Lynn Jorde and Henry Harpending are scientists specialising in human genetics. Since the early 1990s they have been studying mitochondrial DNA using the information to investigate mankind's past. Most of our genetic information is stored in the nuclei of our cells, but a small, separate quantity exists in another component, the part which produces the cells' energy, the mitochondria.

Prof Lynn Jorde (University of Utah): Mitochondria have their own genes. It's a small number of genes, a small amount of DNA, but it's distinct from the rest of the DNA in the cell and because of the way mitochondria are transmitted from one generation to the next, they're inherited only from the mother so they give us a record of the maternal lineage of a population.

Narrator: Mitochondrial DNA is inherited only by the mother. All mutations are passed on from mother to child, generation after generation at a regular rate. Over time, the number of these mutations accumulate in a population.

Lynn Jorde: Every event that takes place in our past, every major event, a population increase, a population decrease, or the exchange of people from one population to another changes the composition of the mitochondrial DNA in that population, so what happens is that we have a record of our past written in our mitochondrial genes.

Narrator: By knowing the rate of mutation of mitochondrial DNA and by a complex analysis of the distribution of these mutations, the geneticists can estimate the size of populations in the past. Several years ago they began seeing a strange pattern in their results.

Lynn Jorde: We expected that we would see a pattern consistent with a relatively constant population size. Instead, we saw something that departed dramatically from that expectation. We saw a pattern much more consistent with a dramatic reduction in population size at some point in our past.

Narrator: This confirmed what other geneticists have noticed. Given the length of time humans have existed, there should be a wide range of genetic variation, yet DNA from people throughout the world is surprisingly similar. What could have caused this? The answer is a dramatic reduction of the population some time in the past: a bottleneck.

Lynn Jorde: We imagine the population diagrammed like this. In the distant past we have a large population, then a bottleneck and then a subsequent enlargement of population size again, so we would have families of people in the distant past with a significant amount of genetic diversity, but when the bottleneck occurs, when there's a reduction in population size perhaps only a few of those families would survive the bottleneck.

We have a dramatic reduction in genetic diversity during this time when the population is very small and then after the bottleneck the people who we would see today would be descendants only of those who survived and they're going to be genetically much more similar to one another reducing the amount of genetic variation.

Narrator: Human DNA is so similar the scientists concluded the population reduction had been catastrophic.

Prof Henry Harpending (University of Utah): It seemed so incredible, you know the idea that all of us, now there's 6 billion people on Earth, and what the data were telling us was that we, our species was reduced to a few thousand. Suddenly it hit us, we had something to say about human history.

Lynn Jorde: Our population may have been in such a precarious position that only a few thousand of us may have been alive on the whole face of the Earth at one point in time, that we almost went extinct, that some event was so catastrophic as to nearly cause our species to cease to exist completely.

Narrator: It is an astonishing revelation, but the key was to find out when and why it happened. Because mitochondrial DNA mutates at an average rate these scientists believe, controversially, that they can narrow down the date of the bottleneck.

Lynn Jorde: Mutations in the mitochondria take place with clocklike regularly, so the number of mutations give us a clock essentially that we can use to approximately date the major event. In the case of a population bottleneck we think that this would have occurred roughly 70-80,000 years ago, give or take some number of thousands of years. So then the real question is: what could have caused such a reduction, an extreme reduction, in the human population down to as few as 5 or 10,000 individuals?

Maybe the Toba supervolcano 74,000 years ago?