In Cryptum i read that the prehistoric humans were hairless.
But in the Halo 4 terminals they have hair, plenty of hair.
[url=http://www.halopedia.org/images/1/12/H4_-_Forthencho_surrenders.jpg]Look at the Lord of Admirals, Forthencho.[/url] At the end of Primordium he is described to have 'Patches of Hair', but in the Halo 4 terminal he looks like a werewolf.
[Edited on 11.18.2012 7:49 AM PST]
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If there's no explanation for this, you should just go with what is established in the Halo 4 terminals.