Though science has long maintained that all complex organisms have mitochondria, an organism without mitochondria has been discovered. All organisms more complicated than bacteria are eukaryotic, defined as having membrane-bound organelles. (An organelle is basically an organ for a cell). All organelles are vital to the proper functioning of a cell, but perhaps no organelle has a more central role than the mitochondria. Sometimes called the "powerhouses of the cell," mitochondria provide energy for the cell in a multicellular organism. Without mitochondria, our bodies would quickly shut down.
What is truly incredible about mitchondria, however, is what they indicate about the evolution of higher organisms. Mitochondria have long stood out from other organelles. Aside from the nucleus, they are the only other part of a cell to contain DNA, suggesting that at one point mitochondria stood alone—mitochondrial DNA, unlike nuclear DNA, is inherited only from mothers. Two billion years ago, most life was prokaryotic and did not have membrane-bound organelles. These simple, unicellular organisms drifted through the primordial ooze without a care in the world. One day, however, one of those organisms decided to eat another through phagocytosis, or engulfing its prey whole. And that's when everything began to change.
There is some evidence that mitochondria are the reason why there are two sexes.
There is one commonly accepted explanation: When that primordial feast took place, the engulfed organism, instead of being digested, took up residence inside its would-be killer. The two formed a symbiotic relationship, with the new inhabitant helping to provide energy for the combined organism. The ingestion of mitochondria was a huge leap forward, introducing internal membranes to cells and making higher organisms possible. Aside from mitochondria, only the photosynthetic chloroplasts in plant cells were acquired in this manner. This explanation of the origin of eukaryotic life is called the "endosymbiotic hypothesis" and no serious evidence has yet been uncovered to contradict it.
Mitochondria are interesting in other ways, too. There is some evidence that mitochondria are the reason why there are two sexes. The explanation is complicated, but basically an analysis of mitochondrial DNA suggests that mitochondria, when they were free-standing, had only one parent. Over time, this leads to conditions that favors distinct sources of DNA, or multiple sexes.
The new finding does not upend the endosymbiotic hypothesis, since apparently this organism (a gut bacteria from a researcher's pet chinchilla, as it turns out) had mitochondria once but then lost them. The organism seems to have evolved an alternative method for creating chemical energy, obviating the need for mitochondria. The discovery does show that while eukaryotes may owe their existence to mitochondria, they do not necessarily owe their identity to mitochondria. Maybe that's our next big evolutionary leap.
Proceedings: Biological Sciences , Vol. 279, No. 1734 (7 May 2012), pp. 1865-1872
BioScience, Vol. 61, No. 8 (August 2011), pp. 594-601
Oxford University Press on behalf of the American Institute of Biological Sciences