We aren't dealing with numbers but with facts.
even if we know that the missing number is 2, but what was 2 for, was it for bananas, oranges, men....etc
The principle is the same. An unknown variable (and yes, facts are variables in logical deduction) being made known based on given information (and yes, facts count as information).
We know that there was a common ancestor between our two species, like we know the number is 2, because there's enough information around it to make this logical conclusion. We don't know what that species
was, like whether the 2 stands for oranges or bananas.
Fact: the scientific consensus is always open to change and correction given compelling new information.
Fact: "I don't know, yet" is perfectly fine in the sciences.
Fact: there is genetic change from generation to generation. You
have to recognize this as true if you recognize that we look different from our parents, that breeding programs exist, etc.
Fact: we have evidence of life on this planet going back
at least 3.7 billion years, because that's the age of the oldest fossil found. We know this because of a variety of accurate measurements, none of which would be good enough on their own but taken together can lead us to reasonably accurate conclusions (always subject to revision given new information). Side note: yes, I find it kinda odd that we have fossils going back that far, when I'd expect plate tectonics to have melted any rock from that far back by now, based on the information
I have personally. EDIT: A bit of research tells me that the exact age of life on Earth is still contested.
Fact: there are lots of genetic similarities between all life on this planet, including humans
Fact: all of this is predicted by the theory of biological evolution via natural selection.
Fact: if biological evolution via natural selection didn't happen, breeding programs wouldn't exist, and neither would modern medicines work.
Based on all these, the only reasonable conclusion is that common ancestors exist between species, and that all life on Earth shares a single common ancestor in the far distant past. Sure there's a few problems with the details of
how, but these problems aren't, in any way, indicative that this process doesn't happen.