"In the field of human genetics, Mitochondrial Eve refers to the matrilineal most recent common ancestor (MRCA) of modern humans. In other words, she was the woman from whom all living
humans today descend, on their mother's side, and through the mothers
of those mothers and so on, back until all lines converge on one person.
Because all mitochondrial DNA (mtDNA) is generally passed from mother to offspring without recombination,
all mitochondrial DNA (mtDNA) in every living person is directly
descended from hers by definition. Mitochondrial Eve is the female
counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they lived thousands of years apart.
Each ancestor (of people now living) in the line back to the
matrilineal MRCA had female contemporaries such as sisters, female
cousins, etc. and some of these female contemporaries may have
descendants living now (with one or more males in their descendancy
line). But none of the female contemporaries of the "Mitochondrial Eve"
has descendants living now in an unbroken female line.
Mitochondrial Eve is estimated to have lived around 200,000 years ago,[2] most likely in East Africa,[3] when Homo sapiens sapiens (anatomically modern humans) were developing as a population distinct from other human sub-species.
Mitochondrial Eve lived later than Homo heidelbergensis and the emergence of Homo neanderthalensis, but earlier than the out of Africa migration.[4] The dating for 'Eve' was a blow to the multiregional hypothesis, and a boost to the hypothesis that modern humans originated relatively recently in Africa and spread from there, replacing more "archaic" human populations such as Neanderthals. As a result, the latter hypothesis became dominant."
(http://en.wikipedia.org/wiki/Mitochondrial_Eve)
Imagine - we all came from Mitochondrial Eve:
