The question is located in the title. I apologize for prior confusion.

I have an extinct xenophobic culture that had made it to fusion and space-age technology. (Reason for xenophobism already thought of) I'll say the tech level in the beginning of Kim Stanley Robinson's Blue Mars.

Their civilization first developed on the Seychelles, Mauritia, and Mascarene Microcontinents, presently located about 1000 kilometers east of Madagascar and 100 meters below. The civilization reached this level by about 14,567 years ago, when an abrupt rise in sea level brought about its end. I can help with maps, such as this and this. All I have left is this speculation on the flora and fauna, and a technology timeline.

I hope these resources are sufficient.

Question: How could a technological civilization have arisen earlier?

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    $\begingroup$ What is the actual question here? I don't see a question. $\endgroup$ – kingledion Dec 24 '17 at 4:47
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    $\begingroup$ At the moment, this question is too broad. Basically it's "I have a civilation 15,000 years ago that had fusion technology... how could that have happened?" That's a wide open question and will get the question closed. Frankly, there's nothing stopping them from doing it the way we did/are doing it... the passage of time. They just evolved first. $\endgroup$ – JBH Dec 24 '17 at 6:45
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    $\begingroup$ There's nothing preventing it other than the fact it didn't happen. You've given us nothing to work with here. $\endgroup$ – Separatrix Dec 24 '17 at 10:35
  • $\begingroup$ The scenario as currently stated does not sound believable. You can do it by handwaving all of the problems away but for me at least it does not suspend disbelief. What made this island so special? Given thousands of years why didn't they spread to the main land where there's more room and how did they get there in the first place? $\endgroup$ – Slarty Dec 24 '17 at 10:40
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    $\begingroup$ I find it hard to believe that a civilization so advanced would not have used more of the Earth's surface and left artifacts elsewhere. Even if the primary location for the civilization was on a handful of low-lying areas, if they were that advanced they would have certainly explored the rest of the Earth's surface and likely needed to make use of other areas to mine raw materials. They would have also had scientific outposts, if nothing else than to observe the more primitive cultures elsewhere on Earth. $\endgroup$ – manassehkatz-Moving 2 Codidact Dec 24 '17 at 16:21