"Human tree" is vague since we know that there are lots of prior species that were not direct ancestors of modern humans. The point is that (1) related species can breed and (2) modern humans are a component of all races.
Since White people have common ancestry with egrets, horses, salamanders, and wheat, I'm pretty sure we also share common ancestry with Blacks. That doesn't mean we aren't evolving to be more and more different from them, though. We are, and we should continue to do so.
"If black people and white people do not share common ancestry"
HOLD UP! who said that? last i checked ALL MANKIND came from THE BLACK MAN.
"....then how come white people can reproduce with black people?"
because no matter what the race, HUMANS can produce with each other. now get this part. a black/African women can give birth to a white baby (look it up). some say that albinos were once ostracized in ancient Africa, and they were forced to move to the caucus mountains. after 100's if not 1000's of years, so call albinos adapted to their environment. straight hair to keep warm. thin nose so too much air can't get in, and whiter skin because the lack of sun. not sure how true this is, but scientist believe this to be true with other animals.
to add on, because black people have the dominant gene/DNA, no matter who a black person have a baby with, the baby will come out BLACK
Modern humans = arose in Eurosiberian wilds
They mated with:
* Denisovans (Asia)
* Australopithecus or similar (Dravidians, Indonesia, Australia)
* Ergaster or Erectus (Africa)
Africans are part modern humans
Different groups from the human tree can breed, e.g. Neanderthalis