At the time sea levels were about 200 feet (61 meters) lower than they are today, and many of the early coastal settlements may now be underwater.
Ancient Monte Verde was situated on a small tributary of a large river, and sat more than 50 miles (80 kilometers) from the coast and about 10 miles (16 kilometers) from a large marine bay.
The site has well-preserved ruins of a small settlement that likely supported 20 to 30 people living in a dozen huts.
Many types of food, including extinct species of llama, have been found at the site.
When Dillehay and his colleagues first excavated the site in 1979, they found several species of seaweed there. (The excavation was partially funded by the National Geographic Society, which owns National Geographic News.)
"This indicated that people living inland had occasionally gone to the coast to exploit marine beach resources," Dillehay said.
"But we didn't give it much thought. We thought it was just one component of their economic realms."
When they went back to analyze material collected in the 1980s, the scientists identified nine species of marine algae recovered from hearths and other cooking features at the site, suggesting the people living there ate the plants.
Using radiocarbon dating, they dated the seaweed samples to between 14,220 to 13,980 years old.
The scientists found pieces of seaweed mixed with other plants and chewed into clumps, possibly for medicinal purposes.
They also found a stone cutting tool with seaweed stuck to the blade.
"Seaweed is difficult to preserve, which means we're just looking at a fraction of what the seaweed contribution to the diet probably was," Dillehay said.
The Monte Verdeans' familiarity with the coast suggests that they may have initially migrated down the coast and perhaps spent some time there before moving inland, he said.
Michael Waters is the director of the Center for the Study of the First Americans at Texas A&M University.
"The new evidence from the Monte Verde site confirms its status as the earliest known human settlement in the Americas and provides additional support for the theory that one early migration route followed the Pacific Coast more than 14,000 years ago," he said.
But Dillehay says the coastal migration may have been slower than is commonly believed.
The discovery of plentiful inland resources at the Monte Verde site, including meat from an elephant-like animal called gomphothere, suggests that the people living there moved back and forth between different ecological zones, he noted.
That kind of movement takes considerable time to adapt to, he explained.
"If Monteverde is indicative of what some early migrants are doing, then I think this migration down the coastline may not have been this blitzkrieg movement" that many scientists suggest, he said.
"It may have been a much slower movement of people exploiting simultaneously coastal and river resources."
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