Recently, I suddenly wondered why we have five fingers. I wondered if there is some meaning behind the body and the five limbs – head, right arm, left arm, right leg, and left leg. Apparently, only the front feet of dogs have five fingers, but most mammals have five fingers, and horses also developed hooves from five fingers. Amphibians have five fingers on their hind feet, which is the opposite of dogs, and most reptiles also have five fingers.
With that in mind, I researched the number 5 and discovered that starfish have five arms radiating out from their central mouth, making them pentaradial. Echinoderms such as sea urchins, starfish, and sea cucumbers seem to have pentaradial symmetry. The shell of a sea urchin excluding its spines is also pentaradial, and it also has five teeth. The number of tentacles on a sea cucumber is also usually a multiple of five. I wonder why it’s five.
Many flowers also have five petals, with about 40% of the 219 families of seed plants having five petals. Plants that are symmetrical in five directions are said to be pentamerous, and examples of this include cherry blossoms, hydrangeas, roses, eggplants, gourds, bellflowers, maples, camellias, grapes, mandarin oranges, and silver vines. The pentagonal cross section of okra is also due to pentamerous. In the evolution of plants, plants with pentamerousity seem to be a relatively new species.
There are various theories as to why it is five, but according to what I have researched,
For creatures that move slowly to catch food from the ground, such as starfish, a 5-radiate shape is more efficient at catching food. Apparently, it is less likely to leave gaps than a 6-radiate shape. I wonder if the same reason applies to the five fingers of animals. On the other hand, creatures that move quickly to catch food seem to be symmetrical. The petals can be thought of as signs to attract insects and as runways for landing. If it were a 6-radiate shape, their heads would bump into each other when they land and try to take off, but with a 5-radiate shape, they won’t.
Regarding these issues, there seems to be a detailed explanation in the book “Sea Urchins are Amazing, Grasshoppers are Amazing” written by Motokawa Tatsuo.
For some reason, I feel like nature mainly chooses shapes with 3 to 6 radiations that are highly efficient for the purpose. 3, 4, 5, and 6 are also components of regular polyhedrons, so there may be some connection. 7 seems to be quite rare.
The property of forming the same figure when rotated 72 degrees, such as five-fold radial symmetry, five-numbered petals, pentagrams, and pentagons, is called five-fold symmetry. Things with symmetry are perceived as beautiful, but the pentagram in particular contains the golden ratio, which may make it seem mysterious and beautiful.
This pentagram was born from the orbit of Venus. Venus orbits faster than the Earth, apparently overtaking the Earth every 1.6 years, which is called a conjunction. Venus conjuncts the Earth five times every eight years, and if you connect the meeting places in order with lines, you can draw a pentagram on the orbit. It was a magnificent figure drawn by the Earth and Venus. Five-fold symmetry is amazing. The relationship between the Earth and the number five is amazing.
