as well as thousands of kilometres to the north, which disproved the single hotspot model for the formation of the Line Islands, and instead suggested the chain was made by a more complex series of volcanic events than the Hawaiian–Emperor seamount chain. During the 1980s, scientists put forward new theories to explain the evolution of the chain, including a transform fracture zone theory (since refuted by paleomagnetic data), and a multiple hotspot model which is still being discussed as of 2020. A combination of hotspot-transform fracture zone phenomena is also possible. Flora and fauna Despite more than three centuries of occasional human impact on Caroline Island, it is one of very few remaining near-pristine tropical islands. The United Nations rated the atoll with a 'Human Impact Index' rating of 1 in 1998, making it one of the most unspoiled islands in the world. Its undisturbed state has led to a proposal that it be designated a World Heritage Site and a Biosphere Reserve. Caroline was visited in 1965 by the Pacific Ocean Biological Survey Program, in 1974 by the Line Island Expedition, and in 1988 and 1991 by the United Nations Environment Programme (UNEP) Wildlife Conservation Unit. A 1991 Kiribati Government expedition, comprising personnel of the Wildlife Conservation Unit and officials of the Ministry of Line and Phoenix Islands Development, agreed that with the exception of South Islet, Long Islet, and Nake Islet, the Caroline Atoll islets should become Wildlife Sanctuaries. In 1988, 90% of Caroline Island had vegetation; two-thirds of the vegetation was woodland, and 89% of the plant species were indigenous. Caroline's islets are made up of seven concentric plant communities, defined by a dominant species. One community—dominated by the coconut palm—was made by humans. The other six are: an outermost "herb mat", comprising mainly the flowering shrub Heliotropium anomalum and the succulent plant Portulaca lutea; "Beach Shrub" (shrubland), consisting of the evergreen shrub Bay cedar Suriana maritima; the minor plant community "Pandanus Forest" (consisting primarily of specimens of the Pandanus tectorius tree); "Tournefortia Scrub and Forest", populated by Heliotropium arboreum, which dominates the wooded islets of the atoll, forming 31% of the atoll's land area; "Cordia Forest", dominated by the flowering tree Cordia subcordata; "Pisonia Forest", consisting of the Pisonia grandis tree, located in the interior of the larger islets, and widely distributed throughout the atoll. Smaller islets lack the central forest and the smallest islets are