Regarding my previous post where I mentioned a paper on the non-African origin of modern humans, I have come across another very interesting paper: "Explaining worldwide patterns of human genetic variation using a coalescent-based serial founder model of migration outward from Africa" by Michael DeGiorgio, Mattias Jakobsson and Noah A. Rosenberg (Proc Natl Acad Sci U S A. 2009 Sep 22; 106(38): 16057–16062. Published online 2009 Aug 17. doi: 10.1073/pnas.0903341106) which offers an interesting model which can explain the greater diversity of Africans and at the same time make them the "most recent" humans instead of the oldest ones.
The usual argument for an Out of Africa origin of Homo Sapiens is the higher diversity in Africa versus that of non-African populations. A typical example used as proof is the graph showing how Heterozygosity decreases with the distance to Africa (there are countless of these on the internet, with the Y axis showing Heterozygosity and the X axis showing the distance from Addis Abbaba), below is one of them:
Notice the negative gradient of the graph (shown by the dashed line in the figure -Fig. B, on the right). Heterozygosity falls from left to right.
The paper by DeGiorgio, Jakobsson and Rosenberg which caught my attention explores a models to explain this fact. In doing so they analyze different alternatives. And the one they discard because it does not fit the current idea of an Out of Africa model, is precisely the one that explains the data, if you accept a non-African origin of mankind. Allow me to explain:
Archaic Persistence Model
The team developed this "archaic persistence model" in which an archaic population evolved-mutated and originates modern humans (Homo sapiens). And as this first original and most ancient Homo sapiens population expands outwards from its homeland, it spreads gradually forming colonies which move into a "collection of preexisting archaic populations". For instance Denisovans, Neanderthals or even surviving yet unknown Chinese hominds -or Homo erectus descendants...
In the words of the authors, this is what they observed as the populations moved away from the original nucleous, the first modern human population:
"Heterozygosity increases, LD decreases, and the ancestral allele frequency spectrum slope increases with increasing colony number. These results can be understood from the fact that in the long time since the initial divergence, the K archaic populations have enough time to develop distinctive localized variants. As the migration wave travels through them, it accumulates diversity, gathering new variants from each population through which it passes. Thus, heterozygosity increases with increasing colony number in the same way that it increases in the archaic admixture model at the population in which admixture occurs. The difference between models lies in the fact that in the archaic persistence model, archaic admixture occurs in every population, so that heterozygosity increases at each step rather than at a single location. This occurrence of archaic admixture at each step also explains the decrease in LD and increase in the slope of the ancestral allele frequency spectrum that occur at each step...
In other words they observed that if the original, the "first real" human group, was in Africa, as we moved further away from there we should find more heterozygosity, which we don't. Hence the model is wrong and despite having originated in Africa we did not go through this type of admixture.
But lets just look at this with a more open mind:
Turn it around!
However if the original home of modern humans (Homo sapiens) was in America and not Africa, then, as they migrated outwards from America, into Asia, Oceania, Europe and went through each of the archaic populations in the old world, adding diversity and finally reached Africa mixing with the archaics there, the heterozygosity would be lowest in America and grow towards Africa, which is exactly what their model finds and which coincides with the observed heterozygosity cline in modern humans.
So It all makes sense if you invert the origin of the first humans: form Africa, to America.
Below is the image showing how heterozygosity grows as you move away from the original H. sapiens population if you assume the Archaic Persistence Model, the authors discard it because the actual observed heterozygosity falls as you move away from Africa:
Now let's turn this same figure around and the result is a graph with a negative gradient which is exactly the same as the first graph we used in this paper (the distance to A. Abbaba & heterozygosity):
Adapted by Austin Whittall
As an engineer, I sometimes get the feeling that Anthropology and those who study the origins of humanity are like the late 1800s Physicists who despite all the evidence kept on pushing the notion of ether or fiddle with the data to uphold classic mechanics. It took Einstein and Plank to change the paradigm with their Relativity and Quantum theories. Something similar took place in the 1500s, with Copernicus explaining with graceful simplicity the movement of the planets, doing away with the complex Ptolomean system. A shift in mindset is needed to look at the same data to find the necessary changes and to avoid adhering to theories that contradict that data.
Sources
[1] Verdu P, Pemberton TJ, Laurent R, Kemp BM, Gonzalez-Oliver A, Gorodezky C, et al. (2014) Patterns of Admixture and Population Structure in Native Populations of Northwest North America. PLoS Genet 10(8): e1004530. doi:10.1371/journal.pgen.1004530
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