Their closest extant relatives (the genera Amblyrhynchus, Conolophus, Ctenosaura, Cyclura, Iguana and Sauromalus[2]) are present in primarily tropical regions of the Americas and islands in the Galápagos and Lesser and Greater Antilles.

One hypothesis to account for this biogeographical puzzle, based in part on an estimated divergence date of 50 million years ago (although more recent studies have revised this to about 35 million years ago[3]), is that these species are the descendants of a more widespread but now extinct lineage of Old World iguanids that migrated overland from the New World to Asia or Australia, and then dispersed by some combination of continental drift, rafting and/or land bridges to their present remote location.

[4] A secondary hypothesis, and the more widely accepted one, is that these species evolved from New World iguanas that rafted 10,000 km (6,200 mi) west across the Pacific Ocean with the aid of the South Equatorial Current.

[5][6] While a rafting voyage of four months or more might seem implausible, the ancestors of Brachylophus may have been preadapted for such a journey by having water requirements that can be satisfied by food alone, as well as comparatively long egg incubation periods.

[1][9] A giant Tongan species, Brachylophus gibbonsi, similar in size and build to an iguana of the genus Cyclura once existed on Lifuka, islands in the Ha‘apai group and Tongatapu but became extinct in prehistoric times due to predation by humans and their domestic animals.