Early Human Migrations from Africa May Have Caused the Accumulation of Harmful Mutations

It turns out that harmful mutations may have accumulated during early migrations out of Africa. Researchers have found that if humans migrated in small bands, then these groups progressively accumulated slightly harmful mutations.

About 150,000 years ago, the first humans appeared in Africa. About 100,000 years later, a few of them traveled to Asia and then further east, crossing the Bering Strait and colonizing the Americas. It's these groups that are likely to have accumulated mutations over time.

In order to test this hypothesis, the researchers used next-generation sequencing (NGS) technology in order to sequence the complete set of coding variants from the genomes of individuals from seven populations within and outside Africa, including the Democratic Republic of Congo, Namibia, Algeria, Pakistan, Cambodia, Siberia and Mexico. Then, the researchers simulated the spatial distribution of harmful mutations according to their theory.

So what did the scientists find? It turns out that the number of slightly deleterious mutations per individual increases with the distance from Southern Africa. This is consistent with an expansion of humans from that region.

The main reason for a higher load of harmful mutations in populations established further away from Africa is that natural selection is not very powerful in small populations. This means deleterious mutations are purged less efficiently in small groups than in large ones. In addition, selection had less time to act in populations that had broken away from their African homeland and settled far later.

"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," said Stephan Peischl, one of the researchers, in a news 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."

The findings are published in the journal Proceedings of the National Academy of Sciences.

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