Jan Bartek - AncientPages.com - According to a new genetic study, something strange happened to our ancestors about 900,000 years ago.
Suddenly, the ancestral population of humans was reduced to very low numbers. As few as 1300 breeding individuals were left. The populations remained that low for the next 100,000 years or more, researchers argue in the study. From the modern perspective, it was as if, suddenly, everyone was gone. Obviously, humans were on the verge of extinction. How could it happen, and how reliable is the study?
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The variation in the genomes of living people was analyzed by researchers from the Chinese Academy of Sciences (CAS), Yi-Hsuan Pan Haipeng Li, and their colleagues, and the study was published in the journal Science.
The results of the study are without doubt as intriguing as surprising, and there are already scientists who question the genetic analysis.
Population bottlenecks occur when an existing population is reduced in size. It can be due to a natural catastrophe, war, pandemic outbreak, or something similar. When one population leaves to find another, there is a sudden loss of genetic diversity.
There is previous evidence of population bottlenecks that took place when humans evolved and moved from one part of the world to another, but such a drastic reduction in size is unusual.
Populations living 1 million years ago are of special interest as many branches of our closest relatives emerged at that time, one of which gave rise to our lineage.
As the journal Science explained, scientists "developed a new statistical approach in this new study. To keep computing costs down and reduce errors that come with winding the clock so far back in time, their model only uses a subset of genes—such as those not subjected to forces like positive selection that would change the mutation rate—to estimate population sizes at different points in time.
Using this method, they tabulated when genetic changes had appeared in the previously sequenced genomes of 3154 individuals from 10 modern African populations and 40 modern non-African populations. Population size and history affect the accumulation of these changes, and scientists can analyze them to figure how many people lived at different points in time. Crunching the timelines, Pan and Li found a very steep decline—of roughly 99%—in the breeding population of our ancestors approximately 930,000 years ago.
The number of reproducing couples plummeted from at least 100,000 to 1280, they report. (Total population, including children and the elderly, would have been higher.) The low population numbers persisted until about 813,000 years ago, when the number of people began to rise again, the researchers report.
It’s not clear what drove our species to the brink of extinction, but Pan and Li suggest long periods of glaciation, cooling sea surface temperatures, or droughts may have played major parts.
According to Joshua Akey, an evolutionary geneticist at Princeton University who was not involved with the work, the new study "fills in a few more pieces of the human evolutionary history puzzle.” “There may be some fine-tuning of the exact time frame and severity,” Akey says, but “the evidence of a severe bottleneck seems pretty compelling.”
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However, it is vital to keep in mind many scientists are skeptical, mainly due to the lack of evidence backing up these low numbers.
Janet Kelso, a computational biologist at the Max Planck Institute for Evolutionary Anthropology, points out that a strong genetic bottleneck signal can be found in present-day African populations but not outside populations. Kelso says the results are intriguing but "should probably be taken with some caution and explored further.”
The study was published in the journal Science
Written by Jan Bartek - AncientPages.com Staff Writer
