New evidence for the so-called Out of Africa hypothesis of modern humans suggests that our ancestors migrated from the continent about 50,000 years ago.

By looking at the individuals' shared, maternally inherited DNA sequences, together with a knowledge of how this genetic material changes over time, the scientists have been able to trace our common ancestry.

As well as showing the evolutionary tree is firmly rooted in Africa, their study even suggests human numbers may have dwindled to just 40,000 at one stage.

Rival theories

There are two competing theories to explain how mankind spread across the globe.

One suggests that between 100,000 and 200,000 years ago modern man (Homo sapiens) emerged from Africa to slowly populate the rest of the world, replacing any species of human that were already there. This is the Out of Africa hypothesis.

The other theory suggests that modern humans arose simultaneously in Africa, Europe and Asia from one of our predecessors, Homo erectus, who left Africa about two million years ago.

In recent years, support for the Out of Africa theory has come from the study of DNA in mitochondria, the energy-generating structures that reside just outside a cell's nucleus. This mtDNA, as it is known, is inherited only from females. It also mutates - errors appear - at a steady rate, meaning it can be used as a "molecular clock" to investigate human history.

Deep branches

Critics argued such analysis was based on a small section, about 7%, of the mtDNA and this might cause problems in determining the genetic distance between individuals. With the data available it was not possible to trace the mtDNA lineage back to sub-Saharan Africa, they argued.

But the Swedish group have overcome some of the original shortcomings by carrying out an analysis of the complete mitochondrial genomes of their 53 subjects.

The new analysis suggests the three deepest branches on the new mtDNA family tree all go back to sub-Saharan Africa and there is another branch that contains both African and non-African mtDNA.

"This is the first study in which the genome is being used in a sufficiently large number of individuals to come up with very strong evidence, in this case supporting the Out of Africa theory," lead researcher Professor Ulf Gyllensten said.

Sports stadia

What seems particularly significant is that the amount of mtDNA diversity among Africans is more than twice as great as the diversity seen among non-Africans.

The data also show some evidence of a "population bottleneck" when the number of humans fell to a low level. It happened about 40,000 years ago when there could have been as few as 40,000 humans - a number less than many national sports stadia would hold.

The researchers may have also identified the stock of people from whom all non-Africans descended.

They write in the journal Nature: "A group of six African (mtDNA) sequences are genetically distant to those of other Africans, but share a common ancestor with non-Africans. These lineages represent descendants of a population that evidently gave rise to all the non-African lineages."

The researchers believe all humans alive today could share common ancestry with a being in Africa who lived about 120,000 to 220,000 years ago.