Capybara relative's 9-million-year-old molar points to a wet interval in the Atacama Desert

A single molar is a thin base for a claim about climate. Its weight comes from the habits of the animal it belonged to. Living capybaras, the largest rodents on Earth, stay close to stable freshwater and eat plants that need water to grow ^[4]. The driest part of the Atacama now gets less than 2 mm of rain a month, with the Chilean Coast Range blocking Pacific rain to the west and the Andes blocking Atlantic rain to the east ^[5][16].

"Finding a capybara tooth in a marine deposit in what today is one of the driest regions on Earth is just freakish," Priscilla Martinez, a University of Arizona geoscientist and co-author of the paper in Scientific Reports, said in a statement ^[7][2].

Samantha Hopkins, who teaches Earth sciences at the University of Oregon and had no role in the research, told Live Science it is unlikely the tooth reached the site without capybaras living there ^[12]. "If you wanted to find a way that this capybara could be here while the Atacama was still as dry as it is today, you'd have a hard row to hoe," she wrote ^[13]. Large carcasses can float for miles, she noted, but "if you're going to invoke a large enough waterway to do this, you've demonstrated that the Atacama wasn't as dry as it is today anyway" ^[14].

The plant record points the same way. Microscopic plant fragments called phytoliths show that palms, grasses, gingers and other flowering plants grew at the site ^[9]. One animal proxy and one botanical proxy agreeing is a stronger design than either alone. Earlier work has reported that the Atacama was once wet enough for crocodiles and freshwater fish ^[10].

Recent research puts the start of the hyperarid core between 47.8 million and 33.9 million years ago, about 20 million years before the Andes formed, and suggests the mountains intensified the dryness without causing it ^[6]. The capybara lived roughly 25 million to 39 million years after that onset ^[17]. The tooth tests whether the desert stayed hyperarid without a break once it began drying, and the evidence from this site says it did not. Live Science frames the result as support for the hypothesis of "a sporadically moist Atacama" ^[11].

How the animals got there is less settled. According to Live Science, the researchers are not sure; the Andes stood as a barrier about 5,500 metres high between the animals' habitats and the fossil site, so the team proposes a route along South America's western coast, following wetlands and rivers ^[19]. The paper's title calls it "a transcontinental dispersal pathway" ^[2]. Capybaras do not live in Chile today, according to the International Union for Conservation of Nature ^[15].

The tooth came from a layer recording a shallow sea between 7 million and 10 million years ago ^[8], a window of three million years ^[18]. The published account does not give the duration of the wet conditions or their extent beyond this one site.