How did we get this symmetrical body structure? – Bilateral Symmetry

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Acoel, First Bilateral Water Organism

This strange organism in the picture is “ACOEL”. 550 million years ago, the Acoel was the first bilateral organism. Bilateral organisms are those which looks symmetrical on both left-hand and right-hand sides with respect to a line drawn vertically in the middle. Organisms are continuously evolving and changing forms since the beginning of life on earth. Life, as predicted by many theories, first came into existence in water bodies. Slowly the first creatures began to move and wander in water. With time and evolution, they came out of the water and started life on the land 330 million years ago.

Ossinodus, Stem tetrapod, Fish with limbs which came out of the water to land.

Unlike swimming in the water, movement on land requires proper balance and structure. So, these organisms developed a bilateral body which is symmetrical about lateral or vertical section. This symmetry along with various physical factors on land like gravity, helped in forming limbs and bones giving a proper structure, eventually resulting in balanced movement on the land surface.

With this many creatures came into existence and adapted themselves according to the living and climatic conditions. This is not an overnight process and took many million years to adapt themselves as a new organism. So, according to the theory of evolution, Fish and other water species could be considered as our ancestors.

Muscular human body showing symmetry with a vertical line drawn.


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