In a recent study published in the journal natureIn this study, researchers explore differences between different demographic models using statistics based on diversity and disequilibrium.
Stady: A poorly organized trunk of human origins in Africa. Image credit: Julius Kielaitis / Shutterstock.com
Where did the human race originate from?
Previous studies indicated that the global population most likely originated from a single phylogenetic group in Africa and could be traced through a tree-like model. However, fossil and archaeological records obtained throughout Africa have not confirmed this theory.
Most genetic models assume a tree-like model of isolation with migration; However, other theories have also been proposed, including population orthology and segmentation or fundamental models. In addition, recent advances in genomics have allowed new population genetic tools to incorporate the ‘ghost’ cohort to improve descriptions of genomic data and how they relate to single-origin models. However, these models also have some limitations, reinforcing the need for ancient DNA samples originating at least 300,000 years ago (Ka) to fully understand early civilization in Africa.
Trying to explain the origin Homo sapiens In the current study, the researchers used linkage disequilibrium and diversity-based statistics to distinguish between different models that have been used to study the evolution of the human species.
The four models considered in the current study included individual population expansion, individual expansion with territorial stability, ancient hominin admixture, and multiregional evolution, along with 290 genomes of individuals from southern, eastern, and western Africa and Eurasia. In addition, samples from British individuals from the 1000 Genomes Project were also included to represent reverse gene flow into Africa and recent colonial admixture in southern Africa. Neanderthal genomes from Vindija Cave in Croatia were also included in the analysis to represent gene flow from Neanderthals to regions outside Africa.
Modeling patterns of migration and diversity
Models with and without migration between stem populations were considered to study two types of gene flow during the expansion phase. In the first model, one stem group expands and migrates symmetrically with the other stem groups. Relatively speaking, in the second model, one or more stem groups expand and receive immediate “pulse” events from other stem groups. This subsequently leads to the formation of modern populations after mergers from different ancestral populations.
The two commonly used models are the continuous migration and multiple merge models, both of which allow migration between stem branches. However, these models differ primarily in the early divergence of stem populations and their relative effective population size (Ni).
According to the continuous migration model, stem 2 turns into lineages that give rise to present-day populations in southern, western and eastern Africa, while stem 2 provides different origins for these populations. Minde populations show the highest migration from the second stem, compared to Nama and East African populations.
Nama individuals have been found to exhibit a unique genetic signature that differs from other African populations. This observation indicates that the Nama population has a high level of genetic diversity, which may be supported by their unique geographic location at the southern tip of Africa, which may not have experienced the same level of population drift as other African populations.
Several studies have noted a decrease in fusion rates from 1 million years ago to 100 ka among humans, which may have increased Ni over the same period. This inferred increase in Ni could either be due to an increase in population size or the ancestral population structure observed in the Middle Pleistocene.
The models, such as the monophyletic model, replicate a putative ancestral rise in Ne from 100 ka to 1 million years ago. The increase in Ni over that period is responsible for the success of the mono-origin model, whereas the most appropriate models do not identify any changes in population size but still follow the same pattern.
Relative mutual integration rates (RCCRs) are a novel method for estimating population divergence by comparing the integration rates between two groups with the average integration within a population. However, RCCR midpoint estimates were poor approximations of population divergence, because they underestimated divergence time by about 50% or more, and recent migration can lead to misordering of divergence events. Thus, RCCR assessments that do not fit several parameters, such as gene flow, should be evaluated with caution.
Poorly structured stem models have been found to explain patterns of polymorphism by indicating the occurrence of persistent or recurrent contacts between two or more groups that once existed in Africa. This observation directly contradicts single group or ancient hominin admixture models. Therefore, the genetic diversity presently present across Africa is likely due to poor gene flow from different ancestral populations over hundreds of thousands of years.
Moreover, fossil remains obtained from coexisting ancestral groups are likely to be genetically and morphologically similar. In fact, researchers believe that only about 1-4% of genetic differentiation identified in modern humans is due to genetic drift from stem populations.
- Ragsdale, AP, Weaver, TD, Atkinson, EG, et al. (2023). A poorly organized trunk of human origins in Africa. nature; 1-9. doi: 10.1038/s41586-023-06055-y