"The use of mitochondrial mtDNA to investigate human history is not without drawbacks.
The rate of mtDNA mutation is not well known. A study by Parsons et al. (1997) found a rate 20 times higher than that calculated from other sources. In an article reviewing mtDNA research, Strauss (1999a) reports that mtDNA mutation rates differ in some groups of animals, and can even vary dramatically in single lineages. Although there are many agreements, some divergence dates for modern animals calculated from mtDNA do not match with what is known from the fossil record. There are suggestions from a few sources that paternal mtDNA can sometimes be inherited, which could affect analyses based on mtDNA.
In 1999 Awadalla et al. published a study suggesting that mtDNA could sometimes be inherited from fathers. If mtDNA is inherited only from mothers, the correlation between different mutations should not depend on how far apart on the genome they were. Instead, their measurements showed that mutations at distant sites on the mtDNA genome were less likely to be correlated than nearby mutations, suggesting that mtDNA from mothers and fathers could sometimes get mixed. However, there is no explanation so far as to how this recombination could be occurring, and the possibility that other phenomena could be causing this effect has not yet been disproved. If it occurs, mixing would mean that the dates from current mtDNA studies would be too old. If mixing is common enough, it could even mean that there was no mitochondrial Eve, because different parts of the mtDNA molecule would have different histories. (Awadalla et al. 1999, Strauss 1999b) Other studies, however, have contradicted these results and argued for strictly maternal mtDNA inheritance (Elson et al. 2001), and, according to Sykes (2001), the Awadalla paper and another paper which also suggested that mtDNA could be inherited paternally were based on incorrect data and were later retracted."