The Toba catastrophe theory suggests that a bottleneck of the human population occurred c. 70,000 years ago, reducing the total human population to c. 15,000 individuals when Toba erupted and triggered a major environmental change, including a volcanic winter. The theory is based on geological evidence for sudden climate change at that time and for coalescence of some genes (including mitochondrial DNA, Y-chromosome and some nuclear genes) as well as the relatively low level of genetic variation among present-day humans. For example, according to one hypothesis, human mitochondrial DNA (which is maternally inherited) and Y chromosome DNA (paternally inherited) coalesce at around 140,000 and 60,000 years ago, respectively. This suggests that the female line ancestry of all present-day humans traces back to a single female (Mitochondrial Eve) at around 140,000 years ago, and the male line to a single male (Y-chromosomal Adam) at 60,000 to 90,000 years ago.
However, such coalescence is genetically expected and does not necessarily indicate a population bottleneck because mitochondrial DNA and Y-chromosome DNA are only a small part of the human genome, and are atypical in that they are inherited exclusively through the mother or through the father, respectively. Most genes in the genome are inherited randomly from either father or mother, thus can be traced to either matrilineal or patrilineal ancestry. Other genes display coalescence points from 2 million to 60,000 years ago, thus casting doubt on the existence of recent and strong bottlenecks.
Other possible explanations for limited genetic variation among today’s humans include a transplanting model or “long bottleneck”, rather than a catastrophic environmental change. This would be consistent with suggestions that in sub-Saharan Africa human populations dropped to as low as 2,000 individuals for perhaps as long as 100,000 years, before numbers began to increase in the Late Stone Age.
Limitations of single locus studies include the large randomness of the fixation process, and studies that take this randomness into account have estimated the effective human population size at 11,000–12,000 individuals.
