The people of Upward Sun River—who lived near Alaska’s Tanana River from 13,200 to 11,500 years ago—left very little behind. No pottery, no permanent architecture, and only a few burials of infants. But now, scientists have reconstructed their diets by using one of the few traces of their presence: their cooking hearths. Seventeen hearths from different time periods have been discovered at the site in central Alaska, all quite well preserved. The scientists wondered whether they could use those hearths in a way similar to archaeologists who have recently reconstructed other ancient diets using chemical isotopes stored in bones or left behind on ceramics. The scientists at Upward Sun River examined the hearths for specific isotopes of carbon and nitrogen and calculated the ratios of one isotope to another. That allowed them to determine whether the food cooked there came from land animals and plants or aquatic ones. The oldest hearths, which were used 13,200 years ago, showed no signs of fish, just waterfowl and mammals. But in the later hearths, dated to 11,800 and 11,500 years ago, high nitrogen values pointed to significant consumption of fish. What’s more, the carbon ratios indicated that those fish came from both marine and freshwater. And because Upward Sun River isn’t anywhere close to the ocean, the marine species must have been salmon, which migrate from the sea into rivers every year to spawn, the researchers report today in the Proceedings of the National Academy of Sciences. Salmon bones had already been recovered from the 11,500-year-old hearths, but not the 11,800-year-old one, making this analysis the earliest evidence of salmon consumption in the Americas.