Intricate cellular components are often cited as evidence of intelligent design. They couldn't have evolved, I.D. proponents say, because they can't be broken down into smaller, simpler functional parts. They are irreducibly complex, so they must have been intentionally designed, as is, by an intelligent entity.

But new research comparing mitochondria, which provide energy to animal cells, with their bacterial relatives, shows that the necessary pieces for one particular cellular machine — exactly the sort of structure that's supposed to prove intelligent design — were lying around long ago. It was simply a matter of time before they came together into a more complex entity.

The pieces "were involved in some other, different function. They were recruited and acquired a new function," said Sebastian Poggio, a postdoctoral cell biologist at Yale University and co-author of the study published Monday in the Proceedings of the National Academy of Sciences.

Mitochondria are descended from free-living bacteria, which several billion years ago were swallowed by complex cells. The mitochondria soon became central to the cells' function.

Mitochondria couldn't have lasted in their new home without the help of a protein machine called TIM23, which delivers other proteins harvested from the cell's body. Bacteria don't possess TIM23, suggesting that it evolved in mitochondria. This seems to pose a cellular chicken-and-egg question: How could protein transport evolve when it was necessary to survive in the first place?

The essential paradox applies to other protein-transporting cell systems, providing disbelievers of evolution with a key part of their critique. As articulated by intelligent design proponent Michael Behe, "This constant, regulated traffic flow in the cell comprises another remarkably complex, irreducible system. All parts must function or the system breaks down."

According to evolutionary theory, however, cellular complexity is reducible. It requires only that existing components be repurposed, with inevitable mutations providing extra ingredients as needed. Flagella, the hairlike propellers used by bacteria to move, are one example of this. Their component parts are found throughout cells, performing other tasks.

Intelligent design mavens once cited flagella as evidence of their theory. Scientific fact dispelled that illusion. The mitochondria study does the same for protein transport.

"This analysis of protein transport provides a blueprint for the evolution of cellular machinery in general," write the researchers, led by molecular biologist Trevor Lithgow at Australia's Monash University. "The complexity of these machines is not irreducible."

When they analyzed the genomes of proteobacteria, the family that spawned the ancestors of mitochondria, Lithgow's team found two of the protein parts used in mitochondria to make TIM23.

The parts are located on bacterial cell membranes, making them ideally positioned for TIM23's eventual protein-delivering role. Only one other part, a molecule called LivH, would make a rudimentary protein-transporting machine — and LivH is commonly found in proteobacteria.

The process by which parts accumulate until they're ready to snap together is called preadaptation. It's a form of "neutral evolution," in which the buildup of the parts provides no immediate advantage or disadvantage. Neutral evolution falls outside the descriptions of Charles Darwin. But once the pieces gather, mutation and natural selection can take care of the rest, ultimately resulting in the now-complex form of TIM23.

"It hasn't been possible up until this point to trace any of those proteins back to a bacterial ancestor," said Dalhousie University cell biologist Michael Gray, one of the researchers who originally described the origins of mitochondria, but was not involved in the new study. "These three proteins don't perform precisely the same function in proteobacteria, but with a simple mutation could be transformed into a simple protein transport machine that could start the whole thing off."

"You look at cellular machines and say, why on earth would biology do anything like this? It's too bizarre," he said. "But when you think about it in a neutral evolutionary fashion, in which these machineries emerge before there's a need for them, then it makes sense."

*Citation: "The reducible complexity of a mitochondrial molecular machine." By Abigail Clements,1, Dejan Bursac, Xenia Gatsos, Andrew J. Perry, Srgjan Civciristova, Nermin Celik, Vladimir A. Likic, Sebastian Poggio, Christine Jacobs-Wagner, Richard A. Strugnell, and Trevor Lithgow. Proceedings of the National Academy of Sciences, Vol. 106 No. 33, August 25, 2009. *

Image: Journal of Cell Science

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