Only 1,280 breeding humans once roamed Earth, gene study shows
Humankind struggled to survive during a 100,000 year period during the early Pleistocene, according to researchers who used a computer model to discover a severe population bottleneck in our species’ ancient past.
The bottleneck occurred between 813,000 years ago and 930,000 years ago, and reduced an ancestral human species to less than 1,300 breeding individuals. The issue persisted for 117,000 years, and aligns with a chronological gap in the African and Eurasian human fossil records in that period. The team’s research on the bottleneck was published today in Science.
Population bottlenecks are events in which a species’ total population is severely reduced, which causes an overall reduction in genetic diversity across the species. The loss of genetic diversity can cause populations to become less healthy. Bioengineers can now synthesize genetic diversity in animal populations through cloning and gene editing.
But it’s not always the case that population bottlenecks threaten populations—look at the flightless, sexually inept kākāpō of New Zealand or the critically endangered vaquita porpoise, whose main threats are human-introduced threats and humankind itself, rather than small genetic pools. Now it appears that an ancestral human species may have been threatened by a similar culling of the population.
The recent team of researchers developed a tool called the fast infinitesimal time coalescent process (FitCoal) to analyze 3,154 present-day genomes from 10 African and 40 non-African populations. The researchers found evidence of a “severe population bottleneck” in each of the 10 African populations that “brought the ancestral human population close to extinction,” as the scientists wrote in their paper. The team posits that the bottleneck may have been due to climatic changes.
Nick Ashton, an archaeologist at the British Museum, and Chris Stringer, a paleoanthropologist at London’s Natural History Museum, commented on the study in a Perspectives article published alongside the new research.
“Whatever caused the proposed bottleneck may have been limited in its effects on human populations outside the H. sapiens lineage, or its effects were short-lived,” Ashton and Stringer wrote. “This also implies that the cause of the bottleneck was unlikely to have been a major environmental event, such as severe global cooling, because this should have had a wide-ranging impact.”
“Nevertheless,” Ashton and Stringer added, “the provocative study of Hu et al. brings the vulnerability of early human populations into focus, with the implication that our evolutionary lineage was nearly eradicated.”
Homo sapiens (our species) doesn’t appear in the fossil record until about 300,000 years ago, meaning that the modeled population bottleneck would have affected our ancestors. “The researchers point out that fossils of Homo heidelbergensis are among the few from Africa that date back to the bottleneck period, which spanned from 950,000 to 650,000 years ago. The team goes as far as to suggest that the bottleneck “possibly marks a speciation event leading to the emergence of the [last common ancestor] shared by Denisovans, Neanderthals, and modern humans.”
Stringer and Ashton note that some studies suggest the last common ancestor was earlier, but in any case, if the bottleneck occurred with the severity modeled by the team, it could have had significant effects on the speciation of hominins.
Genetic modeling is becoming an increasingly useful tool in understanding how ancient human populations dispersed across the globe and mixed with other populations, including other hominin species.
Population bottlenecks in the more recent past have offered hints at how climatic changes impacted local communities, for example. Studying ancient DNA alongside the DNA of modern groups may clarify the proliferation of humankind across the world.
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