Symmetry runs crawling in nature. It’s present wherever mirror images are repeated, like in the right and left halves of elephants or butterflies, or in the repeating patterns of flower petals and starfish arms around a central point. It’s even hiding in the structures of tiny things like proteins and RNA. While asymmetry certainly exists in nature (like how your heart is off to one side in your chest, or how male fiddler crabs have one enlarged claw), symmetrical forms crop up too often in living things to just be random.
Why does symmetry reign supreme? Biologists aren’t sure — there’s no reason based in natural selection for symmetry’s prevalence in such varied forms of life and their building blocks. Now it seems like a good answer could come from the field of computer science.
In a paper published this month in Proceedings of the National Academy of Sciences, researchers analyzed thousands of protein complexes and RNA structures as well as a model network of molecules that control how genes switch on and off. They found that evolution tends toward symmetry because the instructions to produce symmetry are easier to embed in genetic code and follow. Symmetry is maybe the most fundamental application of the adage “work smarter, not harder.”
“People often are quite amazed that evolution can make these incredible structures, and what we’re showing is that it’s actually easier than you might think,” said Ard Louis, a physicist at the University of Oxford and an author of the study.
“It’s like we found a new law of nature,” said Chico Camargo, a co-author and a lecturer in computer science at the University of Exeter in England. “This is beautiful, because it changes how you see the world.”
Dr. Louis, Dr. Camargo and their colleague Iain Johnston began their exploration of symmetry’s evolutionary origins when Dr. Johnston was working on his Ph.D., running simulations to understand how viruses form their protein shells. The structures that emerged were highly biased toward symmetry, cropping up far more often than pure randomness would allow.
The researchers were surprised at first, but it made sense — the algorithms to produce simple, repeating patterns are easier to carry out and harder to screw up. Dr. Johnston, now at the University of Bergen in Norway, likens it to telling someone how to tile a floor: It’s easier to give instructions to lay down repeating rows of identical square tiles than explain how to make a complex mosaic.
Over the next decade, the researchers and their team applied that same concept to basic biological components, looking at how proteins assemble into clusters and how RNA folds.
“The shapes that appear more often are the simpler ones, or the ones that are less crazy,” Dr. Camargo said.
Imagining RNA and proteins as little input-output machines that carry out algorithmic genetic instructions explains the tendency toward symmetry in a way that Darwinian “survival of the fittest” hasn’t been able to. Because it’s easier to encode instructions for building simple, symmetrical structures, nature winds up with a disproportionate number of these simpler instruction sets to choose from when it comes to natural selection. That makes evolution a bit like a “biased game with loaded dice,” Dr. Camargo said, producing disproportionate symmetry because of its simplicity.
While their paper focuses on microscopic structures, the researchers believe that this logic extends to bigger, more complex organisms. “It would make an awful lot of sense if nature could reuse the program to produce a petal rather than have a different program for every one of the 100 petals around the sunflower,” Dr. Johnston said.
While there’s still a gulf between demonstrating the statistical bias toward microscopic symmetry and explaining the symmetry we see in plants and animals, Gábor Holló, a biologist who studies symmetry at the University of Debrecen in Hungary, says he’s excited by the results of the new paper . “To explain how such an inherent and such a universal feature emerges at all in evolution, in nature, that’s something,” said Dr. Holló, who was not involved with the study.
Similarly, Luís Seoane, a complex systems researcher at the Centro Nacional de Biotecnologia in Spain, also not involved in the study, praised the work as being “as legit as it gets.”
“There is a war going on between simplicity and complexity, and we live right at the edge of it,” Dr. Seoane said. The universe tends toward ever-increasing randomness, he added, but these simple, symmetrical building blocks help make sense of that complexity.