The title might be a little misleading for it doesn’t show the first woman in our species, but the most recent mitochondrial ancestor of all humans and everyone today is a decedent from her. But what is mitochondrial DNA? It’s separate DNA outside of the nucleus that located in, you guessed it, the organelle mitochondria. And it doesn’t recombine like nucleic DNA so when it is pasted down through the generation it undergoes very little change. The way mtDNA works is it’s pasted through the maternal line given to daughters and sons, but the her sons significant other will only pass down mtDNA to there children from her own maternal line.
But why is there DNA in mitochondria in the first place? Well it goes far back when there was only single celled organisms and instead of digesting smaller bacterium these single celled organisms made a symbiotic relationship where instead of being digested were copied in the cells reproduction cycle and the first mitochondria where small bacterium. The theory that explains this endosymbiotic theory that is accepted by most scientists today.
MtDNA also lead to whats is called haplogroups, which can look at your ancient ancestry as well as migration of past human populations. The way it works is people in a haplogroup can trace that specific mtDNA to a common ancestor. The changes in mtDNA are created by a mutation and each of these changes in the mtDNA is a different haplogroup. Not everyone has the same mutations in the mtDNA because these mutations happened in different geographical locations at different times. And using mtDNA and archaeological evidence it can be used to track human migration. One example is a person in Northeast Asia can have the same haplogroup found in Native Americans showing a common ancestry among them that created that haplogroup in Asia. And around 15,000 years ago people in Asia migrated to Alaska with the same Haplogroup.
But since mtDNA shows a portion of ancient ancestry you most likely wont find a lost family member if you learn a stranger has the same haplogroup as you since haplogroups origins can be thousands of years old.