The replacement model, or out of Africa theory, proposes that humanity's alleged arboreal (tree dwelling) simian (ape-like) ancestors underwent a single and relatively recent transition from archaic hominins to anatomically correct humans in Africa, followed by a later migration to the rest of the world, mutating or rapidly evolving into MODERN (not just anatomically correct) humans and replacing other archaic hominin populations. This hypothesis has run into problems in light of recent genomic sequencing, as well as ongoing gaps ("missing links") in the fossil record