Advocates of that idea call the new ancestor Eve. The answer has to do with the basis of the theory. Certain of our cells carry building blocks called mitochondria. Both our mother and father contributed to our genetic makeup. But only our mother gave us our mitochondria.
Mitochondria are useful in tracking kinship.
Some years ago, Argentine grandmothers fought to get back children who'd been kidnapped by the dictatorship. They used a mitochondrial DNA test to identify the children. Now ethno-biologists put mitochondrial data into computers. They can do their calculations many ways. But every path leads to one specific ancestor or another.