Researchers theorized that if the modern humans traveled out of Africa as small groups, then these populations should have accumulated marginally harmful mutations along the way. The team described this as a "mutation load" and theorized that the load can somehow measure the distance it has traveled since its departure from Africa.

Experts believe that Homo sapiens or modern humans originated in Africa approximately 150,000 years ago. After 100,000 years, some left Africa and traveled first to Asia. They then traveled further east, crossing the Bering Strait and then occupying the Americas. Theoretically speaking, a person from Mexico should more likely possess potentially harmful genetic variants, compared to a person from Asia or Africa.

Using a next-generation sequencing (NGS) technology, the team sequenced the broad set of coding variants from the DNA of representatives from seven populations outside and inside Africa, namely Mexico, Cambodia, Siberia, Algeria, Pakistan, Namibia, and the Democratic Republic of Congo.

Using this theory, they simulated the harmful mutations' spatial distribution. Their theory was correct: the number of a person's slightly harmful mutations increases with the distance from Southern Africa. The finding is consistent with the modern humans' expansion from Africa.

Natural selection is not as robust in small bands. Compared to larger tribes, harmful mutations in smaller populations are not eliminated as effectively. The findings were published in the journal Proceedings of the National Academy of Sciences.

Stephan Peischl, a Swiss Institute of Bioinformatics (SIB) member and one of the study's main authors, said the slightly harmful mutations progressed as though they were neutral during the periods of expansion. Peischl added that the mutations could have lasted over a thousand generations.

In contrast, similar frequencies of highly damaging mutations are found across worldwide populations. It seemed as if there is somewhat a maximum rate of harmful mutations an individual can tolerate.

Researcher Laurent Excoffier highlighted how amazing it is that marks of 50,000-year-old migrations are still distinguishable in the genetic makeup of the current population.

"Only 5 years ago, this would not have been possible," added Excoffier. However, in order to observe this mark, researchers need to analyze large amounts of data from various populations across the continents. 

Photo: Corie Bidgood | Flickr

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