The Pleistocene Epoch

Before delving into the details of the recent study, let’s discuss the era it pertains to: the Pleistocene Epoch, which lasted from about 1.9 million to roughly 10,000 years ago.

This period is characterized by a cooler climate and the formation of extensive ice sheets and glaciers that sculpted today’s landscapes. It was also marked by severe droughts and widespread food shortages, leading to significant evolutionary shifts among species. Many large mammals, such as mastodons and mammoths, became extinct, while other species adapted and thrived.

As ice sheets expanded, sea levels dropped, revealing land that had previously been submerged—this included the famous Bering land bridge linking modern-day Alaska to Russia. While it was once believed that Central America also served as a land bridge during this epoch, this notion is now debated.

The late Pleistocene Epoch is particularly intriguing as it is believed to be when our Homo sapien ancestors began to emerge and expand, coinciding with the decline of Neanderthals. These land connections likely facilitated human migration into new territories like the Americas and Australia.

However, there is a notable gap in the fossil record of human ancestors from approximately 800,000 to 900,000 years ago. Recent studies suggest that the scarcity of fossils from this period is linked to a significant bottleneck that nearly obliterated the Homo sapiens lineage, which did not appear until about 300,000 years ago.

The Study

A collaborative research effort involving experts from China, Italy, and the United States was recently published in the journal Science, unveiling a remarkable discovery that could clarify the gaps in the fossil records of Eurasia and Africa during the Late Pleistocene Epoch.

The researchers analyzed genomic data from 3,154 individuals across 50 different populations—10 from Africa and 40 from other regions. By examining the emergence of specific genetic traits within these groups, the team was able to trace back characteristics to their most likely ancestral populations and establish an evolutionary timeline.

This genetic data provides insights beyond mere appearance or ancestry; it offers a glimpse into population dynamics spanning hundreds of thousands of years. The research team’s lead Chinese geneticists developed the Fast Infinitesimal Time Coalescent (FitCoal) method to trace genetic mutations back in time, enabling them to estimate population sizes during various evolutionary phases.

Results

The findings from FitCoal yielded surprising results. The researchers reported:

“Results showed that human ancestors went through a severe population bottleneck with about 1280 breeding individuals between around 930,000 and 813,000 years ago. The bottleneck lasted for about 117,000 years and brought human ancestors close to extinction.”

According to Haipeng Li, a population geneticist at the University of Chinese Academy of Sciences in Beijing and co-author of the study, “About 98.7 percent of human ancestors were lost.” Furthermore, the bottleneck may have caused a significant loss—estimated at 65.85%—of modern genetic diversity. Giorgio Manzi, an anthropologist at Sapienza University of Rome and the study's senior author, elaborated that the fossil record gaps in Africa and Eurasia align chronologically with this bottleneck during the Early Stone Age.

Not only does this research explain the limited fossil evidence from that era, but it also reveals that this enigmatic human species persisted in dangerously low numbers for over 100,000 years amid significant environmental changes. When conditions eventually became more favorable for human life—possibly due to the discovery of fire or climate warming—this particular lineage managed to rebound, paving the way for future human evolution, including our own lineage, which saw a remarkable increase in population around 813,000 years ago.

But there's more to the story.

The researchers also proposed that this bottleneck may have played a role in a speciation event. It’s possible that two ancestral chromosomes converged to form what is known today as chromosome 2 in modern humans. If this hypothesis holds, the human species that survived the bottleneck may have been the last common ancestor of Neanderthals, Denisovans, and Homo sapiens.

Why Did It Happen?

With the discovery of the bottleneck, an essential question arises: What caused such a drastic decline in population numbers? While genetic data can't yet provide clear answers, scientists recognize the significant climate shifts that characterized this era. As previously mentioned, the Pleistocene was a time of cold temperatures, ice sheets, and widespread droughts, leading many to believe that climate change was the primary driver behind the drastic reduction in human populations.

A Shift in Perspective

While the implications of this discovery are profound, some experts question whether the population truly dwindled globally to such low numbers. Nick Ashton, a Paleolithic archaeologist at the British Museum who was not part of the study, notes that archaeological evidence from various sites in Eurasia and Africa corresponds to the proposed bottleneck period, suggesting that such a decline may have been localized rather than universal.

Given that there are approximately 8 billion individuals on Earth today, the sample size of around 3,100 individuals used in this study is relatively small. Replicating the research with a larger dataset could yield even more fascinating insights. Regardless, if the bottleneck indeed led to a divergence in lineage that resulted in our existence, the discovery is monumental. Who would have imagined we could trace our speciation event?

The first video, titled "This Killed All But 1,000 Humans 900,000 Years Ago," delves into the significant population decline that nearly wiped out our ancestors, offering a detailed look at the research findings.

The second video, "The Times Humans Almost Went Extinct!" explores various instances in human history when our species faced near-extinction, providing context to the recent findings.

This article was originally published in the author’s free newsletter, Curious Adventure. It was edited and republished on Medium with her consent. Thank you for reading; your interest is appreciated.