In Ancient Times, Humans May Have Gone Nearly Extinct

Scientists have observed a hole in the fossil record of early humans in Africa dating from about 900,000 to 600,000 years ago. That might be because, as a new study contends, 98.7% of human ancestors were wiped out. "The novel finding opens a new field in human evolution by raising many new questions," says Dr. Yi-Hsuan Pan, co-author of the study published Thursday in Science. Researchers analyzed the genomic sequences of 3,154 people from 10 African and 40 non-African populations to better understand how ancestral groups evolved, per the Guardian. In the African populations, there was evidence of a major evolutionary bottleneck that caused the population of human ancestors to fall from about 100,000 to just 1,280 breeding individuals between 930,000 and 813,000 years ago.

Essentially, humans became a species at risk of extinction, says senior author Giorgio Manzi, an anthropologist at Sapienza University in Rome. The bottleneck appears to coincide with climate change marked by long, intense cold periods and potential drought both in Africa and Eurasia. It is also "congruent with a substantial chronological gap in the available African and Eurasian fossil record," according to the study. However, there's some debate whether this event was global or localized. As "there are a number of sites that record evidence of human occupation" during the bottleneck, it's possible that "it only had a localized impact on part of the human population," says Chris Stringer, an expert in human evolution at the Natural History Museum who reviewed the study.

Though "only a weak signal of the bottleneck" was detected in non-African populations, per the Guardian, their ancestors' migration out of Africa much later would conceal the previous bottleneck, thought to have prompted a branching of new human species, potentially including the last common ancestor of modern humans and Denisovans and Neanderthals, with up to two-thirds of genetic diversity lost, per Nature. While ancestral hominins had 48 chromosomes, "probably around the time of the proposed bottleneck, two of these fused ... leaving modern humans with 46 chromosomes," explains Stringer. As Denisovans and Neanderthals also had 46 chromosomes, evidence of the bottleneck in those populations would indicate they split from our ancestors after 813,000 years ago. (More human evolution stories.)