Discover Magazine.

"In the depths of history, a free-living bacterium was engulfed by a larger cell and was neither digested nor destroyed. Instead, it was domesticated. It forged a unique and fateful partnership with its host, eventually becoming the mitochondria of today. All of this happened just once in life's history and all of today's eukaryotes are descended from that fused cell. Indeed, many scientists view the origin of mitochondria as the origin of the eukaryotes themselves."
http://blogs.discovermagazine.com/notrocketscience/2010/10/20/the-origin-of-complex-life-it-was-all-about-energy/#.VVdxrflViko