The message of the gravity is of Life itself. How life is formed through unbelievable circumstances. Think of the whole movie in space as a mother's womb (remember that scene, Sandra's body floating in circle as soon as she gets back to the ship). The final scene is of a baby who takes the step to live.
Now, the bigger question is, where is the choice? Do humans make choice in acquiring life? How? The simple answer may be procreation and reproduction, but is it really a choice?
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