There is genetic variation in the human species. In Africa alone we could divide people into a thousand different "races" if we wanted to, based on various genetic differences. But there would be no point in having a thousand races. If you divide humans into just a few groups, however, then you can build a social hierarchy around those divisions. Besides, skin color varies within races and is consistent between some people of different races.
The biological concept of race has been refuted by evolutionary biologists and geneticists and genomicists for decades. The scientists who led the Human Genome Project made a point of saying human genetic variation isn't divided into races. There's no such thing as black genes or white genes. The amount of genetic variation among people of the same so-called race is greater than the amount of genetic variation between races. You might have genes that can be traced to a certain population somewhere on the globe, but there's no point at which you can draw a boundary line and identify one race on one side and a different race on the other.
All humans originated in African and then migrated outward in groups, each carrying a subset of the genetic variation in Africa. No one has identified a point in human history at which these migrating groups evolved into discrete and homogeneous "races."In other words, we are all one, people.