The near extinction of humanity 900,000 years ago preceded the Great Migration

Even before Homo sapiens evolved, our ancestors were on the brink of extinction. Evidence from various sources supports this theory, but gives conflicting estimates of the timing: one study claims it occurred 1.15 million years ago, while another placed it 200,000 years later. Reconsidering the data supports the later figure – and could reveal one of the most important events in the human family tree.

The genomes of species may contain evidence of times when organisms went through extreme bottlenecks and fell to a small fraction of their previous population. The legacy of inbreeding left by these species may increase the risk of extinction for many generations, but some eventually recover.

In the case of humanity, the bottleneck probably occurred with an ancestor homo erectus, long before we existed as a species, but the legacy can still be found today. However, when it comes to determining the timing of the event, geneticists and paleontologists disagree, as competing papers offer different dates. Solving the question is important because without knowing the timing, it is virtually impossible to determine the cause. New work claims to have resolved the contradiction and provided evidence for an unidentified human migration in the process.

excavation in the open pit of loess paleosol sequence in Kostolac, Serbia.

This loess-paleosol sequence in Kostolac, Serbia, preserves a record of Earth’s temperature cycles over millions of years, including a cold snap that is thought to be causing our genetic bottleneck.

Image credits: Giovanni Muttoni

The case for the 930,000-year-old bottleneck was made last year in a genetic study that found there were fewer than 1,300 hominins on the planet at the time. According to that study, this was not a short-term disaster. Instead, populations remained weakly low for 117,000 years – by modern standards, humans were on the endangered species list. Modern human genetic diversity is almost two-thirds lower than without the bottleneck.

Even as that paper was published, an accompanying commentary cast doubt on certain aspects of the findings. Archaeological evidence suggested that hominids were widespread at the time, the commentators argued, but that most did not contribute to modern genetics for whatever reason.

Even the authors of that paper recognized that genetics in a case like this does not have all the answers and needs archaeological support. Just a few weeks later, the same journal published independent evidence of a severe decline in the number of human-inhabited sites, but placed this from 1,154,000 to 1,123,000 years ago – a considerably shorter and earlier gap.

According to the second study, the disappearance of inhabited places was the result of a sharp increase in climate variability that drove our ancestors from Europe.

Authors Professor Giovanni Muttoni of the University of Milan and Professor Dennis Kent of Columbia University tried to resolve the disagreement. They have concluded that the first major Pleistocene ice age occurred about 900,000 years ago, based on shifts in oxygen isotopes.

This fits well with the genetic interpretation, but what about the archaeological gap? Muttoni and Kent re-evaluated the sites in Europe and the Middle East that would reveal an earlier population crash and concluded that the dating is not as reliable as previously claimed.

Loess section is sampled for integrated stratigraphy in Kraków-Zwierzniec, Poland, leveling the researcher with evidence of early occupation by H. sapiens.

Loess sampling in Krakow-Zwierzniec, Poland. The researcher is on par with evidence of early occupation by H. sapiens.

Image credits: Giovanni Muttoni

There is also evidence of the presence of hominins in East Asia up to 2.1 million years ago, but this is so scarce that Muttoni and Kent argue that it is not really possible to identify population shifts.

On the other hand, the pair argue that hominin habitats appeared across Eurasia about 900,000 years ago. They interpret this data as indicating that the very dry conditions in Africa around this time became so uncomfortable for our ancestors that most became extinct. Meanwhile, low sea levels made it easier for the survivors to migrate out of Africa and become the ancestors of Neanderthals and Denisovans.

Muttoni and Kent argue that many other African animals, such as elephants, made similar migrations at the same time.

The authors are unsure whether other members of the human family were actually present in Eurasia earlier. If they did, Muttoni and Kent argue, they may have been outcompeted by the newcomers or may have become extinct earlier for various reasons. Either way, they left no legacy in the human genome, not even the small contributions that Neanderthals and Denisovans made when the first H. sapiens made another journey from Africa 100,000 years ago.

The research was published in the Proceedings of the National Academy of Sciences.

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