Scientists have long puzzled over how iguanas, a group of lizards  mostly found in the Americas, came to live in the isolated Pacific islands of Fiji and Tonga. Some scientists used to suppose that they must have traveled there on a raft, a journey of around 5,000 miles from South America to the islands. There are documented cases of iguanas reaching remote Caribbean islands and the Galapagos Islands on floating logs. But new research in January by Brice Noonan and Jack Sites suggested that iguanas may have simply walked to Fiji and Tonga when the islands were still a part of an ancient southern supercontinent. The ancient supercontinent was made up of present-day Africa, Australia, Antarctica and parts of Asia. If that's the case, the island species would need to be very old. Using "molecular   clock" analysis of living iguanas' DNA, Noonan and Sites found that, sure enough, _ has been around for more than 60 million years--easily old enough to have been in the area when the islands were still connected by land bridges to Asia or Australia. Fossils   uncovered in Mongolia suggest that iguanid ancestors did once live in Asia. Though there's currently no fossil evidence of iguanas in Australia, that doesn't necessarily mean they were never there. "The fossil record of this continent is surprisingly poor and cannot be taken as evidence of true absence," the authors write. So if the iguanas simply walked to Fiji and Tonga from Asia or possibly Australia, why are they not also found on the rest of the Pacific islands? Noonan and Sites say fossil evidence suggests that iguana species did once inhabit other islands, but went extinct right around the time when humans settled in those islands. But Fiji and Tonga have a much shorter history of human presence, which may have helped the iguanas living there to escape extinction. The researchers say that their study can't completely rule out the rafting theory, but it does make the land bridge theory "far more reasonable than previously thought." What is the main topic of this passage?
The life span of animals living on the ancient supercontinent. The two islands being home to several iguana species in the Pacific region. The fossil evidence suggesting iguanas' ancestors' swimming to Fiji and Tonga By raft or by land -- how did iguanas reach the tiny Pacific islands?