Even if we (humans) had this ability (transdifferentiation), well, I can't even think of how it would work. From my understanding, the jellyfish reverts to a polyp which seems to be the precursor organism for a jellyfish. Wikipedia defines the process as an already differentiated cell changing to a cell outside it's path. Humans seem to complex of an organism for this to work, you can't have nerve cells turning into other stuff, even if they changed to a related cell type like some form of neuroglia. Evolution would have figured a way for it to work (nerve cells wouldn't revert to preserve body function and memory, major body system cells wouldn't undergo the process all at the same time), but you'll always be susceptable to disease, massive trauma and such. You just won't die from getting old. This article
says that transdiffertiation occurs in humans, but only on cells of the esophagus: smooth muscle cells change to goblet cells, which I guess are related since the mucous probably lubricates the esophagus or something.
Smart stuff is fun.
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