From Africa to the World: What Population Genetics Shows About Human Unity

What Population Genetics Actually Shows About Human Unity, and Why the People Who Deny It Understand Less About DNA Than They Think

There is a particular kind of irony built into my professional position, and I have never quite decided whether it is amusing or merely characteristic of how science actually works. When a skeleton arrives in my examination room, one of the first tasks is constructing a biological profile: age at death, sex where determinable, stature, and what the discipline calls ancestry estimation, meaning the assessment of which broad population group the individual’s morphological features most closely resemble. I use measurable variation in skull shape, dental morphology, and postcranial proportions to generate an investigative lead. The medical examiner needs to know whether to search missing persons databases for a person of European ancestry or sub-Saharan African ancestry, because this narrows the search by several orders of magnitude. I supply that assessment because the morphological variation is real and measurable, and because it is useful in the specific, operationally limited context of identification.

And I also agree, without reservation, with the 2019 statement adopted unanimously by the American Association of Physical Anthropologists: race does not provide an accurate representation of human biological variation, humans are not divided biologically into discrete continental types or racial genetic clusters, and the Western concept of race must be understood as a classification system that emerged from and in support of European colonialism, not from biological reality (Fuentes, A., et al., 2019, AAPA Statement on Race and Racism, American Journal of Physical Anthropology, 169, 400-402). Both of these positions are simultaneously correct. The tension between them is instructive, and working through it honestly is more useful than either pretending the tension does not exist or collapsing into the comfortable simplification that race is just a social construct with no biological reference points whatsoever. What the genetics actually shows is simultaneously more interesting and more decisive for the racism question than either side of that debate usually acknowledges.

What Happened in Morocco 315,000 Years Ago

The version of human origins taught in most schools, that Homo sapiens emerged in East Africa approximately 200,000 years ago and spread from there, was already being revised by the time it was fully established in public consciousness. In 2017, Jean-Jacques Hublin and colleagues at the Max Planck Institute for Evolutionary Anthropology published an analysis of newly discovered human fossils from Jebel Irhoud in Morocco, dated by thermoluminescence to 315,000 years ago, plus or minus 34,000 years (Hublin, J.J., et al., 2017, New Fossils from Jebel Irhoud, Morocco and the Pan-African Origin of Homo sapiens, Nature, 546, 289-292). The facial morphology of these fossils, including the mandible and dentition, aligned clearly with early or recent modern humans. The braincase retained a more archaic shape, suggesting that the globularization of the human skull continued to evolve after the rest of the modern human face had already been established. Hublin described the implication directly: where once we thought there was a cradle of mankind in eastern Africa 200,000 years ago, the Jebel Irhoud evidence points to a pan-African origin, with dispersal across the entire continent preceding the later out-of-Africa migration by tens of thousands of years. The story of human origins is not a story of a single population in a single place. It is a story of a species that evolved across the largest continent on Earth, in multiple populations, exchanging genes across enormous distances, before the ancestors of everyone living outside Africa today walked north into the Levant and beyond.

This matters for the racism question because the implicit logic of racial hierarchy has always depended on some version of the idea that certain human populations are more “original,” more “pure,” or more central to the human story than others. The genomic and paleoanthropological record tells exactly the opposite story. Africa is not the origin point of one branch of humanity. It is the source of all of it, including the genetic diversity that all non-African populations carry as a reduced subset of the variation that remained on the continent. The farther a population’s ancestors traveled from Africa, the less of that original diversity they retained, because each founding migration carried only a portion of the population and therefore only a portion of the alleles. This is the founder effect, and its consequences are visible across the genomes of everyone alive.

The Bottleneck That Followed Every Ancestor Who Left

When anatomically modern humans left Africa approximately 60,000 years ago, they did so as a small group whose genetic diversity represented only a fraction of the variation present in the African populations they descended from (Pagani, L., et al., 2016, Genomic Analyses Inform on Migration Events During the Peopling of Eurasia, Nature, 538, 238-242). The genetic signature of this bottleneck is measurable today: African populations harbor more genetic diversity than any other population on Earth, and the genetic diversity found in all non-African populations combined represents a subset of what exists within Africa alone. This is not a statement about cultural or intellectual capacity. It is a mathematical consequence of population genetics. A small founding group carries fewer alleles, and subsequent drift and selection reshape the frequencies of the alleles it does carry. As those founding populations expanded and eventually separated into the geographically distinct groups we recognize today, their genetic profiles diverged further, but the initial compression meant that the total variation between all non-African populations was, from the start, smaller than the variation within a single large African population.

This is the genomic backdrop against which any discussion of human diversity should proceed, and it rarely does, because the intuitive perception of human diversity runs precisely backward. People look at a Swede and a Congolese person and perceive maximal difference because the visible traits, skin color, hair texture, facial proportions, are maximally different. But those visible traits are the product of intense local selection acting on a tiny fraction of the genome, and they are not representative of overall genetic divergence. The overwhelming majority of the human genome varies within every population, not between them.

