The researchers seem to know little about race, because they then say,
“Race, as a psychological category, may be relatively modern in terms of human evolution,” explains Kinzler, now at the University of Chicago. In prehistoric times, “a neighboring group might have sounded different even if they did not look different,” she says. Preference for our own race might have developed later, after the more ancient preference for our own accent.
Historically speaking, race is a very recent concept—tribe is what usually matters to cultures, and an accent is an indicator of tribal belonging.