By Serge Kreutz
I exist because some self-replicating chemical has designed and created me as an instrument for the sole purpose of improving its success rate for self-replication.
The self-replicating chemical is nucleic acid (which comes along in two representations, DNA and RNA).
Because the purpose of my existence is self-replication (of my genes, not of me myself), it logically follows that my central organ system is the reproductive system (and not the brain, or central nervous system).
All other physiological systems are but mere adjacent structures to the reproductive system. We need a skeletal-muscular system to be able to move towards reproductive success, and to withdraw from anything that endangers reproductive success.
We need a digestive system to supply the reproductive system, as well as adjacent structures such as the skeletal-muscular system with energy.
And the reproductive system has designed a central nervous system to evaluate behaviors that are conducive or non-conducive to reproductive success.
The central nervous system is wrongly classified as a single unit.
Certain structures of the central nervous system, such as the cerebellum or large parts of the cerebrum, have only an auxiliary function: they facilitate reproductive behavior: the cerebellum allows an increased precision in movement, and the medulla is crucial for the regulation of heartbeat.
But while much of the central nervous system is auxiliary in function, it also incorporates elements that are more central, and more essential for the self-replication of nucleic acids: it provides reproductive motivation.
Without reproductive motivation, no auxiliary functions of the central nervous system would be needed, and, for that matter, no other organ system. There is no purpose for a skeletal-muscular system, or a digestive system, if there is no reproductive motivation.
Thus, I consider any organ structure as essential that provides the reproductive motivation for any organism.
In mammals, this essential structure probably is the limbic system of the brain, with the hypothalamus standing out. In a very direct way, the hypothalamus, may be considered a person’s most central organ (as did, by the way, Descartes).