The populations that broke off from early out-of-Africa migrants may have progressively accumulated harmful genetic mutations, suggests a new study published this month in the Proceedings of the National Academy of Sciences.

Modern humans, originating in Africa, started migrating out of the homeland towards Asia and the Americas around 50,000 years ago. Theoretical models predict that the expansion out of Africa might have happened through small bands that started expanding into multiple continents.

Population genetics theory says that each population breaking off from these small bands carried a mutational load.

Scientists in this study say that not only did the migrations leave a mark on the genetic diversity of different populations, but they also gave way to classes of harmful alleles that have different patterns across said populations. The farther away from Africa (in other words, the greater the distance covered away from the homeland), the more harmful the mutations or genetic variants are.

To test their hypothesis, the team of scientists sequenced the full genomes and high-coverage exomes from seven geographically divergent human populations from Namibia, Congo, Algeria, Pakistan, Cambodia, Siberia, and Mexico.

The next-generation sequencing technology they used confirmed that the mutations under scrutiny evolved with the migrations, and revealed that the degree of the harm is directly proportional to the distance traveled away from Africa.

“To be able to see this, you need a huge amount of data in many populations from different continents. Only 5 years ago, this would not have been possible,” says study co-author Laurent Excoffier, in comments to Science Daily.