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The key idea here is how genetic variation is distributed across humans. Lewontin’s 1972 analysis of several genetic markers showed that the bulk of variation is found within individual populations rather than between different populations. In practical terms, most genetic differences you observe when you compare any two people come from variation among individuals within the same group, not from differences that separate groups as a whole. This means that two people from the same population can be as genetically different from each other as two people from different populations.

There is some variation between populations, but it accounts for a smaller share of the total diversity. Focusing on mitochondrial DNA or the Y chromosome looks at maternal or paternal lineages, which capture only a fraction of the genome’s variation and therefore do not reflect the overall pattern Lewontin described. So the most accurate takeaway is that most human genetic variation resides within populations.