The mastodon fossils were discovered by its paleontologists in 1992 and showed microscopic damage considered to be evidence of human activity. (Credit theNat)

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SAN DIEGO — Fossils unearthed 25 years ago at the site of a San Diego County freeway project, and reexamined more recently, place early humans in North America far earlier than previously thought, the San Diego Natural History Museum announced Wednesday.

According to the museum, the mastodon fossils were discovered by its paleontologists in 1992 along state Route 54, near the city limits between National City and the San Diego neighborhood of Paradise Hills. The fossils showed microscopic damage considered to be evidence of human activity.

They were buried alongside large stones that appeared to have been used as hammers and anvils, and were dated back to 130,000 years ago, according to a study to be published Thursday in the journal Nature.

“This discovery is rewriting our understanding of when humans reached the New World,” said Judy Gradwohl, the museum’s president and CEO.

“The evidence we found at this site indicates that some hominin species was living in North America 115,000 years earlier than previously thought,” Gradwohl said. “This raises intriguing questions about how these early humans arrived here and who they were.”

Until recently, the oldest records of human sites in North America generally accepted by archaeologists were about 14,000 years old. But the mastodon fossils were embedded in fine-grained sediments that had been deposited much earlier, during a period long before humans were thought to have arrived on the continent, according to the museum.

“When we first discovered the site, there was strong physical evidence that placed humans alongside extinct Ice Age megafauna,” said Tom Demere, the museum’s curator of paleontology.

“This was significant in and of itself and a first in San Diego County,” Demere said. “Since the original discovery, dating technology has advanced to enable us to confirm with further certainty that early humans were here significantly earlier than commonly accepted.”

Three years ago, James Paces, a research geologist with the U.S. Geological Survey, used state-of-the-art radiometric dating methods to determine that the mastodon bones — which were still fresh when they were broken by blows from hammerstones — were 130,000 years old, with a conservative error of plus or minus 9,400 years.

“The distributions of natural uranium and its decay products both within and among these bone specimens show remarkably reliable behavior, allowing us to derive an age that is well within the wheelhouse of the dating system,” said Paces, the study co-author.

The specimens went on display at the museum Wednesday, and a public lecture is scheduled for Saturday night.

Scientists affiliated with the Center for American Paleolithic Research, University of Michigan, University of Wollongong in Australia, Adams State University and Anza-Borrego Desert State Park assisted with the project.

The funding came from the National Geographic Society, the Walton Family Fund of Arkansas, San Diegans Pat Boyce and Debbie Fritsch, and two San Diego- based philanthropic organizations — the James Hervey Johnson Charitable Educational Trust and Downing Family Foundation.