Why evolution favors symmetry

Life loves symmetry. You only have to take a look at the appearance of the vast majority of animals to realize this. But why? What pushes the living to symmetry? This is the question that an international team of researchers in mathematics, theoretical physics, bioinformatics and chemistry set out to answer in an article in the American scientific journal Proceedings of the National Academy of Sciences. Using artificial intelligence, they simulated the random evolution of virtual cells. Surprise : the building blocks of life tend to become symmetrized in one way or another. For nature, it would be a way of operating according to a principle of economy. Explanations.

“A typical sponge lacks definite symmetry; it grows like an irregular mass. Virtually all other animals have a definite form and symmetry which can be described by reference to an imaginary axis passing through the animal’s body. write the biologists Peter H. Raven, Jonathan B.Losos and Kenneth A.Mason in Biology. But how to explain this recurrence of symmetry?

Biologists have offered different answers, faithful to the guiding principle set out by the neurobiologist Jean Pierre Changeux in reason and Pleasure (Odile Jacob, 1994): “The properties of symmetry of living beings result from evolution”, therefore from the natural selection of the fittest. But how then does symmetry constitute an evolutionary advantage over a generally considered common ancestor, summarizes Michael Manuel in “Early evolution of symmetry and polarity in metazoan body plans”like an asymmetrical living thing or a primitive, cylindrical symmetry?

To fully grasp the problem, it is necessary to distinguish two major types of symmetries :

  • Radial symmetry : “The parts of the body are arranged around a central axis. […] A pie, for example, has radial symmetry; the same is true for a sea anemone”but also corals or jellyfish. This form of symmetry is generally considered to be particularly suited to ocean life, in a three-dimensional displacement space where the effect of gravity is not significant.
  • Bilateral symmetry : “The body has right and left halves which are mirror images of each other. […] Bilateral symmetry is an important advance in the evolution of the animal’s body plan. Bilaterally symmetrical animals have the ability to move through the environment in a coherent direction”which is particularly decisive when life gains the earth plane where the movement is essentially two-dimensional. “Along with this ability for directional movement, nerve cells cluster in the brain and sensory structures like eyes and ears develop at the anterior end of the body. This concentration of nerve tissue at the anterior end […] is called cephalization. » Decentralized marine animals give way to organisms whose parts are more clearly differentiated.

The results recently published in PNAS suggest another way. “Symmetrical structures arise preferentially not only due to natural selection, but also because they require less specific information to encode and are therefore much more likely to arise as phenotypic variation through random mutations. »note the scientists. Thus, symmetry is not so much an evolutionary advantage, but a kind of economy of means of nature: “Because symmetric structures require less information to encode, they are much more likely to appear as a potential variation. » Algorithmic study in support, they managed to show “a much higher prevalence of low complexity (high symmetry) phenotypes than what results from natural selection alone”. The simplest solution to a body-structuring problem is almost always the best…or at least the most viable.

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Why evolution favors symmetry

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