For nearly eight decades, the Doomsday Clock has served as a vivid symbol, illustrating how near we are to extinction. And right now, with the clock at a mere 90 seconds to midnight, we are closer to calamity than we’ve been since the clock first ticked in 1947.
However, a recent Genetic study has revealed that if the Doomsday Clock had been around approximately 900,000 years ago, there would have only been one second left.
The study found that, during that era, the Population of our early ancestors shrank to just 1,280 viable breeding adults. In other words, humanity was on the brink of disappearing long before we became the modern humans that we recognize today.
The research, published in the journal Science, delved into the genetic history of 3,154 contemporary humans to reverse-engineer likely population trends. This analysis suggested that ancient humans– our direct predecessors– hit a genetic “bottleneck” between 813,000 and 930,000 years ago.
Though the exact cause remains unclear, it could have been harsh environmental factors that caused their numbers to plummet so drastically. According to the study’s calculations, an astonishing 98.7% of our ancestral population was nearly eradicated, putting our lineage on the brink of complete extinction.
The researchers suggest that this bottleneck could have resulted in more inbreeding, which in turn led to a lasting decrease in human genetic diversity.
They also speculate that this period may have set the stage for the emergence of a significant new hominin species. Interestingly, the timing aligns with other genetic data, suggesting this could be when a new type of hominin emerged—one that could be the last shared ancestor of the three large-brained species from the later Pleistocene era: Neanderthals, Denisovans, and modern humans.
Variations in population sizes, even those that occurred several hundred thousand years in the past, leave imprints that can still be detected in today’s human DNA. To explore these signatures, a group of scientists headed by Chinese geneticists came up with a novel tool named FitCoal.
They applied this tool to the genomic data of over 3,000 individuals from 10 African and 40 non-African populations. By using FitCoal, the team was able to backtrack through numerous genetic mutations and their likelihoods to estimate what population sizes would have been at different points in our evolutionary journey.
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“Our findings indicate that the severe bottleneck brought the ancestral human population close to extinction and completely reshaped present-day human genetic diversity,” the researchers wrote.
Aaron Ragsdale, a population geneticist at the University of Wisconsin-Madison who was not involved in the study, believes the situation might not have been so harrowing.
“I think it’s a bit of a stretch to conclude from these results that human ancestral populations were on the verge of extinction,” Ragsdale said.
He pointed out how “breeding” population sizes calculated by tools like FitCoal are generally smaller than the overall actual population sizes. Moreover, the notion that the population remained small for an extended period could be influenced by shorter intervals of dwindling numbers.
As for how this suggested population plunge was caused, the answers could not be identified within genetic data. However, scientists are aware that this era in time witnessed a significant fluctuation in our planet’s environment.
The mid-Pleistocene era marked a period of considerable climate upheaval, featuring a notable global cooling around 900,000 years ago. This led to expanding glaciers, colder oceans, prolonged droughts, and more intense monsoons. Both African and Eurasian wildlife experienced considerable transformations during this time.
While numerous studies have highlighted the role of environmental changes in propelling significant evolutionary shifts in humans, it seems that this particular era didn’t result in a widespread decline in the population of other hominin species that were not directly related to us.
Remarkably, the research indicates that our early ancestors hung on with dangerously low population numbers for an astonishingly long time—about 120,000 years. However, when the living conditions finally turned favorable again, whether due to positive shifts in climate or, as the researchers speculate, advancements like mastering fire, our ancestors made a quick recovery.
By roughly 813,000 years ago, the study shows that all ten African populations examined had experienced a twentyfold increase in size.
The researchers propose that this extended stretch of time, where our ancestors persisted in minuscule numbers, could have triggered the emergence of a completely new species. This species might be the final shared ancestor of not only modern humans but also our closely related Neanderthals and Denisovans.
Nick Ashton, a Paleolithic archaeologist from the British Museum who was not a part of the study, noted that this theory aligns well with some genetic data– which suggests that the common ancestor for these species could have existed relatively recently, between 500,000 and 700,000 years ago.
“But there are other possible interpretations,” Ashton pointed out.
For instance, studies of fossils, which focus on physical changes such as the shape of skulls and teeth, imply that Neanderthals and Denisovans had already branched off before any bottleneck event. In other words, avoiding its effects.
As for the bottleneck’s estimated timeframe, it’s too ancient for us to retrieve any DNA samples using today’s techniques. The most aged hominin DNA recovered to date is just 400,000 years old. Moreover, Africa’s climate doesn’t bode well for the preservation of ancient DNA.
Physical evidence like stone tools and bones is also sparse, although there are several known sites from this crucial epoch. Future scrutiny of skulls and bones unearthed in places ranging from eastern and southern Africa to China, Indonesia, and Spain could assist researchers in determining if the evolutionary shifts observed in our skeletal structure align with what the genetic models propose.
“The genetic theory needs to be thoroughly tested against the archaeological and human fossil evidence,” concluded Ashton.
“This is best achieved through more refined dating of current and new sites that potentially date to the proposed bottleneck.”
To read the study’s complete findings, visit the link here.
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