Sea Levels
Different ecologies on neighboring islands denote recent depopulation. Huge waves crossed the Pacific about 3,540 yrs. ago, as explained in Chapters 4 & 10. Once again, islands would be repopulated randomly.
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Many low islands were completely depopulated about 3,540 years ago. Others would become populated after seal levels dropped 20 ft. at that time. Read about it at martiangrandpa.com.