Before Homo sapiens ever lived, our ancestors were on the edge of dying out. Several genetic studies and archaeological discoveries show a significant drop in population – known as a bottleneck event – among our early ancestors. One estimate suggests that there may have been only 1,300 breeding individuals remaining around 900,000 years ago.
Recent research provides a more detailed timeline of this bottleneck, which has been linked to early human migration patterns.
An Ice Age almost wiped out our ancestors, but also presented a new opportunity
Scientists have known for a long time that examining specific genetic markers in genomes can give clues about past population bottlenecks. However, determining the exact timing of these events is difficult and not always precise. Some estimates suggest that our ancestors experienced a significant bottleneck 0.9 million or 1.15 million years ago, while others claimed it may have occurred as early as just 200,000 years ago.
The earlier claim that suggested a brush with extinction about one million years ago was made in 2023 by an international team of researchers, including Giorgio Manzi, an anthropologist at Sapienza University of Rome.
“The gap in the African and Eurasian fossil records can be explained by this bottleneck in the Early Stone Age as chronologically. It coincides with this proposed time period of significant loss of fossil evidence,” Manzi said at the time in a press release.
However, this research sparked controversy. Archaeologists were skeptical, pointing to evidence like tools and other artifacts that indicate early humans were still widespread in Africa around the time of the proposed bottleneck.
This difference prompted Giovanni Muttoni from the University of Milan and Dennis Kent from Columbia University to reexamine the data. They studied oxygen isotopes that record Earth’s temperature cycles. This analysis revealed that the first major ice age of the Pleistocene era occurred around 900,000 years ago, which aligns with the genetic bottleneck.
They also noted that evidence from previous studies suggested that early human habitation also began approximately 900,000 years ago across Eurasia, most likely due to climate change. As the ice age progressed, ocean levels would have dropped, providing an easier path out of Africa for early human ancestors, who might have been Homo erectus or a related species. They would have encountered increasingly difficult conditions at home, making the idea of migration appealing. Additionally, the researchers observed that many animals also started leaving Africa around the same time.
“This timing agrees with the independently dated bottleneck from genomic analysis of modern human populations and allows speculations about the relative roles of climate forcing on the survival of hominins,” the researchers wrote in their study.
However, not all early humans left Africa. The direct ancestors of Homo sapiens remained, and our species did not depart the continent until 60,000 to 90,000 years ago. During their migration, they would have encountered other human species that had branched out in the meantime, such as Neanderthals and Denisovans.
The findings were published in the Proceedings of the National Academy of Sciences.
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