Lewontin’s Numbers, Fifty Years Later

In 1972, the evolutionary biologist Richard Lewontin published an analysis of genetic diversity in 17 protein-based polymorphisms across 7 conventionally defined racial groups. He found that 85.4% of the total genetic diversity of the human species was contained within populations, 8.3% varied among populations within racial categories, and only 6.3% of total human genetic variation corresponded to differences between the “races” themselves (Lewontin, R.C., 1972, The Apportionment of Human Diversity, Evolutionary Biology, 6, 381-398). Lewontin concluded the paper with a statement that deserves to be more widely known than it is: “Since such racial classification is now seen to be of virtually no genetic or taxonomic significance either, no justification can be offered for its continuance.”

In the 50 years since that paper, datasets have grown from 17 markers in a few hundred individuals to hundreds of thousands of genetic markers in millions of genomes. The finding has not changed. Edge, Ramachandran, and Rosenberg, reviewing the anniversary of Lewontin’s paper in 2022, confirmed that the 85% within-population finding remains robust across modern genomic methods and datasets of a scale Lewontin could not have imagined when he worked with protein electrophoresis in the early 1970s (Edge, M.D., Ramachandran, S., & Rosenberg, N.A., 2022, Celebrating 50 Years Since Lewontin’s Apportionment of Human Diversity, Philosophical Transactions of the Royal Society B, 377, 20200405). The number has shifted slightly with more comprehensive data, but the fundamental relationship has not: more than 80% of human genetic variation is found within the populations that racists claim to distinguish from each other, not between them.

There is a technical objection to the Lewontin finding, associated with the statistician A.W.F. Edwards, who argued in 2003 that even small between-group differences in allele frequencies can allow probabilistic assignment of individuals to populations when many markers are considered together. This is mathematically correct and forensically relevant: it is part of the basis for ancestry estimation in identification work. But it does not rescue the racism argument. It demonstrates that population structure exists, which no population geneticist disputes, while being entirely compatible with the conclusion that most human genetic variation is within populations and that the between-group variation that exists does not correspond to the discrete, hierarchically organized racial categories that racial ideology requires.

The Neanderthals Were Family

Every person of non-African descent alive today carries approximately 1 to 4% of their genome from Neanderthals, the consequence of admixture events that occurred as anatomically modern humans spread into Europe and western Asia and encountered the populations already there (Green, R.E., et al., 2010, A Draft Sequence of the Neandertal Genome, Science, 328, 710-722; Prüfer, K., et al., 2014, The Complete Genome Sequence of a Neanderthal from the Altai Mountains, Nature, 505, 43-49). The sequencing of the first Neanderthal genome by the Pääbo group established what had long been suspected: the separation was not absolute, the meetings were not merely hostile, and the modern human lineage that eventually replaced Neanderthal populations in Europe and Asia did so while incorporating a detectable fraction of the genetic material it encountered. Populations from Oceania carry an additional 4 to 6% of their genome from Denisovans, a separate archaic hominin group known primarily from a finger bone and several teeth recovered from Denisova Cave in Siberia, whose genome was sequenced by Reich and colleagues in 2010 (Reich, D., et al., 2010, Genetic History of an Archaic Hominin Group from Denisova Cave in Siberia, Nature, 468, 1053-1060).

I find it useful to think about what this genetic record says about the concept of human purity, not because the concept deserves rehabilitation, but because understanding precisely why it is nonsensical requires more than a moral statement. The Neanderthal admixture in living non-African populations means that no one of non-African descent is fully Homo sapiens in the strict sense of descending exclusively from anatomically modern humans. We are hybrids, in the technical genetic meaning of that word. Some of the Neanderthal-derived variants that persist in modern populations have been identified as functionally significant: contributions to immune system function, to keratin expression in skin and hair, to adaptations that may have facilitated survival in the high-latitude environments where Neanderthals had lived for hundreds of thousands of years before our ancestors arrived. We did not simply replace them. We partially became them, at the genomic level, and that fraction of archaic ancestry varies by population in ways that track geography rather than any concept of racial purity. The populations that racists have historically positioned as most “pure” carry detectable archaic hominin ancestry that the populations they have historically positioned as most “primitive” largely do not.

The Forensic Anthropology Paradox, Stated Honestly

I began this article by describing the tension between my professional practice and the scientific consensus on race, and I owe a direct explanation of how that tension resolves. When I estimate population ancestry from skeletal remains, I am using real, measurable variation in skull shape and dental morphology that reflects genuine patterns of population history. The variation is clinal, meaning it changes gradually across geography rather than jumping between discrete categories, and it correlates with ancestry in probabilistic rather than deterministic terms. A skull with a particular combination of features is more likely to have come from an individual whose recent ancestors lived in West Africa than from one whose recent ancestors lived in northern Europe. This is a statistical statement about a biological measurement, not a claim about race in any hierarchical or essentialist sense.

The AAPA 2019 statement, with which I agree, makes a distinction that is important here: race does not provide an accurate representation of human biological variation, but the morphological variation itself is real, and the population history that produced it is real (Fuentes et al., 2019). What is not real is the system of discrete, hierarchically ordered categories that racial ideology imposes on that continuous variation, and what is not real is the inference from phenotypic difference to differential human worth, capacity, or moral status. I use population morphological variation as a tool with a specific, limited investigative function. I do not use it to make claims about anyone’s intelligence, culture, civilization, or inherent characteristics, because the genetics does not support such claims, and neither does any other field of science that has examined the question seriously.

