We share a common ancestor with every single known living thing.

http://en.wikipedia.org/wiki/Last_universal_ancestor

Those are theories. But time and time again, researchers have sought to trace where the evolution path branches off. So far: no dice. And that means a theory of humans coming from another evolutionary path not found on earth still may hold merit. Because it has not been disproven yet.

The odds are heavily stacked in favor of a universal common ancestor. We share like 99% of our DNA with chimps; how do you suppose we got that, if we didn't share an ancestor?

http://www.ncbi.nlm.nih.gov/pubmed/20463738

> Among a wide range of biological models involving the independent ancestry of major taxonomic groups, the model selection tests are found to overwhelmingly support UCA irrespective of the presence of horizontal gene transfer and symbiotic fusion events. These results provide powerful statistical evidence corroborating the monophyly of all known life.