The iguana is a group of reptiles primarily found in Central America, South America, and the Caribbean. It is believed that they traversed from Central America to the Galapagos Islands, approximately 1000 km to the west, by climbing dead trees long ago. Even today, there are sightings of iguanas traversing the water while moving along thick branches or dead tree trunks in the Caribbean region.
Researchers from the University of California, Berkeley (UC Berkeley) and the University of San Francisco revealed on the 17th that iguanas living in Fiji sailed 8000 km after leaving the western coast of North America approximately 34 million years ago and arrived at these isolated South Pacific islands, according to a study published in the Proceedings of the National Academy of Sciences (PNAS). This is considered the longest ocean crossing among terrestrial vertebrates.
Overwater dispersal is the phenomenon of terrestrial organisms crossing the ocean to migrate to other landmasses. Typically, they do not reach their destination at once, but instead traverse multiple steps between islands to cross the sea. This theory is used to explain the process of humans and plants migrating to newly formed islands and the evolution of adapting to entirely new ecosystems. Such colonization processes play an important role in the evolution theory proposed by Charles Darwin based on natural selection.
Before this study, biologists speculated that the iguanas living on Fiji once widely inhabited the Pacific but later became extinct, derived from the only remaining ancestor of iguanas in the southwestern Pacific. Some scholars suggested that iguanas might have migrated from the tropical regions of South America through Antarctica or Australia, but there is still no genetic or fossil evidence to support this.
The research team noted that their new analysis put an end to the existing hypothesis. According to the researchers, the timing of the arrival of the iguana ancestors currently living in Fiji is estimated to be shortly after the formation of this volcanic island, approximately 34 million years ago. This coincides with the divergence time genetically from the close relative that lives in the North American desert, Dipsosaurus.
Reptile expert Simon Scarpetta, an assistant professor at the University of San Francisco, said, 'This is the first time we have discovered that the iguanas of Fiji are the closest relatives to the iguanas living in the North American desert,' adding that 'the timing of their genetic divergence aligns with the period when the island was forming.'
On the other hand, some evaluations regarding the analysis that the iguana crossed a vast ocean suggest skepticism. Co-author Jimmy McGuire, a professor at UC Berkeley, mentioned, 'To say they arrived directly from North America to Fiji sounds insane, but other models do not align with when the iguanas arrived in Fiji,' and added, 'It is a remarkable event that iguanas began living there as soon as the island emerged.'
According to navigation experts, it currently takes about a month to sail from California to Fiji with favorable winds. Iguana ancestors from Fiji are presumed to have taken much longer to traverse the doldrums and cross the equator, moving from Fiji to Tonga. Reptile experts believe that the physical characteristics of iguanas may have favored such long-distance journeys. Iguanas are large-bodied and feed on vegetation, allowing them to survive for long periods without food and water. The researchers suggest that if the drifting raft were a uprooted tree, it could have provided sustenance.
Professor Scarpetta stated, 'One can imagine iguanas living in groups and laying eggs on a tree that fell due to a cyclone, using it as a makeshift raft to cross the sea along the currents.'
There are more than 2100 species of iguanas on Earth. Animals such as chameleons, anoles, bearded dragons, and horned lizards are also included. Most people associate the term 'iguana' with the iguana family (Iguanidae) found in the Western Hemisphere. The widely distributed green iguana Iguana iguana, described by Swedish botanist Carl Linnaeus in 1758, belongs to this family. Currently, there are 45 species of Iguanidae inhabiting tropical, subtropical, and desert regions of the Caribbean, North America, and Central America. This includes the marine iguana, iconic to the Galapagos Islands, and the chuckwalla from the southwestern United States.
The Fiji iguana is classified as a heterogeneous species living in the middle of the Pacific. The four species living in Fiji and Tonga are currently endangered due to habitat reduction, predation by invasive rodents, and exploitation by poachers supplying the exotic pet trade.
Scientists have speculated, based on several fossils discovered in East Asia, that now-extinct ancestors of iguanas lived along the Pacific coast and migrated to the central Pacific. Some scientists propose that they may have moved from the U.S. to land and sea via the Bering Sea, passing through Indonesia and Australia, or traveled from the Pacific coast of the Americas via Antarctica. There is also an analysis suggesting they migrated from South America to the South Pacific by drifting on rafts along the Humboldt Current, the northbound current of the distant waters off the western coast of South America.
Previous genetic studies regarding the relationship between Fiji iguanas and all other iguanas did not clearly reveal their connections. The researchers at UC Berkeley launched a comprehensive investigation into all genera of iguanas prior to this study to create a family tree. They collected genomic sequences from more than 200 iguana specimens housed in museums worldwide and over 4000 genes.
As data analysis began, one result became distinctly apparent. It was discovered that the Fiji iguana, Brachylophus, is the closest relative to the Dipsosaurus genus. Among these, the most widespread species is the North American desert iguana, Dipsosaurus dorsalis, which is adapted to the intense heat of the deserts in the southwestern U.S. and northern Mexico. Another species in this genus also resides on Santa Catalina Island in the Gulf of California.
Professor Scarpetta stated, 'Iguanas and desert iguanas are particularly resilient to hunger and dehydration, and if there was a group that traveled 8000 km across the Pacific on a plant mass, it would likely be ancestors like desert iguanas.' The analysis conducted by researchers indicates that the two lineages of Brachylophus and Dipsosaurus diverged around 34 million years ago, which does not align with previous theories regarding the origins of the Fiji iguana.
Professor McGuire stated, 'If we cannot pinpoint exactly where Brachylophus came from, then it could almost be from anywhere,' and added, 'It would have been much easier to imagine that Brachylophus originated from South America, given that there are marine and terrestrial iguanas in the Galapagos and that they certainly spread from the mainland to the islands.'
However, this study concluded that the likelihood of the iguanas from Fiji originating in South America is very low. Professor Scarpetta noted, 'While iguanas could have lived on other islands beyond Fiji and Tonga, volcanic islands often emerge easily but also disappear easily,' and added, 'Even if iguanas had lived on other Pacific islands, the evidence has likely vanished.'
The research team plans to analyze the complete genome data of iguana lizards and conduct further studies on their evolutionary relationships. They will also continue research to infer movements and interactions over time and space.
References
PNAS (2025), DOI: https://doi.org/10.1073/pnas.2318622122