Erred Into Perfection
Protein errors spark complexity!
Group of impalas (Aepyceros melampus) in Kruger National Park
Over four billion years of evolution, plants and animals grew far more complex than their single-celled ancestors. But a new comparison of proteins shared across species finds that complex organisms, including humans, have accumulated structural weaknesses that may have actually launched the long journey from microbe to man. The study, published in Nature, suggests that the random introduction of errors into proteins, rather than traditional natural selection, may have boosted the evolution of biological complexity. Flaws in the "packing" of proteins that make them more unstable in water could have promoted protein interactions and intracellular teamwork, expanding the possibilities of life. "Everybody wants to say that evolution is equivalent to natural selection and that things that are sophisticated and complex have been absolutely selected for," said study co-author Ariel Fernández, PhD, a visiting scholar at the University of Chicago and senior researcher at the Mathematics Institute of Argentina (IAM) in Buenos Aires. "What we are claiming here is that inefficient selection creates a niche or an opportunity to evolve complexity." "This is a novel bridge between protein chemistry and evolutionary biology," said co-author Michael Lynch, PhD, professor of biology at Indiana University. "I hope that it causes us to pause and think about how evolution operates in new ways that we haven't thought about before." When mildly negative mutations arise in a species with a large population, such as the trillions of bacterial organisms that can fill a small area, they are quickly cleared out by selective forces. But when a new mutation appears in a species with a relatively small population, as in large mammals and humans, selection against the error is slower and less efficient, allowing the mutation to spread through the population. To look at whether these mild defects accumulate in species with small populations, Fernández and Lynch compared over 100 proteins shared by 36 species of varying population size. Though these shared, "orthologous" proteins are identical in shape and function, genetic differences alter them in more subtle ways. Fernández and Lynch focused on design flaws called "dehydrons," sites where the protein structure is vulnerable to chemical reactions with water. Proteins with more dehydrons are more "unwrapped" -- unstable in an aqueous environment, and therefore prone to bind with another protein to protect their vulnerable regions. A computational analysis of 106 orthologous proteins confirmed their hypothesis that proteins from species with smaller populations were more vulnerable in water. The result suggests that structural errors accumulate in large organisms such as humans due to random genetic drift.
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