Why are our bodies born symmetrical?

The question above has recently intrigued me. As human beings, we have two eyes, two ears, two arms, two legs, and even two nostrils. And the right side of our body is a mirror image of the left side. It may seem quite natural to us now. But when you think about it, this is truly amazing. We now know that people associate symmetry with beauty, so does nature think in the same way?

Being curious, I looked for an answer in biology textbook, and found out that living organisms exist in three different types of symmetry.

bodysymm

The first type of organisms does not display any symmetry at all. The only animals that truly belong to this classification are sponges. Asymmetrical animals are the more primitive forms of life on the earth.

symmetry

The second type is radial symmetry. The body plan is based around an axis. Let’s say we put an imaginary line through the centre of the organism, the rest of the body all radiates out from that line. These organisms have a top and bottom, but they don’t have a left or right side or a front and back side. Examples of such organisms include starfish and jellyfish. And most of the species with radial symmetry live in water. Radial symmetry allows organisms to reach out to all directions from one central point.

The last type of symmetry is called the bilateral symmetry. It is when the body plan can be divided along a plane that splits the body into left and right sides that are mirror images of each other. Human and the majority of other animals all belong to this category, e.g. cat, bird, prawn.

Bilateral symmetry gives species evolutionary advantages. For example, it allows the organism to move faster. Also, the organism can put most of the sensory organs and associated nervous tissues in the front of its body instead of spreading them in all directions. This specialization of the foremost part of the body is called cephalization. A more centralized nervous tissue can lead to the evolution of more complex behaviour and greater intelligence.

internal-organs-of-human-body-female.jpg

However, it is also interesting that the symmetry of our body is just superficial; the world inside our body is not symmetrical. For example, we have only one heart on the left side of our body and most of the liver on the right side. Even for organs that seem to be symmetrical like lungs, they are not entirely so – the left lung has two lobes whereas the right lung has three lobes. And that is all because our genes direct the cells to move in an asymmetrical manner as we develop from an embryo.

Knowing about all this, wouldn’t we start to view our bodies in a different light? They are truly beautiful and intricate works by the nature, and all the symmetries and asymmetries are carefully designed so that we stand a better chance of survival.

References:

Alters, S. (2000). Biology, Understanding Life, 3rd ed. Retrieved from

https://books.google.com.sg/books?id=GRDUIbQwGc8C&pg=PA133&dq=bilateral+symmetry&hl=zh-CN&sa=X&ved=0ahUKEwjW7MCSvYLSAhXML48KHdUzAgQQ6AEIJTAC#v=onepage&q=bilateral%20symmetry&f=false

Anjara, A. (2015, June 26). Lesser Known Human Body Symmetry Facts. Retrieved from http://www.geekpause.com/science/lesser-known-human-body-symmetry-facts/

Baron, A. (n.d.) Bilateral Symmetry: Definition, Examples & Advantages. Retrieved from http://study.com/academy/lesson/bilateral-symmetry-definition-examples-advantages.html

Farabee, M.J.(2007, Jan) Biological Diversity: Animals I. Retrieved from https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookDiversity_7.html

wiseGEEK Blog (n.d.) What is Bilateral Symmetry? Retrieved from

http://www.wisegeek.com/what-is-bilateral-symmetry.htm

Picture courtesy goes to:

https://www2.estrellamountain.edu/faculty/farabee/biobk/bodysymm.gif

http://study.com/cimages/multimages/16/Symmetry.png

http://gethumananatomy.com/wp-content/uploads/2016/03/internal-organs-of-human-body-female.jpg

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2 thoughts on “Why are our bodies born symmetrical?

  1. That’s the beauty of nature! Interesting to know the different types of symmetry present in different types of animals. I will highlight radial symmetry and bilateral symmetry to my students when I do teach them an extension lesson on animals!

    Liked by 1 person

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