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The Assimilation model (AM) is a hypothesis of human evolution proposed by the anthropologist Fred H. Smith in 1989 as an intermediate model between the Recent African Origin hypothesis (RAO) and Multiregional hypothesis (MRE). Smith has developed his model in a number of papers (Smith, 1994, 2002, 2010; Smith et al. 2005, 2012).
Assimilation agrees with RAO that anatomical modernity had an exclusive origin in the African continent, but argues migrants from Africa interbred with non-Africans such as Neanderthals in Europe, and that archaic genetic admixture was significant (not negligible or absent in the case of RAO) which has more in common with Multiregionalism.
AM argues that while there was a non-trivial archaic contribution to early modern humans, this admixture was modest and only shows for a small number of morphological traits:
The Assimilation model (AM) was formally articulated in 1989 (Smith et al., 1989); however, elements of it were present earlier. The AM has always asserted that modern humans likely arose in Africa [...] modern humans evolved specifically in East Africa, and then spread, with their distinct morphological makeup from this homeland to other parts of the Old World and ultimately beyond. In these points the AM does not differ from the RAO model. However, whereas the RAO model theoretically accepts the possibility of admixture, this is viewed as being inconsequential in the emergence of modern humans outside Africa. The AM agrees with the MRE model that gene flow occurs between regional populations of archaic and early modern humans outside the African homeland of the latter. However, the AM differs with the MRE model in terms of the likely extent of gene flow... AM holds that evidence of local morphological continuity across the archaic/modern human 'boundary' is much less extensive, being reflected only in limited details of anatomy superimposed on the modern morphological pattern originally derived from Africa (Smith et al., 1989; Smith, 2002; Cartmill and Smith, 2009).
The Multiregional hypothesis instead tends to defend much more archaic admixture, and a larger combination of skeletal features to show regional continuity and interbreeding. Note though Chris Stringer has observed MRE has "shifted closer to the Assimilation model" since 2000-2001, and this distinction maybe no longer as clear as it once was. Throughout the 1990s Assimilation had very limited support, Smith notes that AM was "ignored or lumped in with multiregionalism" and "kicked so much".
In 2007, biologist Darren Curnoe published a study arguing "the present study indicates that the Assimilation model presently offers the best explanation for the origins of Pleistocene Australians", but later "changed his views and no longer considers interbreeding to provide a parsimonious explanation for Aboriginal Australian morphology".
Assimilation (AM) has recently become more popular because the genome sequencing of Upper Paleolithic skeletons in 2014 and 2015 has shown they carry archaic (e.g. Neanderthal) genes that are greater than the trivial amount the RAO hypothesis allows through interbreeding, and simultaneously less than what Multiregionalism predicts.
- Smith et al. 1989.
- "This model [AM] differs from the recent African origin (RAO) model in that the AM holds that the genetic exchange was more than incidental." (Smith et al. 2005)
- Smith et al. 2012.
- "MRE as originally presented (what I call classic Multiregionalism) has been falsiﬁed... recent versions of it, which emphasize gene ﬂow from Africa, are often difﬁcult to distinguish in essentials from AM." (Stringer, 2014)
- Gibbons, 2011.
- Curnoe, 2011
- Holliday et al. (2014).
- "Differences among the samples clearly show that European samples have projecting faces directly above the nose as a typical feature." (Frayer, 1993)
- NOL Nasion-Occipital Length, BNL – Basion-Nasion.
- Cartmill, M., Smith, F. H. (2009). The Human Lineage. Wiley-Blackwell.
- Curnoe, D. "Modern human origins in Australasia: testing the predictions of competing models". Homo. 58(2): 117-157.
- Gibbons, A. (2011). " A New View Of the Birth of Homo sapiens". Science. 331: 392-394. 
- Holliday, T. W., Gautney, J. R., Friedl, L. (2014). "Right for the Wrong Reasons: Reflections on Modern Human Origins in the Post-Neanderthal Genome Era". Current Anthropology. 55(6): 696-724.
- Smith, F. H., Falsetti, A. B., Donnelly, S. (1989). "Modern human origins". Yearbook of Physical Anthropology. 32: 35-68.
- Smith, F. H. (1994). "Samples, species, and speculations in the study of modern human origins". In: Nitecki, M. H. & Nitecki, D. W (eds.). Origins of Anatomically Modern Humans. Plenum Press.
- Smith, F. H. (2002). "Migrations, radiations and continuity: Patterns in the evolution of Middle and Late Pleistocene humans". In: Hartwig, W. (ed.). Primate Fossil Record. Cambridge University Press.
- Smith, F. H., Janković, I., Karavanić, I. (2005). "The assimilation model, modern human origins in Europe, and the extinction of Neandertals". Quaternary International. 137(1): 7-19. 
- Smith, F. H. (2010). "Species, populations and assimilation in later human evolution". In: A Companion to Biological Anthropology. Larsen, C. (ed.). Wiley-Blackwell.
- Smith, F., Hutchinson, V., Janković, I. (2012). "Assimilation and modern human origins in the African peripheries". In: Reynolds, S. C. & Gallagher, A. (eds.). African Genesis: Perspectives on Hominin Evolution. Cambridge University Press.
- Stringer, C. (2014). "Comment". In: Holliday, T. W., Gautney, J. R., Friedl, L. "Right for the Wrong Reasons: Reflections on Modern Human Origins in the Post-Neanderthal Genome Era". Current Anthropology. 55(6): 715-716.