Inside each cell in your body is a nucleus that contains your DNA, your unique genetic material. But you also have another type of DNA in your cells. This DNA is found outside the nucleus, in your mitochondria, structures in the cell that provide energy. Largely unchanged from generation to generation, this genetic material gives researchers a way to “see back” through generations.
Unlike ordinary DNA, which you inherit from both parents, mitochondrial DNA comes only from your mother. Since it passes directly from mother to child, mitochondrial DNA doesn’t usually change from one generation to the next. Occasionally, however, a “typo,” or mutation, will appear in an individual’s mitochondrial DNA. If that individual is a woman who has children, the alteration will get passed on to her children, her children’s children, and so on.
It’s these mutations in mitochondrial DNA that allow scientists to track how human populations have migrated over time. How?
Imagine this: A group of people living in a certain region all share the same mitochondrial DNA. One day, a small band of people leaves the group and travels far away to a distant island. Eventually, a mutation appears in the mitochondrial DNA of one of the women on the island. The mutation spreads to future generations, and eventually everyone living on the island has it. Now the mutation is a marker that separates the two populations.
This example imagines people on separate islands, but islands aren’t required for genetic isolation. Back when traveling just meant walking or, perhaps, paddling, great distances created genetic islands, zones across which interbreeding was nonexistent.
Since mutations in mitochondrial DNA occur at rate that is roughly predictable, they can also be used to estimate the amount of time since two populations split apart. Using evidence from the mitochondrial DNA of individuals around the world today, researchers have postulated that we are all descended from a group of people who fanned out of Africa roughly 50,000 years ago. Traced back through the generations, this evidence points to a single woman—sometimes called “mitochondrial Eve”—whose mitochondrial DNA was the basis for all modern human mitochondrial DNA.