[ad_1]
About 900,000 years ago, humans almost became extinct.
According to the results of a genomic study published last year, the ancestors of modern humanity were reduced to a breeding population of just 1,300 individuals in a devastating bottleneck that brought us to the very brink of annihilation. Now, a new study has found that a mass migration of humans out of Africa occurred at the same time.
It is a discovery that confirms previous dating of population decline and suggests that both are linked to a common denominator; an event known as the Middle Pleistocene Transitionin which the Earth’s climate suffered a period of total upheaval, destroying many species.
The movement of early humans to and through Europe and Asia from Africa is difficult to reconstruct. The best evidence we have consists of a sparse record of bones and mostly stone artifacts, which can be difficult to date. However, the evidence suggests that it was not a one-time event, but multiple waves of early hominids and human ancestors who packed up their lives and embarked on long journeys to new environments.
Two recent studies have linked human migration to a demographic bottleneck, based on different types of analyses. A close read of the human genome found that a population bottleneck caused a loss of genetic diversity about 900,000 years ago. A second study, published a few weeks later, studied the first archaeological sites in Eurasia and dated the bottleneck until 1.1 million years ago.
This discrepancy makes it difficult to identify the climate event that may have caused or at least contributed to the temporary drop in numbers, so geologists Giovanni Muttoni of the University of Milan and Dennis Kent of Columbia University embarked on a effort to limit the bottleneck moment.
First, the researchers reassessed records from early hominin settlement sites in Eurasia and found a group of sites that reliably date back to 900,000 years ago. In comparison, dating at older sites used as evidence of a population bottleneck was more ambiguous and therefore debatable.
They compared their findings to records of marine sediments, which preserve evidence of changes in climate in the form of oxygen isotopes. The proportions of oxygen trapped in sediment layers indicate whether the climate was warmer or colder at the time the minerals were deposited.
The genomic data and dating of hominin sites together suggest that the bottleneck and migration were simultaneous. During the Middle Pleistocene transition, Global ocean levels fell, and Africa and Asia dried out, with large areas of aridity. Hominids living in Africa would have faced horrible conditions, depriving them of food and water. Fortunately, with the fall in sea level, land routes to Eurasia became available and could be circulated, according to the researchers’ model.
This does not mean, they carefully point out, that hominids had not migrated before. Rather, the demographic bottleneck in the ancestor of modern Homo sapiens and their migration occurred at the same time as a consequence of the climatic upheaval that was occurring about 900,000 years ago.
“We suggest that the greater aridity during marine isotope stage 22 which caused the expansion of the savanna and arid zones in much of continental Africa gave early impetus Homo African populations adapt or migrate to avoid extinction,” they write on their paper.
“Rapid migration in response to a severe climatic trigger and the concomitant means of escape is what may explain the… migration out of Africa 0.9 million years ago and contribute to modern genomic evidence in modern African populations of the bottleneck”.
The findings have been published in the proceedings of the National Academy of Sciences.