we, human, may not be the only intelligent life in the universe.
Efforts to search for another life in the universe are now being done by the SETI (Search for Extraterrestrial Intelligence) Institute, a private, non-profit organisation dedicated for scientific research, education and public outreach in exploring, understanding and explaining the origin, nature and prevalence of life in the universe. In a simpler statement, what SETI does is to send and seek for any "signals" of intelligence extra terretrial life out there or in other word they try to communicate with "alliens" by using sending/accepting light beams or listening to radio signals.
The idea behind this "we're not alone" theory is the "fact" that human life evolved from the simplest form in a right condition for a certain period. It means, in another universe, given a similar condition and simplest micro-organisms exist and in a similar period of evolution, there may be another life of our kind. Whether it is as "civilised" as us or worse/better, we don't know.