A tiny bone fragment pulled from Denisova Cave in Siberia’s Altai Mountains has yielded only the fourth complete Neanderthal genome ever sequenced — and it’s already rewriting what scientists thought they knew about how this species spread across Eurasia.
The fragment is roughly 110,000 years old. The individual it belonged to, a male, turns out to be a distant relative of another Neanderthal whose remains were found in the same cave — one who lived there approximately 10,000 years earlier. That connection, separated by millennia yet rooted in shared ancestry, points to something far more complicated than a stable, continuous population.
The Fourth Complete Neanderthal Genome
The bone fragment, labeled D17, was recovered from Denisova Cave — a site already known for yielding genetic evidence of both Neanderthals and Denisovans across nearly 300,000 years of intermittent occupation. Researchers extracted a full, high-coverage genome from it, making D17 only the fourth complete Neanderthal genome ever sequenced. The study appeared in PNAS in March 2025.
The individual was male. His genome was compared with three other complete Neanderthal genomes to map how populations related to one another across time and geography — and that comparison produced results that surprised even researchers familiar with the species’ fragmented history.
Distant Relatives, Same Cave, 10,000 Years Apart
The most striking comparison was with D5, a female Neanderthal from the same cave dated to roughly 120,000 years ago — about 10,000 years before D17. They weren’t in a direct ancestor-descendant relationship, but they shared a common ancestor and belonged to closely related lineages.
That connection across such a long interval suggests Neanderthals returned to this landscape repeatedly rather than occupying it in one unbroken stretch. “It is likely that Denisova Cave was part of a broader landscape used repeatedly by these Neanderthal populations over time, rather than a site occupied by a single, continuous group,” said lead author Diyendo Massilani, a genetics professor at the Yale School of Medicine. The cave functioned as a waypoint — not a permanent home.
Tiny Groups, Extreme Inbreeding
The genomic data also illuminates just how small these populations were. Genetic markers indicate that Altai Neanderthals lived in groups of 50 or fewer, possibly as few as 20. Long stretches of identical DNA in the analyzed individuals suggest their parents were closely related — as close as first cousins.
Earlier research points in the same direction. A 2022 study estimated one Altai Neanderthal community at around 20 individuals; another identified a group that had been isolated for roughly 50,000 years. Inbreeding and isolation have long been cited as factors in Neanderthal extinction, but the new data complicates that narrative: these populations also survived for extended periods under exactly those conditions, suggesting the relationship between isolation and extinction was anything but straightforward.
A Patchwork Species: Genetic Drift at Extraordinary Speed
Perhaps the most consequential finding concerns how genetically distinct different Neanderthal populations became — and how fast. D17 and D5 were more closely related to each other than either was to European Neanderthals or to later Altai populations. Groups separated by an average of only about 50,000 years had diverged to a degree comparable to modern human populations separated for roughly 300,000 years, such as people from Central Africa and Papua New Guinea.
That rate points to genetic drift: in small, isolated groups, random genetic changes accumulate faster and spread more readily. The result was a species that, despite sharing a name and a basic biology, was quietly fracturing into genetically distinct fragments across its range.
“We already knew that Neanderthals were not a single, homogeneous population spread across Eurasia, but a patchwork of groups shaped by complex demographic processes, including divergence, migration, local extinctions and replacements,” Massilani said. “What is striking in our results is just how differentiated these populations could become.”
What Fragmentation Meant for Neanderthal Survival
High genetic separation between groups may have had real consequences for the species’ long-term resilience. Massilani suggests this fragmentation could have limited Neanderthals’ ability to adapt to shifting environments — a structural vulnerability embedded in their demographic reality, not simply a byproduct of competition with arriving Homo sapiens.
Outside researchers have noted the value of having two sequenced genomes from such a geographically close location. “To have two sequenced Neanderthals in such a close geographic place does bring new and more fine-grained insight” into their population, said Léo Planche, a population geneticist at Paris-Saclay University’s Interdisciplinary Laboratory for Digital Sciences, who wasn’t involved in the study. He added that the field is only now accumulating enough genomes to make meaningful claims about population structure: “Populations are groups of individuals, so the more data the better.”
With only four complete genomes in hand, scientists are still working from a sparse record. Even so, that handful is enough to reveal a species not moving steadily toward extinction but quietly fragmenting — group by group, valley by valley — long before modern humans arrived. That portrait of Neanderthals as isolated, drifting communities rather than a unified people may be the most significant revision this field has produced in years.