I've heard it said that all living things on the planet share a common ancestor. My question is how sure are we of this? I am not asking this from a religious standpoint.

My thought is that if way back, molecules started reproducing themselves, could this not be an automatic mechanism of chemistry--that under the right circumstances life happens?  If so, could there not be multiple trees of life who's foundational DNA look similar?

What gives us the idea that all life shares a common ancestor. It seems that it would be far more likely that life began multiple times. Do I have incorrect information?

see below. In addition most scientists agree that life arose independently multiple times.
http://www.askabiologist.org.uk/answers … p?id=10761
https://en.wikipedia.org/wiki/Evidence_ … on_descent