Sol is right that if we were different species we wouldn't be able to reproduce as we do. Also I've heard/read over and over that compared to many species humans aren't that genetically diverse. We're all basically the same, differences in outward appearance aside. Humans just tend to make a really big deal of small differences (they seems like BIG differences apparently to many).
https://bigthink.com/philip-perry/hu...or-our-species
The idea that you would be a different species to someone in the same family who's "P" instead of "J" doesn't make sense to me.
Since humans and Neanderthals/Denisovans could apparently interbreed it means they were already very closely related, so much so they could perhaps even produce offspring that could interbreed with humans.
An example of species similar enough to breed:
https://en.wikipedia.org/wiki/Mule