The Impact of Genetic Mixing on Ancestry
- Throughout evolution, human populations have mixed. This mixture of various genetic populations (genetic pools) is called admixture.
- Admixture results in mixed ancestry. If admixture is between different species, it can cause introgression of DNA. Introgression means the transfer of DNA from one species to another.
- In modern human populations, DNA introgression signatures from Neanderthals can be seen.
- Two new research papers have been published examining the role of admixture in modern humans. One paper examined this in the populations of the Americas while the other examined Africa.
- In the Americas, the amount of introgression from Neanderthals and Denisovans was proportional to the amount of Indigenous American or European ancestry in each examined population.
- In Africa, ancient introgression from archaic “ghost” populations of human-like species that are now extinct contributed approximately 4–6% of the ancestry of present-day Khoe-San, Mbuti, and western African populations.
Human history has been shaped by migration, isolation, and admixture. Admixture refers to gene flow between individuals from different populations, blending genetic lineages and increasing genetic diversity within populations. Ancient humans also interbred with other human species like Neanderthals and Denisovans, passing down fragments of their DNA through a process called introgression. Two recent studies explore admixture patterns in Africa and the Americas, shedding light on how this process has influenced modern human genomes 12.
Effect of genetic mixing in the American Continent on modern humans
In the first research paper, researchers examined the effect of admixture on modern humans in the ancestral populations of the Americas.
The first people to enter the American continent were Indigenous Americans who migrated from Siberia. Subsequently, the migration of Europeans and Africans due to European colonization and the Transatlantic slave trade resulted in admixture. In this paper, researchers showed that the more Indigenous American or European ancestry a population had, the greater the amount of introgression from Neanderthals and Denisovans. Both European and Indigenous American ancestry in these admixed genomes had approximately the same proportions of Neanderthal variants. However, Denisovan variants were found primarily in Indigenous American tracts.
The researchers then searched for archaic genes present at high frequency in admixed American populations in comparison to East Asian populations. By doing so, they were able to identify several genes as candidates for adaptive introgression. These genes were related to multiple pathways including immunity, metabolism, and brain development. The researchers propose that these genetic differences could have potential implications for the health of individuals in these admixed populations. However, as the researchers point out in their paper, additional research would be required to test this proposal.
Effect of genetic mixing in Africa on modern humans
In the second research paper, researchers examined the effect of admixture on modern humans in the ancestral populations of Africa.
The most striking result of this paper is the introgression of archaic “ghost” species into the African population. They are called “ghosts” as the species are untraceable. They contribute up to 6% of the ancestry of present-day Khoe-San, Mbuti, and Western African populations.
In recent times, various demographic events within the past 10,000 years have led to admixture among modern Africans. These events include gene flow among different click-speaking Khoe-San populations, the expansion of pastoralism from eastern to southern Africa, and the migrations of Bantu speakers across the continent.
In this paper as well, the researchers suggested that a better understanding of genetic architecture can help predict disease risk in a population. They went on to add that this could help inform clinical decision-making for individual patients. Again, further research is required to thoroughly test this suggestion. The researchers also suggested the study of ancient DNA could help to better understand the genetic architecture of modern African genomes.
In summary, these two studies have shown that admixture has played a significant role in shaping human evolution, both in Africa and in the Americas.
- Aaron Pfennig et al., Evolutionary Genetics and Admixture in African Populations. Genome Biology and Evolution. 15, (2023).
- Kelsey E Witt et al., The Impact of Modern Admixture on Archaic Human Ancestry in Human Populations. Genome Biology and Evolution. 15, (2023).