Through the annals of life on Earth and far beyond, the symmetries of the natural world seem to transcend time and space: from the intricacies of a crystalizing snowflake to spiral galaxies that span half a million light years, we live in a universe of symmetries both great and small. Whether dictated by the selective pressures of evolution or the molecular structure of abiotic compounds, the natural world displays a plethora of patterns steeped in the history of deep time.
What is Radial Symmetry?
When we look into the mirror or at a loved one’s face, we can imagine a line dividing the portrait down the middle; with eyes, ears, nose and mouth falling on both sides. We share this bilateral symmetry with 99% of animals (kingdom Animalia) today, as well as with many flowering plants. But in the vast diversity of life on Earth many body symmetries exist, including radial symmetry, or the harmonious arrangement around a central axis, and asymmetry in which no such pattern can be discerned.
Symmetry in Service of Life
While the architecture and design fields popularized the saying “form follows function,” the relationships between form and function has been an ongoing 3.5 billion-year saga in the evolution of life on Earth. Body symmetry in the natural world is not simply a matter of aesthetics, but rather it is critical to biological function. Body symmetry determines the placement of organs within an organism, how bodily fluids circulate, how the creature moves, and how it can interact with its environment.
Radial symmetry, as seen in sunflowers and sea stars, was once considered a “more ancient” form of body symmetry. However, bilateral symmetry, the symmetry we see in human faces, first appeared in ancestral bilaterian over 500-million years ago. Because of this deep history, understanding how symmetry has changed through time, space, and different animals is challenging. Much like crafting a compelling image, answering these burning questions involves identifying patterns and knowing where to look.