Scientists have calculated that for a period lasting one million years and beginning 1.2 million years ago, at a time when our ancestors were spreading through Africa, Europe and Asia, there were probably between 18,500 to 26,000 individuals capable of breeding (and no more than 26,000). The actual population would have been about three times as large, or 55,500. This made them an endangered species with a smaller population than today’s species such as gorillas which number 25,000 breeding individuals and chimpanzees (21,000).
From the composition of just two human genomes, geneticists have computed the size of the humanfrom which everyone in the world is descended.
In biological terms, reports Nicholas Wade for The New York Times, it seems humans were not a very successful species, and the strategy of investing in larger brains than those of their fellow apes had not yet produced any big payoff. Human population numbers did not reach high levels until after the advent of agriculture.
In 2011, researchers proposed a number of explanations , such as events in which a significant proportion of the population is killed or prevented from reproducing. One such event was the Toba super-volcano in Indonesia that erupted around 70,000 years ago, triggering a nuclear winter. Only an estimated 15,000 humans are thought to have survived although recent research suggests that the eruption thought to have nearly driven humanity extinct may not have endangered the species after all,. Another explanation is that the numbers of humans and our ancestors were chronically low throughout the last two million years, sometimes with only 10,000 breeding individuals surviving.
The research was concerned with the entire genome rather than specific genetic lineages studied in the earlier research work. Using a new method of studying genetic markers of DNA in the genome has allowed geneticists to study the genetics not only modern humans, but also our early ancestors such as Homo erectus (thought the most likely to be our direct ancestors), H. ergaster and archaic H. sapiens. Remarkably, they found there was enough information in only two human DNA sequences to estimate the ancient population size.
Human geneticist Lynn B. Jorde and colleagues at the University of Utah studied parts of the genome containing mobile elements called Alu sequences, which are sections of DNA around 300 base-pairs long that randomly insert themselves into the genome. This is a rare occurrence, but once inserted, they tend to stay in place over generations, and act as markers, rather like fossils, for ancient parts of the genome..
They calculated there was more genetic diversity in our early ancestors than there is in modern humans. They also came to the conclusion that there had been a catastrophic event around one million years ago that was at least as devastating as the Toba volcanic eruption, and which had almost wiped out the species.
The human population a million years ago was represented by archaic species like Homo ergaster in Africa and Homo erectus in East Asia. The Utah team says its estimate of 18,500 implies “an unusually small population for a species spread across the entire Old World.”
But that estimate would apply to the worldwide population only if there were inbreeding between the humans on the different continents. If not, and if modern humans are descended from just one of these populations, like Homo ergaster in Africa, then the estimate would apply only to that.
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