How Do Our Bodies Know Left From Right?

Our bodies may be (nearly) symmetrical on the outside, but our insides are a different story. Your heart, your stomach and spleen are all on the left side of your body, your liver on the right. But in one in 20,000 people, the sidedness of the organs is reversed.

Does organ symmetry have any effect on overall fitness – and how does your body know the difference? Joe Hanson explains, in an awesome new installment of It's Okay To Be Smart.

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To quote from video

"these people [people with sinus inversus] usually show no negative effects from their reversed innards"

Well... Except for about 25% of people with sinus inversus who have primary ciliary dyskinesia, wherein their cilia don't function properly. This can lead to issues with the lungs and in males to infertility.

Also "symmetry existing because it's attractive" only works for animals that see each other when they mate. That's not the case for some animals that are symmetrical such as tuna and sardines which just spawn in the ocean and see what happens. If there's an underlying reason behind external symmetry, it's more likely due to symmetrical organ systems (particularly muscles) functioning more efficiently than asymmetrical organ systems.