Approximately 900,000 years ago, humans came very close to disappearing from the face of the Earth. Our ancestors were reduced to just 1,300 individuals, stuck in a devastating bottleneck that brought the species to the brink of extinction.
A bottleneck is a demographic process that a group suffers when it experiences a drastic reduction in its population, either due to natural causes (earthquakes, fires…) or due to anthropogenic causes (excessive hunting, loss of habitat, etc.) .
Nearly a million years ago, at a time known as the Middle Pleistocene Transition, the Earth’s climate experienced a period of complete upheaval, wiping out many species during the first great ice age. Climate change caused “widespread aridity in Africa” and meant that humans had to adapt to new circumstances to survive.
The solution they found was to start an “important migratory pulse of fauna, including hominids, as a consequence of the opening of land routes from Africa after the great drop in sea level,” the researchers write in an article published in the journal Proceedings of the National Academy of Science (PNAS).
The movement of early humans to Europe and Asia has always been difficult to reconstruct. The best evidence there is consists of a scant record of bones and a series of stone artifacts. Research by Giovanni Muttoni of the University of Milan and Dennis V. Kent of Columbia indicates that there were multiple waves of early hominids and human ancestors who packed up their lives and made long journeys to new environments.
In 2023, an analysis of the human genome determined that the bottleneck of 900,000 years ago caused a loss of genetic diversity. Although another study, also from a year ago, on the first archaeological sites in Eurasia indicated that the mass depopulation of the human species occurred a little earlier, 1.1 million years ago.
Hence geologists Muttoni and Kent embarked on an effort to limit the time at which the bottleneck occurred. First, they reassessed records of early hominid settlement in Eurasia and found a group of sites that reliably date back to 900,000 years ago.
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, creating large arid areas.
Ancient humans living in Africa would have faced horrible conditions, being deprived of food and water. Fortunately, with the fall in sea level, land routes to Eurasia became available and could be circulated, according to the model now presented by the researchers.
This does not mean, however, that hominids had not migrated previously. Rather, the demographic bottleneck of the ancestor of modern Homo sapiens and its migration occurred at the same time as a result of the climatic upheaval that was occurring about 900,000 years ago.
“We suggest that this greater aridity caused the expansion of savannah and arid zones in much of continental Africa, which pushed the first populations of Homo on the African continent to adapt or migrate to avoid extinction,” they write in their article.