Is complex life rare in the cosmos? The idea that it could be rests on the observation that the existence of life like us - with large, energy hungry, complicated cells - may be contingent on a number of very specific and unlikely factors in the history of the Earth. Added together they suggest that places like this could be exceedingly unusual across the universe. As I've pointed out in a previous post, there are some potential flaws to such arguments, largely because of the ways in which we make post-hoc inferences.
But one of the more potent pieces of this 'rare Earth' idea comes from the suggestion that our cellular power-plants, the mitochondria, are due to a very low probability merger of simpler microbial (prokaryotic) organisms some 2 billion years ago. In this scenario the complex cellular structure of Eukayrotes (us, plants, insects, fungus...) essentially began with that exceedingly lucky - and therefore cosmically rare - mutual engulfment and resulting symbiosis.
However, a new piece of research offers an intriguing (albeit equally unproven) alternative. Zhang Wang and Martin Wu of the University of Virginia make an argument that instead of a chancy cellular merger, or engulfment of one prokaryote by another, the mitochondrial machinery actually comes from a parasite. What started out as a bacterium stealing chemical energy eventually became an organism providing chemical energy - in return for an evolutionary advantage.
This proposal comes from a deep look at the genetic relationships between modern mitochondria and 18 closely related free-ranging bacteria. The researchers in effect attempt a reconstruction of the likely metabolic processes of the earliest mitochondria and their immediate precursors. They find that these critters were more likely to have been chemical energy parasites, and probably mobile too - with genes for the bacterial 'tails' or flagella that propel many microbes.
Why would this change the rare-complex-life argument? Parasites are rampant in nature. In fact, as the science writer Carl Zimmer pointed out in his book Parasite Rex, life on Earth can, in many respects, be considered to be dominated by parasites. And the beauty of parasites (yes, all things are beautiful in the proper context!) is that their very survival hinges on insinuating themselves into other organisms.
In other words, if mitochondria and complex cells are the later outcome of a parasitic infestation, their existence is perhaps not so unlikely - there was a strong incentive for those ancestral mitochondrial species to find hosts, and the rest is just natural selection.