First Ice Age May Have Led to Epic Hominin Migration 900,000 Years Ago

About one million years ago there was a mass exodus of hominin species out of Africa and into Eurasia. Human ancestors fled their home continent in droves, raising questions about why this would have occurred. According to a new study just published, this huge hominin migration was likely a response to the first major glaciation of the Pleistocene epoch and an associated population collapse, which the evidence shows coincided perfectly with the exodus.

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Population Collapse, Mass Hominin Migration, and a Catastrophic Ice Age

In their article, published in the Proceedings of the National Academy of Sciences (PNAS), planetary scientists Giovanni Muttoni from the University of Milan and Dennis Kent from Columbia University explain how they obtained an accurate estimate of the time of the first Pleistocene ice age. With this information in hand, they were able to cross-reference it with genetic evidence about a hominin population bottleneck, or sudden population collapse, that also took place in the Pleistocene, which lasted from 2.58 million to 11,700 years ago.

Previous research had suggested that both the mass migration out of Africa and the huge die-off of hominins occurred sometime between approximately 1.1 million and 900,000 years ago. It has long been suspected that the event responsible for the population bottleneck was what triggered the migration, as the survivors of the catastrophe sought refuge in other parts of the world. What the two scientists hoped to do in this case was correlate the three related events more precisely, to give scientists a much better understanding of these history-changing developments.

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A record of Earth's temperature cycles over millions of years is preserved in this loess-paleosol sequence in Kostolac, Serbia, including a cold snap suspected of causing our genetic bottleneck. (Giovanni Muttoni/PNAS)

This was essential, because there have been disagreements among geneticists and paleontologists about the exact timing of the population collapse, with different studies reaching different conclusions.

One recent research project produced evidence to suggest that the population crash took place 1.15 million years ago, which would have likely made it remote enough in time to not be connected to the first Pleistocene ice age.

However, a recent genetic study published in the journal Science in September of last year reported something different, namely that the population collapse among hominins had occurred approximately 930,000 years ago, after which there were only 1,300 humans still alive on Earth. The authors of this study provided evidence to suggest that hominin populations remained perilously low for the next 117,000 years, only escaping the endangered species list after that.

Deployment of piston corer used to recover 16 meter-long, deep-sea sediment cores. (Dennis Kent/PNAS)

New Method Confirms Ice Age Date

To begin their analysis, Muttoni and Kent studied changes in oxygen isotopes observed in rock sediment layers from prehistoric times. Using this methodology, they were able to confirm that the first Pleistocene ice age started about 900,000 years ago. This was obviously in line with the findings of one of the aforementioned studies but not the other, leaving the researchers with a conflict to resolve.

After looking more closely at the study that placed the population collapse in 1.15 million BC, they concluded that its results were not reliable. They concluded that population numbers at that time might very well have been higher, but included people who were not being detected or counted. The further back in time one travels the more difficult it is to find archaeological sites that could confirm the presence of hominin settlements or encampments, and they may be why it seemed like there was a population crash more than one million years ago when there really wasn’t.

For Muttoni and Kent, the key discovery was that the first Pleistocene ice age overlapped so completely with the hominin population bottleneck and the time of the first great migration of human ancestors out of Africa. Archaeological work has found evidence of hominin presence in the lands of Eurasia 900,000 years ago, so that is further evidence that hominins were on the move at that time.

In fact, the evidence shows that hominins were spreading out far and wide across Eurasia in 900,000 BC. This is consistent with a mass exodus from Africa, which would have been precipitated by a dramatic drying out of the climate on that continent that made it increasingly inhospitable to life. The same ice age conditions that led to reduced rainfall in Africa also led to much lower sea levels, and Muttoni and Kent note that this would have made it easier for the survivors of sudden climate change to migrate to far-off lands.

From Neanderthals to Denisovans to Homo Sapiens, Eurasian History at a Glance

Among those who left Africa 900,000 years ago were the ancestors of the Neanderthals and Denisovans, who were able to establish a long-term presence in Eurasia. Muttoni and Kent point out that many African animals were known to have migrated to Eurasia during this time frame as well, as would have been expected if drought conditions were severe in their homelands.

Collecting loess samples in Krakow-Zwierzniec, Poland, the researcher stands at a site indicating early H. sapiens occupation. (Giovanni Muttoni/PNAS)

It should be noted that the movements of direct human ancestors like  Homo erectus into Eurasia 900,000 years ago did not have an impact on the subsequent development of  Homo sapiens. While the ancestors of our Neanderthal and Denisovan cousins gained a secure foothold in their new Eurasian lands, these human ancestors lost the competition for resources and died out, leaving no fingerprints behind in the genetic inheritance of modern humans.

But this situation was reversed when  Homo sapiens (modern humans) evolved in and later  migrated out of Africa to Eurasia, starting approximately 80,000 to 100,000 years ago. In this case, it was the humans who prospered in their new homelands while the Neanderthals and Denisovans struggled to compete, and it was the latter species that were destined to disappear forever into the mists of prehistoric time.

Top image: An artist's impression of ice age Earth at the Pleistocene era. Source: Ittiz/ CC BY-SA 3.0

By Nathan Falde