Early humans living about one million years ago were extremely close to extinction. Evidence from a novel genetic approach, one that probes ancient DNA regions, suggests that the population of early human species back then, including Homo erectus, H. ergaster and archaic H. sapiens, was 55,500 individuals, tops.
Lynn Jorde, a human geneticist at the University of Utah, and his colleagues came to this conclusion after scanning two completely sequenced modern human genomes for a type of mobile element called Alu sequences, which are short snippets of DNA that move between regions of the genome. They shift with such low frequency that their presence in a region suggests that it is quite ancient. Because older Alu-containing portions have had time to accumulate more mutations, the team could also estimate the age of a region.
The scientists then compared the sequences in these old regions with the overall diversity in the two genomes to come up with an ancient census figure of 55,500. (Population geneticists actually calculate the so-called effective population size, which is an indicator of genetic diversity and is generally much lower than absolute population numbers; in this case, the effective population of humanity 1.2 million years ago was 18,500, which Jorde used to estimate the total population number.) The findings appear online in the January 19 Proceedings of the National Academy of Sciences USA.
The small number is surprising because, according to the fossil record, members of our Homo genus were spreading across Africa, Asia and Europe, suggesting that the hominin numbers should be expanding, Jorde says. A major setback must have occurred back then, he thinks, as devastating as a purported supervolcano thought to have nearly annihilated humans 70,000 years ago. “We’ve gone through these cycles where we’ve had large population size but also where our population has been very, very small,” he observes.
A new hypothesis about recent human evolution suggests that we came very close to extinction because of a “volcanic winter” that occurred 71,000 years ago.
Some scientists estimate that there may have been as few as 15,000 humans alive at one time.
“This estimate does not preclude the presence of other populations of Homo sapiens sapiens (modern man) in Africa, although it suggests that they were probably isolated from each other genetically,” they say.
The really interesting thing about a population bottleneck is the effect it has on evolution. With a small population, mutations get passed through a very large percentage of the species’ members. Detrimental mutations could be devastating and lead to outright extinction. Beneficial mutations, however, could cause fairly fast shifts in the population. And if you imagine some kind of tribal arrangement in which a few dominant males were responsible for a lot of the procreation going on, this situation becomes even more pronounced. An entirely new species might be created within a few generations. Anthropologists have proposed that such bottlenecks were responsible for the rapid development of hominids.
A catastrophe induced bottleneck has another factor affecting evolution. It isn’t just a bottleneck, it’s a bottleneck under pressure. The kinds of dire circumstances that you can imagine would follow a supervolcanic eruption take “survival of the fittest” to a much higher level. Now, that beneficial mutation (say a larger brain that makes it easier to hunt sparse game and build crude shelters) still spreads through a large percentage of the species, but in addition, every genetic line without that mutation dies off (or moves away to somewhere they can hack it). The result: rapid speciation.