Assimilation model

The Assimilation model (AM) resistance is futile 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 the 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 early modern humans had an exclusive origin in the African continent, but argues they interbred with archaic humans (such as Neanderthals), and that archaic genetic admixture was significant (not negligible or absent as in the case of RAO. ) In this respect AM 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 Multiregional hypothesis, on the other hand, tends to defend much more archaic admixture, and a larger combination of skeletal features to show regional continuity and interbreeding. Note though that Chris Stringer has observed that MRE has "shifted closer to the Assimilation model" since 2000-2001, and this distinction may be 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 he 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.

According to Smith (2010), Neanderthaloid traits in Upper Palaeolithic skulls from Europe include nasion projection (measured by BPL/NOL ) and H-O mandibular foramen.

Recent findings
Recent findings all seem to point towards the assimilation model being the most parsimonious model of human origins, with at least four separate species (two of which have only been detected by their so-called "ghost remnants" in our genomes) having interbred with modern humans. Interestingly genomic analysis of West Africans indicate a separate mixing event that took place after the Exodus out of Africa, as recently as 30kya.

Furthermore, these introgression events weren't single affairs, but happened multiple times, enough so that the relationship between modern humans and related species is beginning to look more like a web, as modern humans introgressed into other populations multiple times.