A novel genomic analysis technique has helped reveal the reasons for a ‘bottleneck’ in the growth of the human population that almost wiped out the chance for humankind as it exists today, scientists reported in a study published in Science.
The findings indicate that the early population of human ancestors went through a period in which approximately 1,280 breeding individuals were able to sustain a population for about 117,000 years, the researchers from China, Italy, and the U.S. said.
They were able to determine demographic characteristics using modern-day human genome sequences from 3,154 individuals and a new analytical method called fast infinitesimal time coalescent process (FitCoal).
FitCoal helped the researchers calculate a probable population size through history based on contemporary sequences. They reported as a result that early human ancestors experienced extreme loss of life and, therefore, loss of genetic diversity.
Giorgio Manzi, an anthropologist at Sapienza University of Rome and a coauthor of the paper, said that the gap in “the African and Eurasian fossil records” can be explained by this “bottleneck in the Early Stone Age”.
The reasons suggested for this downturn in the size of the human ancestral population include glaciation events, leading to changes in temperature and severe droughts, and the loss of other species, potentially those that were food sources for ancestral humans.
An estimated 65.85% of humans’ current genetic diversity may have been lost in this period, in the early to middle Pleistocene era (from two million to 11,000 years ago), and the prolonged period of a minimal number of breeding individuals could have threatened humankind as we know it today, the researchers said in their study.
However, the bottleneck also seemingly contributed to a speciation event in which two ancestral chromosomes may have converged to form chromosome 2 in modern humans, the researchers said.
“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 [climatic] changes, and whether natural selection during the bottleneck accelerated the evolution of the human brain,” said senior author Yi-Hsuan Pan, a researcher of evolutionary and functional genomics at East China Normal University, China.
Now that there is reason to believe that humans grappled with a struggle 930,000-813,000 years ago, researchers can continue digging to reveal how such a small population persisted in presumably dangerous conditions, the study said.
Humankind’s rebound is expected to have been due to the use of fire and the arrival of more favourable climates.