There is a funny argument brought up often when an Atheist and a Gnostic get into a scientific debate over the beginning of modern humans, it goes like this:
"We know there is an 'Eve' because all mitochondrial DNA comes from one source"
Now, that is a compelling argument. It is difficult to discern how the unified mitochondrial DNA, that we all share, got it's beginnings. What we do know is that if you trace every living human's mitochondrial DNA back, there is one source, a human female. We know this to be truth because mitochondria is only passed from mother to child and not from the father's genetic material.
Here is what I am going to suppose and I may be off base but some research has pointed in this direction: What if, the strain of mitochondria our oldest ancestor received was more beneficial to her and allowed for an evolutionary advantage? This unfortunately doesn't explain how she got that initial difference from her peers or how it was an improvement but lets open that up for discussion.
Mitochondrial DNA has been traced to about 200 thousand years ago. The first human to have it wasn't necessarily the very first human, just the one we all descended from. This means that the first human with this specific DNA strand in they're mitochondria had offspring that were successful enough to replace all other hominids on the planet.