Approximately one million years ago, the Earth was populated by a few hundred thousand or so ancient human ancestors. But about 900,000 years ago there was a sudden and dramatic crash in this population, which caused the number of archaic human breeding couples to drop down to below 1,300.
Or at least, this is the latest claim from a team of Chinese and Italian genetic researchers, who’ve just published a comprehensive historical analysis of the evolution of the human genome and its relation to past population levels in the journal Science.
To detect the genetic traces of what was nearly an extinction-level population crash, the scientists analyzed genetic data collected from 3,154 individuals in 10 modern African populations and 40 non-African populations. These people all had their genomes sequenced completely, which made it possible for the researchers to assess the differences and correlate them with long-term population patterns.
Using this form of complex analysis, the researchers were able to detect a steep decline in the breeding population of our distant human ancestors of nearly 99 percent about 930,000 years ago. By measuring the lack of diversity of genetic material passed on from that time period, they were able to calculate that the number of reproducing couples among archaic humans dropped from around 100,000 to just 1,280.
Astonishingly, human population numbers remained extremely low for about 117,000 years after that, showing little signs of improvement. Only around 813,000 years ago did they suddenly begin to rise again.
The new study claims that the population of our human ancestors dropped to just 1,280 couples about 930,000 years ago. (Robert Kneschke / Adobe Stock)
Genetic Time Travel Reveals a Dangerously Depopulated Earth
This groundbreaking genetic study was led by a pair of researchers from the Chinese Academy of Sciences, Yi-Hsuan Pan and Haipeng Li. Going in, the scientists knew that human genetic diversity was much lower than that of other primate species, which suggested some kind of depopulation event or bottleneck had occurred that had dramatically reduced genetic diversity among our most immediate Homo sapiens ancestors.
With the help of powerful computers, Pan and Li were able to untangle the web of genetic diversity amongst our human ancestors that both united and divided their 3,154 human specimens, creating a detailed timeline for when various genetic traits emerged. When they arrived at the period between 900,000 and one million years before the present, they were able to zero in on a time when the evolution of fresh diversity slowed dramatically, even catastrophically.
This is when the population crash occurred, the genetic researchers asserted. Its scope and impact would have been truly astounding.
If the conclusions of this new study are correct, it raises two important questions. Number one, what caused the astonishing population decline? And number two, how did our human ancestors manage to avoid extinction with such a low breeding population?
As for the first question, Pan and Li are virtually certain it had something to do with calamitous climate change. They hypothesize that long periods of glaciation, low temperatures in the sea, and massive global-scale droughts could have put ancient humans in grave peril. Following this thesis, perhaps a tipping point was reached where survival became incredibly difficult.
And yet, whatever happened did not lead to the total extinction of archaic humans, who eventually evolved into Homo sapiens. “The novel finding opens a new field in human evolution because it evokes many questions, such as the places where these individuals lived, how they overcame the catastrophic climate changes, and whether natural selection during the bottleneck had accelerated the evolution of the human brain,” Pan Yi-Hsuan told China Daily.
Pan and his fellow researchers explained that they believe that an improving climate might have helped spur population increases around 813,000 years ago, ending the severe demographic crisis. Before that time, humans would have only been able to survive in a few isolated outposts, presumably fairly close to the equator, where the glaciers didn’t reach and where droughts and temperatures may not have been as extreme. If they survived elsewhere they would have needed to have shelter and the ability to start and control fires, to keep themselves warm on an unusually cold Earth.
The study argues that these human ancestors survived catastrophic climate change which led to a period of glaciation. (Agustin / Adobe Stock)
Skeptics Address Population Crash Amongst Human Ancestors
The results of this new study are being met with some skepticism. What is being contested is that the population crash or bottleneck was a global rather than a regional event.
Janet Kelso, a computational biologist at the Max Planck Institute for Evolutionary Anthropology, noted that the genetic signal suggesting an ancient population plunge amongst human ancestors is strong only in modern-day African populations, and not in those who live elsewhere.
While she admits a stunning population decline did occur 930,000 years ago, she doubts it was experienced everywhere. The conclusions of the Chinese researchers, she told an interviewer from Science, “should probably be taken with some caution and explored further.”
Other researchers ground their skepticism in the archaeological and anthropological record. “The hypothesis of a global crash does not fit in with the archaeological and human fossil evidence,” stated Nicholas Ashton, a Paleolithic archaeologist from the British Museum who was asked to assess the new study.
Fossil of Homo antecessor discovered around the time of the supposed near-extinction of human ancestors. Some scientists have argued that at the time of the bottleneck, not all human ancestor populations were affected in the same way. (Public domain)
Concurring with this opinion was Chris Stringer, a British anthropologist who researches human evolution. “During the period of the bottleneck, there are a number of sites that record evidence of human occupation,” he commented in a review of the new study published by the UK’s Natural History Museum. “This could mean that there wasn’t a bottleneck, but could equally mean it only had a localized impact on part of the human population.”
Responding to such objections, Li Haipeng was quick to emphasize that his team’s work is not finished and that they plan to do more research to prove their case. “These findings are just the start,” he explained. “We plan to paint a more complete picture of human evolution during this transition period to unravel the mystery of early human ancestry and evolution.”
It is likely the true nature of the event that caused the population of our human ancestors in at least some parts of the planet to decline precipitously and rapidly will never be known. If it was in fact global in nature, it would have to be classified as miraculous that our ancestors were able to avoid extinction when their population numbers were so low. And, of course, without that miracle modern humans would never have evolved.
