Rats are playing an ever more important role in the understanding of the early human travels through the Pacific.

That's because, according to the best available evidence, rats were passengers or cargo on virtually all voyages of discovery by Polynesians. Nearly every Pacific island inhabited by the early Polynesians was also populated by the Pacific rat, Rattus exulans.

Once they got there, rats did not play a benign role in the environment. In Hawai'i, the rise in the rat population matches the decline in plant species susceptible to rats. Pollen studies and archaeological evidence of dense roots in old soil show that the Hawaiian Islands were once densely forested with the native Pritchardia fan palm, loulu, which also diminished during the same period. Even today, it's difficult to find viable loulu seeds, because rats eat them.

On remote Rapa Nui, or Easter Island, the dominant tree was also a palm, and archaeologists say that nearly every old seed they've found has been munched by rats. It's possible the rats, as much or more than humans, were responsible for the loss of the tree cover there.

Modern DNA testing of rats also gives clues to the past. In Hawai'i, tests of pre-European Polynesian rat remains appear to confirm Hawaiian legends about multiple voyages back and forth to other island groups.

"The rat DNA from Hawai'i shows multiple lineages — suggesting multiple rat introductions, from some different islands, thus multiple migrations is a likely interpretation," said University of Hawai'i archaeology professor Terry Hunt. "One migration could bring two or more lineages, but as they link to different islands, say in Tahiti and the Marquesas, then separate migrations make better sense."

Hunt is co-author of a new paper in the Journal of Archaeological Science on rat DNA tests from early archaeological sites in Rapa Nui. There, the authors write, the evidence of limited genetic variation — although it's not conclusive—argues for a single migration.

"Rapa Nui appears to have remained relatively isolated following initial colonization," the paper's authors write.

Other evidence of a single very limited visitation: It appears that pigs, as well as breadfruit and coconuts, hadn't been established on Rapa Nui — although they are there now. Since these species are closely associated with Polynesian life, if there had been numerous voyages, it's reasonable to assume these voids would have been filled.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.