Different human populations have different histories of inbreeding and outbreeding, HBD Chick explains:
For instance, the Arabs have been regularly and frequently marrying their first-cousins since well before Muhammad’s time, probably since the time of Christ or even before. Arabs, with their tribalistic societies, exhibit some of the greatest amounts of familial altruism of any human population on the planet. Society operates almost exclusively around the extended-family, the clan and the tribe; nepotism and corruption are the norm; and liberal democracy, which is based on individual freedoms and rights, is difficult if not impossible to implement in these societies.
The Arab form of cousin marriage, what is known as father’s brother’s daughter marriage, spread to the populations of the Maghreb, the Mashriq and parts of South Asia during the Middle Ages, and today these other societies behave tribally just as the Arabs do. Father’s brother’s daughter marriage is almost exclusive to this part of the world. It is the most incestuous of the cousin marriage forms since both mother and father come from the same (paternal) lineage.
The most common form of cousin marriage in the world is mother’s brother’s daughter marriage and it has a very long history in China going back to at least the third century B.C. This form of cousin marriage involves less inbreeding than the Arab type since parents come from different lineages, but it is still a form of inbreeding. That the relatedness of family members in Chinese populations is not as close as in the Arab world is reflected in the shape of Chinese society versus Arab society: the extended family and the clan is important, but society is not fractured along tribal lines. Nepotism and corruption are still rampant, however, and again liberal democracy is difficult to implement. The influence of familial altruism is still too strong in Chinese society.
Due to an historical accident, namely the introduction of Christianity, the one area of the world in which human populations have been outbreeding for a significant amount of time is Europe, more specifically Western Europe, and even more specifically Northwestern Europe. Starting as early as the fourth century A.D., the Roman Catholic Church banned cousin marriage in Europe (and civil codes often backed up these bans). Which cousins you could or could not marry according to the Catholic Church, and later the Eastern Orthodox and Protestant churches, has varied over the centuries; but from the 1200s through the 1800s, marriage up to third cousins was forbidden in the Catholic Church (although dispensations have been available to different degrees at varying times).
In other words, for a good 800 to 1600 years, Europeans have not been inbreeding. The conditions which, as described above, can promote the spread of familial altruism genes in a population were removed from European populations. Not surprisingly, European societies today are not tribalistic and very few are clan-based or even centered around the extended family. European societies, especially Northwestern European societies, are founded upon the individual and the nuclear family. Nepotism and corruption are much less frequent. It was here that liberal democracy, based on the rights and obligations of individuals in reciprocally altruistic relationships to one another, was born.
Wouldn’t the closest cousins be mother’s sister’s child? Mitochrondria would be the same.