All life on earth has a common ancestor
The term "species" is an old taxonomic designation. A whale is a species and dolphins are a different species. However, whales and dolphins can interbreed to produce "wolphins" A species is simply a snapshot of evolution in progress. Chihuahuas are the same species as great Danes. If we killed all of the other breeds and only had those two left, they would be separate species because they could not interbreed.
Even the extinct species that left no descendants were intermediate. They were just killed before they could speciate further.
Said with such confidence.
Why only one ancestor? Why couldn't life have come about from non-life more than once? Because that's just too hard to believe? Don't have sufficient faith for that?
Sure they can. They just probably require a little logistical assistance.
We had a neighbor's miniature poodle get into the pen with our in-heat Rottie.
Funniest thing I ever saw. Despite active cooperation on her part, the mechanics of the thing just weren't working out.