Researchers believe they've traced the original genetic ancestors for most modern humans by studying the male, or Y, chromosome. Nearly all modern men can trace their lineage back to one man who roamed Africa between 125,000 and 156,000 years ago, according to LiveScience. And the same is true for women, who are linked to one woman who lived at the same time. But "Adam and Eve," as these two ancient people are being called, likely never met, let alone mated and populated the earth. Instead, according to LiveScience, both humans simply had DNA that survived, unbroken, resulting in many modern heirs of their mitochondrial or Y-chromosome DNA.
Again, survival of the fittest DNA. Evolution at work. And for you who are theists, there is no reason why a higher being (god for want of a better word) might not have put this in motion.