Mitochondrial Eve - Female and Mitochondrial Ancestry

Female and Mitochondrial Ancestry

Without a DNA sample, it is not possible to reconstruct the complete genetic makeup (genome) of any individual who died very long ago. By analysing descendants' DNA, however, parts of ancestral genomes are estimated by scientists. Mitochondrial DNA (mtDNA) and Y-chromosome DNA are commonly used to trace ancestry in this manner. mtDNA is generally passed un-mixed from mothers to children of both sexes, along the maternal line, or matrilineally. Matrilineal descent goes back to our mothers, to their mothers, until all female lineages converge.

Branches are identified by one or more unique markers which give a mitochondrial "DNA signature" or "haplotype" (e.g. the CRS is a haplotype). Each marker is a DNA base-pair that has resulted from an SNP mutation. Scientists sort mitochondrial DNA results into more or less related groups, with more or less recent common ancestors. This leads to the construction of a DNA family tree where the branches are in biological terms clades, and the common ancestors such as Mitochondrial Eve sit at branching points in this tree. Major branches are said to define a haplogroup (e.g. CRS belongs to haplogroup H), and large branches containing several haplogroups are called "macro-haplogroups".

The mitochondrial clade which Mitochondrial Eve defines is the species Homo sapiens sapiens itself, or at least the current population or "chronospecies" as it exists today. In principle, earlier Eves can also be defined going beyond the species, for example one who is ancestral to both modern humanity and Neanderthals, or, further back, an "Eve" ancestral to all members of genus Homo and chimpanzees in genus Pan. According to current nomenclature, Mitochondrial Eve's haplogroup was within mitochondrial haplogroup L because this macro-haplogroup contains all surviving human mitochondrial lineages today, and she must predate the emergence of L0.

The variation of mitochondrial DNA between different people can be used to estimate the time back to a common ancestor, such as Mitochondrial Eve. This works because, along any particular line of descent, mitochondrial DNA accumulates mutations at the rate of approximately one every 3,500 years. A certain number of these new variants will survive into modern times and be identifiable as distinct lineages. At the same time some branches, including even very old ones, come to an end, when the last family in a distinct branch has no daughters.

Mitochondrial Eve is the most recent common matrilineal ancestor for all modern humans. Whenever one of the two most ancient branch lines dies out, the MRCA will move to a more recent female ancestor, always the most recent mother to have more than one daughter with living maternal line descendants alive today. The number of mutations that can be found distinguishing modern people is determined by two criteria: firstly and most obviously, the time back to her, but secondly and less obviously by the varying rates at which new branches have come into existence and old branches have become extinct. By looking at the number of mutations which have been accumulated in different branches of this family tree, and looking at which geographical regions have the widest range of least related branches, the region where Eve lived can be proposed.

The date when Mitochondrial Eve lived is estimated by determining the MRCA of a sample of mtDNA lineages. In 1980, Brown first proposed that modern humans possessed a mitochondrial common ancestor that may have lived as recently as 180,000 years ago. In 1987, Cann et al. suggested that mitochondrial Eve may have lived between 140-280 thousand years ago.