The movie was'nt about Ponce De Leon or had anything to do with anyhing supernatural like reincarnation or anything.
BAsically all the stuff about Ponce, the quest, queen isabella and the tree of life was all just the BOOK that Izzy was writing, "The Fountain". The reason she was writing it was because she had an interest in that period of time and she saw herself as the queen and her husband( Dr. Thomas Creo( Hugh Jackman) ) as her conquistador seeking out a metaphorical "fountain of youth" for her to save her life through his work in science to cure her tumor.
The whole part of him in the bubble in the future was mearly symbolism of where he saw himself( alone, in pain, slaving away, obsessed, destroyed, isolated), still with the tree he had planted @ her grave which was sybolic for being part of her.
The movie was actually too simple and basic imho. I thought it was going to be much more than what it was. And i thought it would have been much much better if they had of followed the more fictional " Fountain" mythology. But they didnt and the movie was a turd
save ur Money