A new study conducted by researchers at the University of Berne found that early modern humans, Homo sapiens, migrating out of Africa brought harmful mutations with them that accumulated over the years. The team reached this conclusion after the analysis of the genomes of humans across four continents, contrary to previous studies that only focused on two populations.
Although modern humans are believed to have emerged in Africa approximately 150,000 years ago, scientists believe that 100,000 years later, these same humans left the continent and headed east, making their way through Asia and then eventually settling in the Americas.
Using theoretical models, the team of researchers came to the conclusion that if these migrations took place in small groups and some populations broke off from the original family, these small populations would accumulate harmful mutations in a progressive manner. Furthermore, they believe that this accumulation, also called a mutation load, should be a marker for measuring the distance that the populations covered since leaving Africa.
The team used next-generation sequencing (NGS) technology in order to sequence the genomes of individuals from seven populations, both inside and outside of Africa. After simulating the spatial distribution of harmful mutations, as predicted by their theory, they found that the number of harmful mutations in each individual increases with their distance from Southern Africa, which confirms their predictions.
The main reason for this increase in mutation load is due to the fact that natural selection is weaker in smaller populations, meaning these deleterious mutations are not pushed out as hard by the process. Furthermore, the selection process had less time to act in the smaller populations that left their homeland and settled somewhere else at a later date.
"We find that mildly deleterious mutations have evolved as if they were neutral during the out-of-Africa expansion, which lasted probably for more than a thousand generations," Stephan Peischl, one of the study's main authors, said in a press release. "Contrastingly, very harmful mutations are found at similar frequencies in all individuals of the world, as if there was a maximum threshold any individual can stand."
"It's quite amazing that 50 thousand year-old migrations still leave a mark on current human genetic diversity, but to be able to see this you need a huge amount of data in many populations from different continents. Only five years ago, this would not have been possible," Laurent Excoffier, who participated in the research, concludes.
The findings were published in the Nov. 13 issue of the Proceedings of the National Academy of Sciences.
