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Eve, Mitochondrial

Mitochondrial Eve is the name given to the idea that the mitochondrial DNA in all modern humans can be traced back to a single genetic lineage, carried by a woman who lived in Africa approximately 250,000 to 150,000 years ago. This idea has been misunderstood by people who incorrectly think that it means that all modern humans can be traced exclusively to one single human ancestor, like the biblical “Eve.” This is incorrect because “mitochondrial Eve” is not the ancestral person from whom we can trace our entire nuclear DNA, the DNA that carries the codes to make humans.

Mitochondrial DNA

Mitochondria are small, energy-producing organelles found in eukaryotic cells. They have their own DNA (called mitochondrial DNA or mtDNA),separate from nuclear DNA, and replicate by themselves. They are thought to have entered into a symbiotic relationship with protoeukaryotic cells earlier than 1.5 billion years ago.

MtDNA is much easier to study than nuclear DNA. There are hundreds of mitochondria in the cytoplasm of each cell, making it easy to extract DNA for study. Human nuclear DNA contains the instructions to make a human being and is 3 billion base pairs long; mtDNA, which primarily contains the information to make the mitochondria, is much shorter, only 16,000 base pairs long (about 0.05% as long). However, there are two major features of mtDNA that make it particularly useful for evolutionary analysis. First, mtDNA accumulates mutations at a faster (but fairly constant) rate than nuclear DNA; these mutations may more often be neutral, instead of advantageous or disadvantageous, so mutations persist instead of being deleted. Second, mtDNA is inherited solely from the mother, so it does not recombine like nuclear DNA.

This latter point requires some explanation. A person inherits nuclear DNA equally through the egg and sperm of the parents. When sperm fertilizes an egg, it contributes almost exclusively nuclear material; any mitochondria-containing cytoplasm from the sperm does not survive entry into the mother's egg. The two nuclei combine to create a person equally related to each parent. However, the parental mitochondria do not combine; the person inherits mtDNA exclusively from the mother.

Matrilines and “Mitochondrial Eve”

So, while males and females inherit nuclear DNA equally from each parent and pass it on equally, both males and females get their mtDNA only from their mother. Going back one generation, a person is equally related to all four grandparents through nuclear DNA, but only one grandparent through mtDNA (the maternal grandmother). This direct inheritance of the mtDNA lineage makes it possible to reconstruct phylogenetic (or evolutionary) trees using these lineages. This means that the mtDNA of all living people are copies(with some mutations) of the mtDNA from one woman who lived thousands of generations ago.

Sometimes, lineages can be lost. In each generation, some women will not leave mtDNA descendants, as they will have either no children or exclusively sons. This lineage loss is analogous to the family name loss experienced in some cultures where women take their husband's family name; if she has no children or exclusively daughters, the name does not continue. As mtDNA lineages slowly die out over time, there are fewer lineages; eventually, there will be only one lineage remaining: the originator of this lineage has been dubbed “mitochondrial Eve.” This name is misleading: “Mitochondrial Eve” may be the most recent common ancestor of our mtDNA, but in the grandparents example above, she is only one of our countless other nuclear DNA ancestors who lived alongside her.

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