Not many people read the original article then?
To sum it all up: Cells take a lot of energy to power. Our ancestor cells at one point in time have managed to absorb another entire species into its own cells. It takes a lot of energy to reproduce DNA, which cells must to do at a constant rate, meaning they cannot grow larger than a certain size unless they have a constant supply of energy surplus to DNA reproduction. This living creature that was absorbed are what we now call mitochondria and they are little engines that provide your body with a shitload of energy to actually grow larger than bacteria.
That jump in evolution has only happened once in the history of the earth, and evidence shows that it took a huge amount of time for simple cells to evolve into larger creatures, and it was due to absorbing these ancestral mitochondria. Early mitochondria were not as efficient as what eukaryotes have now, and they have evolved greatly to increase efficiency. Essentially, the little engines seem to be products of a very unlikely event, and also seem to be necessary for any living creature to grow large.
What was once another species of living creature, is now fully absorbed and stripped of all remnants of itself, except for the energy producing properties. It is now an organelle that has been fine tuned by evolution, and appears to be the only solution to the energy barrier. So any intelligent life that we encounter in the universe will probably have one thing in common with us, mitochondria, as long as they are carbon based life forms.
Mitochondria: