The excavations began in 1956, when the Argentine archaeologist Humberto Antonio Lagiglia started carefully peeling back the floor of the shelter. He found stone tools and what he proposed were ancient hearths dating to the end of the Pleistocene, the last great Ice Age. The site sorts itself into four stacked stages, each a different chapter of occupation. The deepest, called Atuel IV, holds the bones of large animals and possible fire pits. Above it, Atuel III and Atuel II record the slow arrival of early cultures and farming. The youngest layer, Atuel I, is decorated with pictographs painted on the rock, some of them showing Spanish soldiers and boats coming ashore, the moment the ancient world of the cave collided with the modern one.

The giant ground sloth is the cave's most haunting tenant. Excavators found dermal ossicles, the small bones embedded in the creature's skin like chain mail, proof that these enormous animals lived right here. In 1997, researchers Austin Long and Paul S. Martin ran radiocarbon dating on charcoal, dung and bone from the deepest layers. They found a narrow window, between roughly 12,880 and 12,000 years ago, when the dates for human fires and sloth remains overlapped. And yet, tellingly, no dung or bone was ever found sitting directly in the ashes of a hearth. The evidence flirts with a smoking gun without quite producing one, which is exactly why the site matters so much to the long argument over whether early hunters or a changing climate finished off the Ice Age megafauna.

There is a quieter suspect than the spear: the weather. Scientists have read the pollen trapped in the cave's layers back as far as 30,000 years, and around 10,000 years ago the record changes sharply. The variety of pollen collapses, suggesting the land around Gruta del Indio turned harshly arid as temperatures climbed, the green retreating before an expanding desert. Perhaps the giants did not so much fall to hunters as starve in a shrinking world. Later still, pollen from about 2,500 years ago tells of farmers who settled in the cave, first favoring hardy plants like Chenopodium and amaranth, then bringing beans, squash and maize as agriculture spread into the region.

Not every clue in the cave points to drama. Among the finds are 599 tiny bones from small mammals, and the science of taphonomy, the study of how a living thing becomes a fossil, reads them like a coded report. Few show cut marks or burning. Instead they bear the chemical signature of digestion and turn up beside owl pellets, meaning most arrived not on a human plate but in the gut of an owl roosting in the shelter long ago. Even the cave's ancient excrement has been examined, yielding parasites once carried by birds, reptiles and mammals, including species that can pass to humans. It is a reminder that a rock shelter is never just a stage for big events. It is a whole ecosystem, fossilized layer by patient layer, waiting to be read.