Ancient Arrival: DNA Shows Iguanas Colonized Mexican Island Millennia Before Humans

*Scientists overturn a long‑standing “invasive‑species” label after genetic analysis reveals the reptiles reached the island hundreds of thousands of years ago.*A team of researchers from the National Autonomous University of Mexico (UNAM) and the Smithsonian Institution has upended a decades‑old assumption about the iguanas that dominate the rocky shoreline of Isla del Sol, a small landmass off the coast of Baja California. While the lizards have been treated as a recent, human‑introduced invader, fresh DNA evidence demonstrates that they colonized the island long before the first people set foot there—by at least 300,000 years.A surprise from the labThe study, published this week in *Molecular Ecology*, compared mitochondrial DNA from living iguanas with fragments extracted from sub‑fossil bones recovered in a series of cave deposits on the island. The genetic signatures, calibrated with radiocarbon dating of associated charcoal layers, placed the earliest iguanas on Isla del Sol at roughly 300,000 years before present, a timeline that predates the earliest archaeological evidence of human activity in the region by more than 250,000 years.“Finding such an ancient lineage was completely unexpected,” said Dr. Mariana López, the project’s lead author and a herpetologist at UNAM. “We originally set out to test whether the island’s population was genetically distinct from mainland relatives, not to rewrite its entire colonization history.”From invasive to nativeFor years, conservation officials classified the iguanas as an invasive threat to native flora and to the island’s nesting seabirds. The label prompted periodic culling campaigns and strict biosecurity measures aimed at preventing further introductions. The new genetic data, however, recasts the reptiles as a native component of the island’s ecosystem, having evolved in situ for hundreds of millennia.“This is a textbook example of how molecular tools can reshape our perception of biodiversity,” noted Dr. James Patel, a senior scientist at the Smithsonian’s National Museum of Natural History, who co‑authored the paper. “It forces us to rethink management policies that were based on an incomplete historical narrative.”How did they get there?The researchers propose that the iguanas arrived via natural rafting—clinging to floating vegetation or debris carried by ocean currents during storm events. Such long‑distance dispersal, while rare, has been documented in other reptile groups and is supported by the genetic similarity between the island iguanas and a mainland population from the nearby Sierra de la Laguna.Implications for conservationThe reclassification carries immediate practical consequences. Under Mexican environmental law, native species receive protection, whereas invasive taxa can be legally removed. The Ministry of Environment has announced a review of its current management plan for Isla del Sol, with a view to halting culling operations and focusing instead on habitat preservation.Local stakeholders, including ecotour operators who market the island’s “unique reptile population,” have welcomed the findings. “Our visitors love the iguanas,” said Carlos Méndez, owner of a small dive shop in La Paz. “Knowing they’re a natural part of the island’s history makes the experience even richer.”Future researchThe team plans to broaden its investigation to other islands in the Gulf of California, where similar assumptions about invasive reptiles persist. By constructing a more comprehensive phylogeographic map, scientists hope to clarify the patterns of natural colonization across the archipelago and to refine criteria used to label species as invasive.“This case reminds us that nature often has a deeper timeline than our human records,” Dr. López concluded. “Recognizing that depth is essential for making informed, ethical decisions about wildlife management.”