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Scientists from the University of Berne have discovered that Homo sapiens, the early modern humans, emigrated from Africa, bringing harmful mutations that collected over the years.

The genomes of humans across four continents, unlike earlier studies focusing on just two populations, has been studied.

Even though modern humans emerged in Africa about 150,000 years ago, researchers feel that just 100,000 years later, they left the continent and went towards the east, travelling through Asia and settling in the Americas.

Hence, these migrations happened in small groups, with a few populations breaking off from the original family, but collecting harmful mutations. This mutation load is viewed to be a marker for assessing the distance they covered after leaving Africa.

With the help of next-generation sequencing (NGS) technology to sequence the genomes of individuals from seven populations in Africa, the scientists found that "simulating the spatial distribution of harmful mutations" made them realise that there was an increase in the number of harmful mutations in every person, with the increase in distance from Southern Africa.

Natural selection is found by the scientists to be weaker in smaller populations. Therefore, the harmful mutations were not excluded by the process.

"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, 2015 issue of the Proceedings of the National Academy of Sciences.