Ancient Capybara Tooth Rewrites Atacama Desert Climate History

A tooth no bigger than a fingernail has upended what scientists thought they knew about one of Earth's driest places. Researchers working in northern Chile's Atacama Desert pulled a fossilized capybara tooth from sediment layers at El Morro, a site rich with ancient marine life. The tooth is nearly 9 million years old—and it should not be there.

Capybaras are the world's largest living rodents, semi-aquatic creatures that spend their lives in and around water, grazing on wetland plants. Today, none live in Chile. The Atacama itself is a parched coastal strip, so inhospitable that it has been held up as a model of continuous aridity stretching back roughly 40 million years. Yet here was evidence that a capybara population once thrived in what is now one of the planet's most hostile environments. Priscilla Martinez, a doctoral student at the University of Arizona and lead author of the study published in Scientific Reports, described the discovery plainly: "Finding a capybara tooth in a marine deposit in what today is one of the driest regions on Earth is just freakish."

The tooth came from sediments laid down in a shallow sea between 10 and 7 million years ago, part of the Bahía Inglesa Formation—a fossil-bearing stretch of coast that also contains Cerro Ballena, or Whale Hill, home to the world's largest concentration of whale fossils. This is the oldest and first capybara remains ever found in coastal Chile. Local residents had long known the area held bones of creatures that no longer inhabited the desert, and some had dug up specimens to sell. Among those who received donations from merchants interested in understanding the finds was Carolina Gutstein, head of paleontology at the Chilean mining company Fosfatos de Caldera and a professor at Universidad Santo Tomás in Santiago. Gutstein's working relationships with local fossil experts helped bring these specimens into scientific hands.

But a single tooth, however surprising, might have been dismissed as an anomaly. The sediments told a fuller story. Embedded in the rock were phytoliths—microscopic silica structures that form inside plant cells as roots absorb dissolved minerals from groundwater. Their shapes reveal what plants once grew in a place. Caroline Strömberg, a professor at the University of Washington who specializes in reading this botanical record, found evidence of palms and other flowering plants, including dicots, a major group of flowering species. "Think sunny Southern California full of palm trees," Martinez said. "You don't really expect the Atacama Desert to have palms, and we're not sure if they were blooming extensively there, but they were definitely there." The same deposits held fossils of freshwater fish and gavials—freshwater crocodiles—animals that require flowing water and stable wetland ecosystems.

Mark Clementz, a paleobiology expert and head of the University of Wyoming's Department of Geology and Geophysics, saw in this combination something decisive. "We have multiple lines of evidence indicative of really wet conditions that are typically associated with capybara today," he said. The presence of water, vegetation, and aquatic fauna suggested not a fluke but an ecological community robust enough to sustain capybara populations over time. The question then became: how did they get there?

Capybaras and their close relatives, guinea pigs, are native to South America. Today they inhabit low-elevation wetlands east of the Andes, with their range stretching from parts of Venezuela and Colombia south to northeastern Argentina and Uruguay. The Andes, reaching elevations of up to 18,000 feet, had already formed a formidable barrier by the time capybaras appeared 9 million years ago. For an animal dependent on water, crossing the dry, high plateau would have been nearly impossible. "Not only are the plateau lands of the Central Andes very tall, they're also extremely arid," Martinez said. "So, if capybaras originated east of the Andes in this foreland region, how in the world could these short-legged rodents make it across?"

The researchers propose a route around the mountains. Capybaras move through corridors of suitable habitat along creeks and rivers. Connected low-elevation wetlands may once have allowed them to spread along the coast in a counterclockwise path around northern South America—from present-day Venezuela through Colombia, Ecuador, and Peru into northern Chile. The discovery raises a larger question: when did this part of the Atacama become persistently dry? Clementz noted that evidence of a wetter past had been overlooked under the prevailing view of an ancient desert. "No one really argued with that paradigm, even in the face of these other fossil finds suggesting that things were different in the past," he said. "Finding this capybara—which I think everyone can agree is not a desert animal by any means—at that location is going to challenge many of the ideas about the Atacama's history."

The implications extend beyond paleontology. Deserts are sensitive to climate shifts, including changes in monsoon intensity. Understanding how the Atacama transformed in the deep past, when human activity played no role, provides a baseline for predicting how such landscapes will respond to future climate change. As Martinez put it: "The more we understand deserts, which are extremely sensitive to changes in climate, the better we can predict how they fare long term."