Question

2.Briefly (1-2 sentences) explain both the RAOWH (recent African origin with hybridization) and the AM (assimilation model) theories, and how they differ?

Answer 1

Recent African Origin With Hybridisation

Evidence indicates that Neanderthal and Denisovan traits emerge in Eurasia, while Homo sapiens traits emerge in Africa. Africa and Eurasia are isolated until H. sapiens disperses and interbreeds with the other two (and possibly some other unknown archaic species). Modern humans essentially absorb and replace the fragmentary local populations. The low percentage of Neanderthal and Denisovan genes found in living humans indicates a replacement process was most likely.

New fossil discoveries suggest that modern human physical traits did not emerge as one suite but were gradual. A skull from Jebel Irhoud in Morocco dated to about 300,000 years old, controversially assigned to Homo sapiens, had a modern-looking face but an elongated, archaic-looking braincase. This suggests our globular braincase evolved later and not as part of a fully modern suite of features.

Based on this and other early Homo sapiens remains, including those from Herto and Omo Kibish in Ethiopia, the African Multiregionalism Model of H. sapiens evolution was proposed in 2018 by a group including Eleanor Scerri and Chris Stringer (who proposed the original Out of Africa model). They suggest that early Homo sapiens showed great diversity and that rather than a single origin, our species emerged from admixture among numerous populations within Africa.

Assimilation Model

While most researchers agree with the RAOWH model, there are some that propose a different theory of interactions among human species and the role these interactions had in modern human origins. This theory differs in the way it explains how the DNA of Homo sapiens mixed with local populations outside Africa. Essentially, while some H. sapiens traits originated in Africa, it was when populations spread into Eurasia and extensively interbred with Neanderthals and Denisovans, that the evolution of new modern traits occurred. Thus, this model proposes, modern human origins involve a high degree of assimilation with archaic Eurasian populations within the last 100,000 years.

The two theories differ in their reconstructions of the processes by which the DNA of dispersing H. sapiens populations mixed with those of other populations outside of Africa. AM emphasizes demic diffusion, in which populations of African-derived H. sapiens and Eurasian Neandertals and Denisovans would have mixed over wide areas. Genes would have flowed gradually between these regional populations, catalyzing genetic and anatomical changes and leading to the spread of modern traits. In contrast, RAOWH envisages H. sapiens genes as largely entering and traversing Eurasia within the bodies of dispersing humans of African origin. Along the way there were successful hybridization events with indigenous populations, but these were effectively absorbing fragmented populations of indigenes in a relatively rapid replacement process, where they overlapped. Debate continues over what constitutes necessary or sufficient evidence for either theory.