Quirks & Quarks

The reason why most animals are symmetrical has to do with their locomotion

Most animals are symmetrical and although scientists don't know why, they do know bilateral symmetry has evolutionary advantages
Some animals have radial symmetry with five axes, like starfish. (Pixabay)
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This week's question comes from Meiko Sakamoto in Tokyo who asks:

"Why do all creatures — dogs, humans, fish, even insects — look symmetric? Even our imaginary aliens are often depicted with a symmetric face and body, but isn't it possible that creatures from other planets would look totally asymmetric?"

Emily Standen, assistant professor of biology at the University of Ottawa, says that although it's not entirely clear why symmetry exists in animals, there are some physical laws that explain why it persists. 

As a biomechanist by training, Standen explains that the movement of all living things in their environment is governed by the laws of physics. If we consider that life began in water, it makes sense to look at the way fish swim, contracting one side of their body and then the other, to understand why symmetry would be desirable for living things.

Having bilaterally symmetrical bodies (the same on both sides along an axis) permit them to propel forward in a straight line. In evolutionary history, animals that moved quickly and efficiently from point A to point B would have been more successful and selected for than those who were ineffective at locomoting.


But there are other approaches to addressing this question. For example, Standen says that symmetry is guiding even the fundamental process of cell division. An organism starts as a single cell and has to organize itself and its genetic material in a symmetrical way to ensure that each daughter cell has a copy of each gene before it splits.

Importantly, she notes,  not all animals are bilaterally symmetrical. Some animals have radial symmetry with four or five axes, like starfish, jellyfish and sea urchins. The only creature on Earth who is not symmetrical in any way is the sponge.

And why are  E.T. and other aliens we humans conceive of symmetrical? Standen thinks it's likely because the symmetry that surrounds us in the real world guides human creativity and imagination. What's more, if we consider that the laws of physics that govern our locomotion are constant, we may actually be correctly predicting that symmetry would also  be advantageous for aliens navigating in our shared universe and that they would hence also exhibit bilateral symmetry.