Evolutionary scientists believe that all humans on the earth originated from a small group in Africa over 200,000 years ago. This group included "mitochondrial Eve." Researchers of human origins believe that the ancestry of humans can be traced by analyzing mutations of the DNA contained in the mitochondria of every cell. This mitochondrial is assumed to be transferred only from mother to offspring in the egg cell. The mitochondria in the sperm do not enter the egg, so they don't become a part of the offspring's cells.
Assuming that the mtDNA sequence of two females should be more similar the farther back in time you go, researchers calculated how long ago different people groups separated from each other. The African group had more differences from the other groups, so it is assumed that they have had more time to accumulate the mutations. The date was also calibrated by using the assumed divergence of chimps and humans to calculate the rate of mutation.
The mitochondrial Eve idea is only valid if humans receive mtDNA only from the mother and if the rate of mutation is constant and known. Since none of these assumptions are known, the dating method may be invalid. Since recent research indicates that there is mixing of paternal and maternal mtDNA, no conclusion about the rate or origin is reliable- mitochondrial Eve appears to be dead.
The idea that mutation rates are constant and can be used as a "molecular clock" has also been called into question. The dates arrived at by molecular analysis are much older than the dates given when paleontologists interpret the fossil evidence. Many studies have shown that there are different rates of mutation in different sections of the mtDNA. This makes the dating very speculative.
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