David Reich thinks Neanderthal genetics requires a replacement event that strains credibility
The Neanderthal mitochondrial DNA and Y chromosome sit far closer to modern humans than the rest of the Neanderthal genome does to Denisovans. David Reich argues that gap demands an explanation the field has not yet fully reckoned with.
David Reich, the Harvard geneticist whose lab has reshaped the field of ancient DNA, is pressing on a tension in Neanderthal genomics that is hard to dismiss once it is stated plainly. The Neanderthal mitochondrial DNA and Y chromosome diverged from those of modern humans only around 300,000 to 450,000 years ago. The rest of the Neanderthal genome, by contrast, diverged from Denisovans roughly 700,000 years ago. Those two numbers do not sit comfortably together, and Reich is pushing on why.
The gap points toward a specific historical event. At some point, a population related to modern humans introgressed into Neanderthals, replacing their mitochondrial DNA and Y chromosome entirely while leaving the rest of the genome largely untouched. That replacement material, on Reich’s account, amounted to only about 5 percent of the total Neanderthal genome.
What follows from that figure is what makes the claim arresting. A 5 percent contribution is a modest one. It is the kind of signal that, in most contexts, leaves only a faint statistical trace. Yet the mitochondrial DNA and Y chromosome it carried did not linger at low frequency. They went to fixation. Every sequenced Neanderthal carries them. As Reich frames it: “What the current genetic work is asking us to believe is that even though this is only 5% of the whole genome, it introduces mitochondrial DNA and Y chromosomes, and they jump up to 100% frequency.”
What the current genetic work is asking us to believe is that even though this is only 5% of the whole genome, it introduces mitochondrial DNA and Y chromosomes, and they jump up to 100% frequency. David Reich
That is a steep climb for a small founder signal. In population genetics, the path from rare introduction to complete fixation requires either very strong selection or a severe bottleneck, and sometimes both. Natural selection acting on mitochondrial DNA and the Y chromosome is not impossible to invoke, but it is not a light assumption either. A male-line and a maternal-line replacement happening together, from a single limited admixture event, and then spreading to every member of the population: each step is individually arguable, but the chain of steps is long.
Reich is not asserting that the scenario cannot have happened. He is pointing out what the evidence is asking the field to accept. The divergence dates are what they are. The 5 percent figure is what it is. The universality of the replacement in the Neanderthal record is what it is. The question is whether a coherent mechanism connects them without requiring assumptions that would, in any other context, be treated as extraordinary.
That question matters beyond Neanderthal prehistory. The methodology used to reconstruct admixture events, detect introgression, and date population splits is the same methodology applied across dozens of ancient human studies. If the Neanderthal case exposes a structural puzzle, it is worth knowing whether the puzzle is a rare edge case or a sign that the models need adjustment. Reich is one of a small number of researchers positioned to see that question clearly. His willingness to state the tension in plain terms, rather than absorb it into technical hedging, is itself a form of useful pressure on the field.