Could there be a biological reason why we pat each other on the back as a showing of good will? This is highly speculative but hopefully fun.
Could it be that when you pat someone on the back you also put light pressure on the left lymphatic duct, which is the largest lymphatic vessel. It starts from the second lumbar vertebra and extends to the root of the neck, where it drains into the circulation system for removal. By patting or messaging that area you help push the lymph through the duct and into the blood circulation system to excrete out. Remember that the lymph system, unlike the circulation system does not have a pump, the heart.
So could it be that patting someone on the back helps them fight and remove pathogens that have been caught up in the lymph system?
Tom Hendricks
BIOLOGY HYPOTHESIS
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BIOLOGICAL SPECULATIONS Through The Years.
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UV PAPER
http://www.daviddarling.info/encyclopedia/U/UV_origin_of_life.html
Catabolic and Anabolic evolved, but they did not blend.