Humanity’s ancestors nearly went extinct between 930,000 years and 813,000 years ago, when their numbers dwindled to about 1,280 breeding individuals, according to a new paper.

The study blames climatic conditions during the Mid-Pleistocene Transition, when encroaching glaciers and drought killed off many of the early humans’ food sources.

Generations that lived through the stark 117,000-year period helped to rebuild hominid populations in Europe and Asia to a relatively stable level of about 27,160 breeders.

Genetic Bottleneck

For the study, researchers from China, Italy and the U.S. built a unique “fast infinitesimal time coalescent process (FitCoal)” model that used 3,154 modern-day genomes to peer deep into the genetic past. Prior to the population collapse, the model estimated a population of about 98,130 early humans that survived until about 930,000 years ago. By about that time, the Mid-Pleistocene Transition had taken effect and lowered global temperatures and thickened the glaciers.

The drastic bottleneck threatened our distant ancestors with extinction, the paper says.

“The gap in the African and Eurasian fossil records can be explained by this bottleneck in the early stone age,” said senior author Giorgio Manzi, an anthropologist at Sapienza University of Rome, in a statement. “It coincides with this proposed time period of significant loss of fossil evidence.”

Read More: Why Did Neanderthals Disappear?

The Fusion of Chromosome 2

The winnowing eradicated about 66 percent of early human genetic variation, the study estimates. But something else happened during those dark days: two of our ancestors’ chromosomes fused to form just one, what is now known as Chromosome 2 in modern-day humans. This may have created a new species that served as the last common ancestor of Homo sapiens, Neanderthals and Denisovans.

Scientists have long known that modern humans possess 46 chromosomes whereas the Great Apes and likely their early descendants, such as Homo erectus, had 48. The reduction happened through the fusing and creation of Chromosome 2, but scientists don’t know exactly when this happened. The current study estimates the time at somewhere between 930,000 years and 600,000 years ago.

Read More: Who Were the Denisovans?

How Did the Genetic Fusion Affect Early Humans?

After the fusion, Homo sapiens, Neanderthals and Denisovans all had 46 and could interbreed. Meanwhile, dropping to 46 likely made mating between the older Great Apes and Homo erectus impossible, which would have affected the evolution of the newer species.

“These findings are just the start. Future goals with this knowledge aim to paint a more complete picture of human evolution during this Early-to-Middle Pleistocene transition period,” said senior author Li Haipeng in a statement, a theoretical population geneticist and computational biologist at the Chinese Academy of Sciences.

Read More: Did Early Humans Interbreed? These Scientists Made a Map to Prove It