Higher diversity is normally associated with a effective populaton size, and that is why the Out of Africa migrants are said to have a lower diversity: they moved in a small group and this carried less diversity than the original population (a founder effect and a bottleneck for the migrants),
However, a paper published in Nature by Jakobsson et al., 2025 studied the evolution of modern humans using genetic information garnered from "ancient southern African genomes" and found that the African population size wasn't all that big. In fact it was the same size as the Out of Africa band!
Below I quote the relevant passage in the paper, my comments in brackets, and I highlighted some interesting parts of the text:
"Long-term large population size
... Heterozygosity (HO) for ancient southern Africans (mean across genomes; HO = 0.80 × 10−3) was similar to other ancient Africans, only surpassed by an ancient western African individual (HO = 0.93 × 10−3), indicating a large Holocene population size in southern Africa. A multiple sequentially coalescent approach shows that the effective population size (Ne) was large for several hundred thousand years, up to Ne ≈ 30,000 around 200 ka, similar to other African groups. The large Ne at ≥300 ka for all humans was potentially caused by population subdivision. [interesting! so small populations divided into many give the appearance of a large Ne when in fact it isn't] We note a decline in Ne for ancient southern Africans from around 100–50 ka, to Ne ≈ 10,000 by the Last Glacial Maximum (20 ka), similar to non-African groups and the ancient northern Africans [So at the time of the supposed Out of Africa Event 100,000 to 50,000 years ago southern & northern Africans and non-Africans had the same population size! So where is the OOA bottleneck?]
Runs of homozygosity (ROH, where greater numbers and total length of ROH segments indicate a smaller population size) show that the ancient southern Africans were at the upper tail of the distribution of modern-day Africans, but less extreme than most non-Africans—a pattern attributed to the out-of-Africa bottleneck. [homozygosity is attributed to small populations, and inbreeding, loss of diversity or heterozygosity. So the supposed most ancient humans, the South African San people have the highest ROH among Africans — but lower than non-Africans.] This indicates a smaller population size (relative to, for example, western African groups) in the relatively recent history of each individual, but still larger compared with non-Africans and ancient northern Africans. Most ancient southern Africans are shifted towards greater total segment ROH length without affecting the total number of ROH segments, in particular the Great Brak River (2,355–2,310 cal. bp) and the Matjes River 1 (7,845–7,690 cal. bp) individuals [this date is extremely recent! well after the OOA event]. This pattern indicates a smaller recent ancestral population size, possibly with elements of inbreeding, indicating isolation and fragmentation among ancient southern Africans during the Holocene. Ancient southern Africans south of the Limpopo River therefore consisted of a large, stable population for many millennia, with a modest decline since around 50 ka, and a possible fragmentation and further decline during the Holocene."
Admixture with archaics!
As usual the paper also points out that: "Population stratification between southern Africa (the region south of the Zambezi River) and the rest of Africa probably existed for at least 300 thousand years (kyr), perhaps up to a million years. Such deep stratification may result from admixture with an unknown archaic African group predating the divergence of Homo sapiens from Neandertals and Denisovans, and//or from isolation from other groups." The isolation would mean that an archaic form of humans didn't admix with the others and carried ancient, unshared alleles. The admixture option seems more plausible.
When discussing diversity, the authors note that: "Cumulatively, the genomes of the ancient southern Africans show that this group displays many Homo sapiens-specific variants (and variable positions) at amino acid-altering sites, also reflected among the modern-day San people. This observation cannot be explained solely by a large, stable southern African population, which retained derived variants to a greater extent compared with other groups. The ancient southern Africans were probably also isolated from other African groups for long periods. The derived variants unique to southern Africans may also signal low-to-modest gene flow from an unknown/unsampled group of genetically differentiated humans." Indeed, introgression from archaics.
Notice how reluctant mainstream scholars are. Instead of digging deeper into the archaic admixture hypothesis, they set it aside. In this case, the authors seem to agree with this option, but, politely wrote: "Irrespective of cause, the many variable amino acid-altering sites among the ancient southern Africans point towards a genetic model in which different protein variants can be combined to viable outcomes... The many Homo sapiens-specific variants found in southern African genomes point to a combinatorial genetic model of human evolution in which there are many possible combinations of genetic variants that lead to ‘genetically modern’ Homo sapiens." Yes, a combination brought about by mating with archaics within Africa.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2026 by Austin Whittall ©
No comments:
Post a Comment