From Halopedia, the Halo wiki
The Denisovans (Denisova hominin) were a prehistoric species of the genus Homo, closely related to humans and Neanderthals. The Denisovans were genetically similar enough with these groups to interbreed with both species. They were tall and lean in stature, had chocolate-brown skin, reddish hair and square heads. Male Denisovans had abundant facial hair. An unknown number of Denisovans were present on Installation 07, along with several other species of human.
After the firing of the Array, Denisovans were re-populated on Earth where they would later interbreed with humans and die out of unknown causes. Before their species died out, several Denisovans would mate with Neanderthals and Homo sapiens with their genetic lineage surviving in modern-day humans.
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