No. 100: Jul-Aug 1995
At one point during the last 400,000 years, the human population worldwide was reduced to only about 10,000 breeding men and women -- the size of a very small town. What caused this population "bottleneck"? Did a population crash engulf the entire globe. If not, who was spared?
Such questions arise from a surprising observation: Human DNA is remarkably uniform everywhere humans are found. This hidden genetic uniformity is difficult to believe if one strolls through a cosmopolitan city like New York or Paris. Nevertheless, compared to the DNA of the great apes, whose mutation rates should be close to ours, human genes on the average show far fewer mutations. Human DNA from Tokyo and London is more alike than that from two lowland gorillas occupying the same forest in West Africa. Harvard anthropologist M. Ruvolo has commented: "It is a mystery that none of us can explain."
The clear implication is that humans recently squeezed through a population bottleneck, during which many accumulated mutations were wiped out. In a sense, the human race began anew during the last 400,000 years. Unfortunately, DNA analysis cannot say where the very grim reaper came from.
(Gibbons, Ann; "The Mystery of Humanity's Missing Mutations," Science, 267:35, 1995.)
Comment. The hand that wiped the slate clean, or nearly so, might have been a meteor impact, a pandemic, the Ice Ages, a flood, volcanism, etc. Whatever it was, it seems to have largely spared Africa. The chimps and gorillas there apparently did not pass through the bottleneck. Even more interesting is the observation that the DNA of Subsaharan Africans does shows more variability and therefore seems older than that from humans elsewhere on the planet. (See BMG9 in our catalog volume: Biological Anomalies: Humans III.) Or perhaps Subsaharan DNA only seems older because it was not forced through that bottleneck. There are implications here for the African Eve theory.