The monogenetic theory states that humans originated in one location. From there humans evolved and subsequently migrated out to populate the world. Once these first humans found their new home, adaptions to these new environments, through evolution, cause a divergence in the characteristics of the first humans. This divergence over time initiated the differences we see in people from region to region across the world today.
The idea monogenetic theory can be taken one step further with the writing of Stephen Oppenheimer. The writing I am refering to, is his book The Real Eve: Modern Man’s Journey Out of Africa. In this book he talks of the out of Africa Theory and of the mitochondrial Eve. The mitochondrial eve is the women who is theoretically the one common human ancestor for the entire world population today. This was found because you can trace the mitochondrial DNA from generation to generation, which is passed down from mother to daughter. By taking samples from all over the globe and much research, the DNA has lead back to one common female human ancestor about 150,000 years ago who lived in Africa. For more information the full documentary on The Real Eve can be seen below.