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Humans inherit mitochondrial DNA only from the mother. So, in order to pass on mtDNA
from generation to generation, the mother must have an unbroken link of female descendants.
Unlike nuclear DNA which undergoes recombination, much of mtDNA is passed from mother to daughter
relatively unchanged. If we take a sample of mtDNA from a population and try to reconstruct
their matrilineal ancestry using the very slow mutation rate of mtDNA we just might
arrive at a single female common ancestor! A mother whom we could all relate to from
our mother's side. This is exactly what Cann, Stoneking and Wilson did in 1987 and found
out that, in fact, all humans alive today do have a common female ancestor who probably
lived about 200,000 years ago in Africa. They named her mitochondrial Eve.
Analogous to this, tracing Y-chromosomal DNA which is only inherited from father to son
points to our common father, Y-chromosomal Adam. However there's no reason to believe
that mitochondrial Eve and Y-chromosomal Adam lived at the same time. Likewise they were
not the only humans living at that time but their contemporaries failed to produce a direct
matrilineal or patrilineal descent. Also, mitochondrial Eve and Y-chromosomal Adam were
only the most recent common ancestors. Going further back in time, we can even find common
ancestors of species all the way back to the first single celled organisms. What's more,
our actual common ancestor following both mother's and father's side probably lived
as early as 5,000-10,000 years ago. All of this shows we are so closely related than
we could ever imagine.