When the sperm enters the ovum it finds food. Interwining the two parent's DNA strands, building someone entirely new. We're all parameciums at that point. Sex doesn't occur until the first cell division. I think that 5 stem cells are the first cell divisions. Some of those divisions may be: brain, pineal (or perhaps other similar glands), bone, organs, and nervous system (or perhaps muscle).
So to make permanent changes to DNA (such as life extension), modifying the DNA in the stem cells might work, but would likely create conflict with that original paramecium. I wonder if the original paramecium is contained in the pineal itself. Locked away until emotional trauma is entirely healed.
My theory is the original paramecium provides a gateway before significant changes to genes will by accepted by the organism. Any attempt to modify the stem cells will be "policed" by that. Emotional congruity will likely be important to acceptance of those changes by the organism of large changes (such as life-extension).
When do we ever get back to that original paramecium (self)?