Human evolution was shaped by interbreeding

The ones on the eastern side can breed successfully with the ones in the center.

The ones on the western side can also breed successfully with the ones in the center.

The ones on the eastern side are different enough from the ones on the west that they can't breed successfully.

How many species are there? :-)

It's a continuum, not a binary relation.

It's very similar to the way that in historic times you could travel the Germanic or Romance language areas of Europe and the people in one village could always understand the people in the next village, but after a couple of hundred miles the languages would be mutually incomprehensible (this is not nearly as prominent today due to the rise of strong national governments enforcing "standard" languages through schools).

There are a lot of other issues involved, too. See https://en.wikipedia.org/wiki/Species_problem for some discussion.