The population of our ancestors may have plummeted to as low as 1300 around 900,000 years ago, possibly as a result of our ancestral species splitting from other early humans.
That is the conclusion of an analysis of the variation in the genomes of living people by at the Shanghai Institute of Nutrition and Health and his colleagues. However, while not dismissing the idea outright, independent experts say it isn鈥檛 supported by other lines of evidence.
Population bottlenecks occur when an existing population is reduced in size, for instance as a result of a catastrophe聽or when a small number of individuals leave one population to found a new one. This results in a sudden loss of genetic diversity.
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There have been numerous bottlenecks of varying scales as humans evolved and moved around the world. For instance, there was a major bottleneck when a small number of modern humans left Africa around 60,000 years ago, which is why there is still much more genetic diversity among people of African descent than in everyone living in the rest of the world combined. Much more recently, there was a series of bottlenecks as Polynesians settled island after island in the Pacific.
Past bottlenecks can be uncovered by looking for the reductions in genetic diversity they cause, but more ancient bottlenecks are harder to detect than recent ones. Li鈥檚 team developed a new method for estimating past changes in population size and applied it to the genomes of more than 3000 people from around the world.
According to the researchers鈥� findings, the population of our ancestors fell by 98 per cent to around 1280 鈥渂reeding individuals鈥� around 930,000 years ago, and the population remained very low until around 815,000 years ago.
The early humans alive at this time have been assigned to a number of different species, including Homo heidelbergensis, Homo rhodesiensis, Homo antecessor and Homo bodoensis, and it is unclear which of these is our ancestor. There is also debate about whether they were indeed separate species.
Li and his colleagues think this bottleneck was most likely due to climate change, with global cooling around this time leading to severe drought in Africa and Eurasia. This 鈥渃ould explain the extreme scarcity of the available hominin fossil record in Africa and Eurasia鈥� at the time, they write in their study.
Li says this is referring to previously published studies that have reported a gap at this time. 鈥淲e didn鈥檛 find the fossil gap in this study,鈥� he says. 鈥淥ur findings actually explain the fossil gap.鈥�
But in an , at the British Museum and at the Natural History Museum in London have put together a list of sites in Africa and Eurasia with evidence of continued human habitation during this time.
鈥淭he combined evidence, using several independent methods, seems strong,鈥� says Ashton. 鈥淎 global event seems unlikely as this would have affected populations in Eurasia as well as Africa.鈥�
鈥淭he data of human presence that we marshalled suggest that its effects must have been limited in time and space,鈥� says Stringer.
Li鈥檚 team also cites a paper by at the Australian National University as evidence of drought in Africa and Eurasia at the time of the bottleneck. 鈥淲e said nothing about aridity in Africa,鈥� says Pillans. 鈥淪o, in a way, the reference to our paper is not really correct.鈥�
at the University of Wisconsin, who wasn鈥檛 involved in the study, points to a paper from earlier this year suggesting that early humans in Africa were with only occasional migrations and mergers between them. It is possible that this population structure resulted in the appearance of a bottleneck, says Hawks.
Li鈥檚 team notes that the time of the bottleneck coincides with estimates for when two existing chromosomes fused to form chromosome 2. This is why humans have only 23 pairs of chromosomes rather than the 24 of chimpanzees and gorillas.
Another explanation for the bottleneck is that rather than there being a sudden reduction in population due to drastic climate change, it reflects a speciation event where a small number of individuals split away from other early humans after chromosome 2 evolved.
鈥淭he possible link to chromosome 2 is very interesting, and I think it may be true. But I would not assume that the bottleneck is real until we have a better understanding,鈥� says Hawks. 鈥淚t would be great to see more ancient DNA data that could get us back into this time period.鈥�
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