Anatomically modern humans arose in Africa around 200,000 years ago. Most scientists think that a small group left Africa between 50,000 to 100,000 years ago and colonised the rest of the planet.
But the question is, why did they wait so long and were there other migration waves?
A triad of studies by three international teams of scientists have now mapped the genetics of hundreds of genomes from people around the world and reached an answer.
They have mapped more human genetic variation than ever before and collectively, they conclude that all people alive today outside of Africa are the descendants of a single wave of migration that occurred less than 80,000 years ago.
One of the studies also found evidence for an earlier wave of migration more than 100,000 years ago, as a trace of ‘old’ DNA embedded within the genomes of Aboriginal people from Australia and the Philippines.
The discovery shows how deep and complex our shared history is and may even have included other, older, migrations for which no genetic evidence survives.
“The articles help to give us a qualified basis which is necessary to understand where we come from and to determine where we’re going,” says Professor Peter Kjærgaard, Director of the Danish Natural History Museum, Copenhagen. He was not involved in any of the new studies.
All three articles are published in the scientific journal Nature.
People’s long history written in DNA
The studies have collectively mapped 787 new genomes from more than 280 different populations.
They have covered as many different languages and cultural groups as possible, and included data from often overlooked aboriginal groups including Basques, Pygmies, Bedouins, Pima Indians, Sherpa, and Australian Aborigines.
Their results show that all non-African peoples today are descendants from early humans who left Africa in a single wave of migration.
Old African DNA embedded in chromosomes
But there are still nuances to the story.
“We find a small footprint--at least two per cent--of a previous migration in the genomes of people from Papua New Guinea,” says one of the co-authors Mait Metspalu, Director at the Estonian Biocentre, University of Tartu, Estonia.
This suggests that people from Papua New Guinea and the Negritos from the Philippines have an earlier split time than other non-African groups.
“If everyone alive today came out of Arica at the same time, then we should all have the same split-time,” says Metspalu.
The two per cent comes from an older population of humans who probably lived around 120,000 years ago.
Climate allowed people to migrate
In another new study, also published in Nature, a team of scientists investigated the timing of this migration. The scientists behind this study analysed climate data from around the world since the last ice age and produced models of how and when groups of people migrated, according to climate conditions, desertification, and primary productivity of the land.
They discovered pulses of migration at 100,000; 80,000; 55,000; and 37,000 years ago from humans moving in and out of Africa, through the Arabian Peninsula and the Levant.
These pulses coincide with 20,000-year-cycles in the Earth’s orbit, where the planet wobbles on its axis, causing changes in sunlight, monsoons, and desertification.
“It’s fascinating that astronomical effects have been such an important pace-maker of where and when Homo sapiens migrated,” says Axel Timmermann, co-author on one of the other new studies.
Small seemingly insignificant changes in the planet’s climate history could have had huge implications for the history of our species, says Kjærgaard.
“We might not even be here,” he says. “But we are here, and unlike other animals we have the tools to understand our own history and it’s consistency with other species, as well as our species’ deep dependence on the conditions for life that the world has given us.”
“This is where we are today in the science of human development. And this is where we stand today with the ability to make some crucial choices about our continued shared history in what we call the Anthropocene era,” says Kjærgaard.
Jakobsen, Rasmus Kragh. 2016. “Humans spread around the world in a single wave of migration”. Science Nordic. Posted: October 3, 2016. Available online: http://sciencenordic.com/humans-spread-around-world-single-wave-migration