Recent genomic research has unveiled a startling revelation about our ancient human ancestors. Between 800,000 and 900,000 years ago, a severe evolutionary bottleneck brought them dangerously close to extinction, with only a tiny population of around 1,280 breeding individuals for a staggering 117,000 years. This finding has sparked intriguing discussions among scientists, shedding light on the potential causes and the resilience of early humans.
Genomic analysis, conducted on over 3,000 living individuals, led scientists to this astonishing conclusion. They hypothesize that an extreme climatic event may have triggered this bottleneck, pushing our ancestors to the brink of extinction. The numbers are reminiscent of species currently at risk of vanishing from our planet.
Professor Giorgio Manzi, an anthropologist from Sapienza University of Rome and one of the senior authors of this research, commented, “The numbers that emerge from our study correspond to those of species that are currently at risk of extinction.” However, the story doesn’t end with near-extinction; it takes an unexpected turn.
Despite the existential challenges they faced, this bottleneck may have played a pivotal role in the emergence of a new species known as Homo heidelbergensis. Some scientists believe this species is the common ancestor of modern humans and our close relatives, the Neanderthals and Denisovans. Homo sapiens, the species to which we belong, is estimated to have emerged around 300,000 years ago.
“It was lucky [that we survived], but … we know from evolutionary biology that the emergence of a new species can happen in small, isolated populations,” remarked Manzi.
Professor Chris Stringer, head of human origins at the Natural History Museum in London, who was not involved in the research, emphasized the fragility of such a small population: “It’s an extraordinary length of time. It’s remarkable that we did get through at all. For a population of that size, you just need one bad climate event, an epidemic, a volcanic eruption, and you’re gone.”
This population decline aligns with significant global climate changes, characterized by prolonged glaciations, declining sea surface temperatures, and potential droughts in Africa and Eurasia. Notably, this period also corresponds to a sparse fossil record, both in Africa and Eurasia.
Manzi explained, “We know that between about 900,000 and 600,000 years ago, the fossil record in Africa is very scarce, if not almost absent, while both before and after we have a greater number of fossil evidence.”
However, Stringer raised questions about a global “blank” in the fossil record, suggesting that the bottleneck might have been a more localized phenomenon. “Maybe this bottleneck population was stuck in some area of Africa surrounded by desert,” he speculated.
The study, published in the journal Science, analyzed genomic sequences from 3,154 individuals across 10 African and 40 non-African populations. This analysis provided valuable insights into the growth and decline of ancestral populations over time.
While evidence of the bottleneck was observed in all African populations, the signal was weaker in the 40 non-African populations. This weaker signal is likely due to the more recent population bottleneck experienced by non-African populations during the out-of-Africa migration.
This intriguing discovery aligns with the timing when our last shared ancestor with Neanderthals and Denisovans roamed the Earth. Scientists are now eager to explore whether genetic samples from these ancient cousins contain evidence of the same bottleneck, potentially unlocking new insights into the timing and causes of species divergence.
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