Mitochondrial DNA (or mtDNA) is the DNA we inherit from our Mothers, and is held in a unique circular strand in small tubular packets known as mitochondria that function in a similar way to batteries in the cytoplasm of cells. If you were to trace the matrilineal line all the way back to the most recent common ancestor you’d reach the DNA of Mitochondrial Eve. In other words, Mitochondrial Eve is the most recent woman from whom all living humans descend on their mother’s side.
Mitochondrial Eve is thought to have lived approximately 140,000-290,000 years ago, and despite the obvious similarities in name with Biblical Eve, that is where the similarities end. Mitochondrial Eve was not the only living female of her time, but merely the only of her contemporaries to produce a direct unbroken female line to any living woman in the present day. Therefore, as all mtDNA is generally passed from mother to child without recombination, all mtDNA in ever living person is directly descended from Mitochondrial Eve.
Similarly, if you were to trace paternal lines back to the most recent ancestor, you would reach a common ancestor known as Y-Chromosome Adam or Most Recent Common Ancestor, who is believed to have lived between 237,000 and 581,000 years ago. Once again however, the name is where the biblical similarities end. It is incredibly unlikely that mtEve and Y-Adam would have lived in the same time in history, let alone ever met. They are just the furthest back a direct ancestor has been traced for mtDNA and Y-DNA.
Despite Eve being our most common maternal ancestor, we do not share exact mtDNA with Mitochondrial Eve, with the changes in the DNA, known as mutations, making it possible to trace our ancestral line. The mutations in mtDNA are said to occur every thousand generations, and as a result mutations are rare within a family line. This means that within one generation 1 million mothers could have more than a thousand daughters, each with a different mutation.
There are several theories as to why it is Mitochondrial Eve’s DNA that is common with our ancestry, and not one of her contemporaries, but the one that appears most likely is that Eve lived in a time where an evolutionary bottleneck appeared. An evolutionary bottleneck is where world population suddenly drops dramatically in size, with very few survivors from a species. It is thought Eve was one of these survivors, and lived through a world event similar to the supereruption from Mount Toba, in Sumatra, Indonesia.
These mutations are then tracked as part of ancestral testing, such as in the DNAme test, and it is the discovery of these mutations that allow us to track deep lying ancestors, and plot a map of their likely route out of Africa.