Did Darwin state in his theory of Evolution that he though we all came from a single organism? Or did he say that the orginal species were already present and that new species just evolved from these?

Evolutionary theory doesn't delve into how live began, only how it changes over generations. Abiogenesis explores the possibilities of life from non-life. I don't know whether or not Darwin had an opinion on it.

"All earthly living things are certainly descended from a single ancestor" -Richard Dawkins, River Out Of Eden. Terribly interesting stuff.

I did find this quote:

Charles Darwin "It is mere rubbish to talk about the origin of life; one might as well talk about the origin of matter."

Don't know if that means he didn't think it could be answered, or if it was a moot point, ie creation was the answer.