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Why do some scientists classify Neanderthals as a subspecies of Homo sapiens?

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Neanderthal Artist's rendering of Homo neanderthalensis, who ranged from western Europe to Central Asia for some 100,000 years before dying out approximately 30,000 years ago.

Some scientists classify Neanderthals as a subspecies of Homo sapiens—specifically H. sapiens neanderthalensis—primarily due to evidence of interbreeding between the two groups. While many traditional classifications treat them as distinct species, genetic research has revealed that modern Eurasians carry approximately 1–4 percent Neanderthal nuclear DNA, indicating that they successfully mated and produced offspring.12

Furthermore, when examining a broad range of individuals rather than single specimens, the morphological variation observed does not isolate Neanderthals as a group that is completely distinct from modern humans for every physical trait. Although Neanderthals possess unique craniofacial features, such as a low-vaulted cranium and prominent brow ridges, they share many physical characteristics with early modern humans. This overlap in biological and genetic data challenges the strict separation of the two groups, leading some researchers to argue that they represent different populations of the same species rather than two separate biological entities.13