I just throw this in, because I don't think this possibility has been seriously discussed.
There are two possible rational scientific hypotheses (ok, a bit on the 'fringe' of science, but with some basis in rationality);
- The island could be a manifestation of the Strong Gaia Hypothesis. This follows on from the theories of Teilhard de Chardin who claimed that the earth itself was evolving through a physical, biological and spiritual process to the Omega Point. You have probably heard of the weak form of the Gaia Hypothesis, in the version proposed by James Lovelock. In this version, the Earth is simply treated as a self-organising system, which manages to regulate itself through automatic processes. In the strong version of the theory, these self-organisations give rise to different levels of purposeful activity. In its extreme form this becomes the 'Selfish Bicosm' argument, that the entire universe is evolving towards a self-aware intelligence.
- The island could be a Boltzmann Brain. This is a structure randomly created through chaotic behaviour. For a readable version of what the theory actually proposes read this NY Times article. Normally the theory would suggest that the universe is large enough for 'random' movements of gases to produce some highly improbable scenarios, one of which is that such 'chaotic' systems have the ability to self organise, and through the phenomenon known as emergence, produce a consciousness.
As I said, both of these are on the finges of science. However, given that some very strange things have happened on the island, and very many of the characters anthropomorphise about the island: "The island told me", "Its what the island wants", "He was a sacrifice the island demanded", "I thought the island could heal me"), maybe we ought to consider this seriously.