The history of my discipline is not unblemished on this point. Biological anthropology contributed directly to the construction of scientific racism in the 19th and early 20th centuries, producing craniometric studies whose conclusions were determined by their conclusions, measuring skull shapes and reporting results that invariably confirmed the racial hierarchies that the investigators arrived with. The AAPA’s 2019 statement acknowledges this history explicitly and accurately. Scientific credibility requires acknowledging it rather than treating the past as irrelevant to the present practice of the discipline.

What Actually Varies, and the Physics Behind It

Skin color is the visible trait most strongly associated with race in ordinary perception, and it provides perhaps the clearest illustration of how superficial the “differences” that racial ideology emphasizes actually are. UV radiation damages DNA and destroys folate, a B vitamin essential for fetal neural development. In high-UV environments near the equator, high concentrations of melanin in the skin protect against both effects. In low-UV environments at high latitudes, insufficient melanin production would prevent the skin synthesis of vitamin D, causing deficiency with severe metabolic consequences. Natural selection acted rapidly on this gradient: populations that have lived at high latitudes for many generations carry variants in a handful of genes, including SLC45A2, SLC24A5, and OCA2, that reduce melanin production. This adaptation evolved independently in multiple lineages, which is itself an indication of how powerful the selective pressure was. The traits that trigger the immediate perceptual categorization of race are skin-deep in the most literal sense: they are the product of perhaps a dozen genes out of approximately 20,000 in the human genome, responding to a physical environmental gradient, with no implication for the thousands of other traits that make a person who they are.

The person with dark skin and the person with light skin differ, on average, in the specific genes that regulate melanin synthesis. They do not differ, at the population level, in cognitive capacity, in the emotional architecture of their nervous systems, in moral sensibility, or in any of the traits that have historically been claimed to correlate with race. This is not a hopeful statement designed to make people feel better. It is what the genomic data shows when examined without the intent to confirm a predetermined conclusion.

A Warning Before the Final Word

Racism does not require biological evidence to sustain itself, and it has historically not required coherence either. When Lewontin published his finding in 1972, he must have believed that demonstrating that 85% of human genetic variation was within rather than between populations would materially reduce the scientific credibility of racial ideology. It did not, because racial ideology had never actually rested on genetic evidence. It rested on power structures that found genetics useful when it appeared to confirm them and dismissed genetics when it did not. The persistence of racism in the 50 years since Lewontin’s paper is not a failure of scientific communication. It is a demonstration of the degree to which the ideology operates independently of the evidence base it claims.

What science can do, and what it has done with increasing precision over the 50-plus years since Lewontin’s paper, is remove the legitimate scientific grounds for racial hierarchy, one by one. The claim that races are biologically distinct: refuted by population genetics. The claim that certain races are cognitively superior: refuted by cognitive science and the established effects of socioeconomic environment on every measurable cognitive outcome. The claim that human populations have separate origins in different parts of the world: refuted by paleoanthropology. The claim that racial mixing is genetically harmful: refuted by the genomic record of our species, which shows that admixture has been continuous, universal, and biologically beneficial throughout our evolutionary history. What remains after these refutations is not a weakened form of scientific racism. What remains is an ideology with no scientific foundation, sustained by social and political forces, continuing to cause measurable harm to people who deserve to have the scientific record stated clearly.

The Skull in the Examination Room, Reconsidered

The skeleton on my table does not know what race it is. It carries morphological patterns that reflect the populations its ancestors belonged to, embedded in bone shapes and dental measurements that I can read and interpret with reasonable precision for the specific purpose of investigative identification. What it does not carry, and what no skeleton has ever carried, is evidence that its owner was worth less than another human being, was less capable of thought, was less entitled to dignity, or belonged to a category that should be treated differently under law, medicine, or any other framework that claims to rest on biological reality.

The genetics of our species, read carefully and honestly from the 300,000-year-old faces at Jebel Irhoud through the Neanderthal sequences in living non-African genomes to the contemporary population datasets that confirm Lewontin’s 50-year-old conclusion, tells a coherent story. We are one species, of African origin, with continuous variation across our geographic spread, shaped by local adaptation and archaic admixture and the mathematics of population founding events, carrying within every one of our populations more variation than exists between all of our populations combined. Anyone who tells you otherwise is not reasoning from the evidence. They are using the vocabulary of science to dress an argument that the evidence has already answered.

References

• Edge, M.D., Ramachandran, S., & Rosenberg, N.A. (2022). Celebrating 50 years since Lewontin’s apportionment of human diversity. Philosophical Transactions of the Royal Society B, 377, 20200405. https://doi.org/10.1098/rstb.2020.0405
• Fuentes, A., Ackermann, R.R., Athreya, S., Bolnick, D., Lasisi, T., Lee, S.-H., McLean, S.-A., & Nelson, R. (2019). AAPA Statement on Race and Racism. American Journal of Physical Anthropology, 169, 400-402. https://doi.org/10.1002/ajpa.23882
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