Neanderthals, also known as k'tamanune (plural k'tamanush) to prehistoric humans and the Forerunners, were a human species native to Europe and parts of Central and Western Asia on Earth. Physically, Neanderthals differed greatly from Homo sapiens: they possessed slightly larger braincases, smaller foreheads, thicker brow ridges and lacked a chin, as well as having a shorter but much more robust skeleton than their taller, more lithe relatives. Neanderthals appear in the fossil record approximately 130,000 years ago, and disappear in Asia approximately 50,000 years ago before finally disappearing 30,000 years ago. The reasons for their disappearance remain unknown - theories alternately suggest that they were unable to adapt to a changing climate, were displaced by more adaptable modern humans through direct or indirect competition, or were absorbed into the newer human population. Between 1% and 4% of the non-African genepool is Neanderthal DNA, evidence that the two species interbred at some stage, likely in the Middle East as Homo sapiens migrated out of Africa.
Neanderthals were one of a number of human species indexed by the Librarian as part of the Conservation Measure and reseeded on Earth after the activation of the Halo Array, released in Africa near the Portal and presumably relocated to their former northern homelands. Eventually Neanderthals would mate with Homo sapiens and Denisovans. Several modern-day humans are genetically related to both species. Sometime later the Neanderthal species would go extinct to due several factors.