"Techniques from computer science may
help explain the tendency in biology for structures to repeat themselves.
Symmetry runs rampant 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, Holló Gábor, 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.”
