Did Genetic Degeneration Really Doom the Neanderthals?
One of the last high-quality Neanderthal genomes shows a population that was vulnerable—but not spiralling into mutational meltdown.
In Goyet Cave in Belgium, Neanderthal bones were highly fragmented and marked by human modification.
Some fragments carry marks consistent with butchery. Several bones from what once appeared to be different individuals turned out to belong to the same people. One woman survived in this collection not as an intact skeleton but as a right-femur fragment and two tibial fragments.
Her DNA survived much better.
Researchers have now reconstructed her genome at an average coverage of 22.4 times, making it only the fifth high-coverage Neanderthal genome available. The woman, designated GN1, lived about 45,000 years ago—within a few millennia of the Neanderthals’ disappearance.[1]
She offers a direct test of one of the most compelling extinction stories.
Neanderthals lived in small populations. Small populations lose genetic diversity, expose harmful recessive variants and struggle to remove mildly damaging mutations. If that process intensified near the end, the last Neanderthals should show a genomic decline: more close-relative mating, longer homozygous stretches, less diversity and a rising burden of damaging variants.
GN1 does not show that decline.
She was still genetically less diverse than a present-day human. Neanderthals had long maintained a low effective population size. The new result is not that they were genetically comfortable, but that the measured burden did not demonstrably worsen as extinction approached.
Some of the last Neanderthals were vulnerable. They were not visibly spiralling into genetic collapse.
The extinction-vortex story
The genetic argument for Neanderthal extinction begins with a real problem: numbers.
Neanderthals occupied an enormous territory across Europe and western Asia, but their effective population size was low. “Effective” population size is not a headcount. It describes how many people were contributing genes to later generations. A population can contain more living individuals than its genetic effective size suggests if it is divided into small groups, if some people leave no descendants, or if repeated contractions eliminate lineages.
In a large population, natural selection is relatively efficient at removing harmful variants. In a small one, chance becomes stronger. Mildly deleterious mutations can persist or even become common simply because genetic drift overwhelms weak selection. If close relatives have children, long stretches inherited from the same recent ancestor can meet in both chromosome copies, exposing recessive mutations.
This can create an extinction vortex. Lower survival or fertility makes the population smaller. Greater isolation then increases mate scarcity and inbreeding, which can lower fitness further.
Several discoveries made this scenario plausible for Neanderthals. Their genomes contain much less diversity than those of living humans. Some Neanderthal DNA introduced into Homo sapiens appears to have been removed by selection, indicating that at least part of it carried a fitness cost.[4,5] Demographic models show that inbreeding, difficulty finding mates and random fluctuations could, in principle, extinguish a sparse population without a single catastrophic event.[6]
The strongest image came from the Altai Mountains.
At Chagyrskaya Cave, DNA revealed a father and daughter, other close relatives and extensive homozygosity. One high-coverage individual had long identical stretches indicating recent shared ancestry between her parents.[2,3] It was easy to turn that cave into a portrait of the entire species: scattered family bands, cut off from one another, breeding themselves into oblivion.
But vulnerability is not the same as terminal decline. An isolated eastern group is not every Neanderthal population. The question is whether genomic damage became progressively worse as the species approached its end.