“Evolution of increased complexity in a molecular machine”
We can all think of a phenomenon in biology that is so incredible and complicated that if we didn’t know better, the hands of God would be a reasonable hypothesis to account for its origin. In one of his essays published in Natural History, Stephen Jay Gould marveled at his favorite bizarre and complex “perfection” in a certain species of Lampsilis mussels. Believe it or not, these clams figured out a way to mount a fake fish on their rear-end!
What’s remarkable about these sort of things is that it doesn’t matter who you are, — child or adult — we’re all struck with the same wonder-evoking question: how the heck did that happen?
For biologists like Gould, who understand the process of evolution, the difficulty lies in imagining the the intermediate steps it took to make such a complicated thing. Gould commented on this in his essay: “Natural selection has a constructive role in Darwin’s system: it builds adaptation gradually, through a sequence of intermediate stages, by bringing together in sequential fashion elements that seem to have meaning only as parts of a final product. But how can a series of reasonable intermediate forms be constructed?”
This problem isn’t limited to tissues and organs. There are also plenty of examples in molecular biology of complicated “molecular machines” that are composed of multiple protein subunits that perform exceedingly sophisticated processes. Think, for example, of the ribosome and the process of protein synthesis. Like the fake fish on the clam’s back, we’re struck with the same question: how did such complexity evolve?
A paper published recently in Nature from Joe Thornton’s group at University of Oregon gave us our first glimpse into how complex molecular machines can evolve. They addressed this question in a multisubunit protein complex that pumps protons in and out of cellular structures. Part of the pump is composed of a protein ring, which in most eukaryotes consists of two subunits. In fungi, however, the ring has evolved to be a bit more complicated, and consists of three subunits. Thornton’s group wanted to figure out how the three-component ring evolved.
Their approach was to first infer the evolutionary history of the two- and three-component ring pumps, then ‘resurrect’ the ancestral proteins by gene synthesis, and do biochemical and genetic experiments in yeast to understand the molecular mechanisms governing the evolution of the three-ring system.
What they basically found was that that the three-ring pump evolved in fungi by a duplication event in one of the components of the ancestral two-ring system. These two paralogues, which were initially identical, were then differentiated and specialized to become obligate components of the ring, thus forming the three-ring complex. Remarkably, Thornton’s group demonstrated that the specialization events in the two paralogues were not due to gains of novel functions, but rather from complementary degenerative losses of function in the duplicated subunits. In other words, what was originally bestowed upon one member of the two-ring system, was subsequently partitioned between two members of the three-ring system via degenerative, but complementary mutations.
Now, you might be wondering: why did this happen to begin with? Was there an adaptive advantage to evolving the three-ring system? The answer seems to be no. The system appears to have been driven toward complexity through neutral evolution; it just happened, with no selective “purpose” behind it.
This paper was cool not only in what it demonstrated, but also in helping to “demythologize” a common argument among creationists regarding the evolution of complexity — the so-called “irreducible complexity” argument, which has surely been already debunked a long time ago. Gould said it best in concluding his essay on the problem of perfection with a quote from Darwin:
“When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei [the voice of the people is the voice of God], as every philosopher knows, cannot be trusted in science.”