Just offshore from the chock-a-block development of Southern California, archaeologists have discovered some of the oldest sites of human occupation on the Pacific Coast.

On Santa Rosa Island, one of the Channel Islands just 65 kilometers from Santa Barbara, nearly 20 sites have been found that reveal signs of prehistoric human activity, from massive middens of abalone shells to distinctive stone points and tool-making debris.

At least nine of the sites have what archaeologists say is “definitive evidence” of ancient Paleoindian occupation, about half of them having been dated to 11,000 to 12,000 years ago — making their inhabitants some of the earliest known settlers of North America’s West Coast.

“Finding these sites and the definitive evidence for early occupation is crucial and tells us that people were there, occupying the landscape at the end of the Pleistocene,” said Dr. Torben Rick of the Smithsonian Institution, who led the survey that uncovered the sites.

The discovery adds hefty new data to the already mounting evidence that maritime Paleoindians — also known as Paleocoastal peoples — lived along the California coast at the end of the last ice age.

Such finds have important implications for the history of human migration, suggesting that at least some of America’s earliest settlers moved south from Alaska along the coast, rather than farther inland, where retreating glaciers are thought to have allowed passage to the continent’s interior.

Uncovering hard evidence of this coastal migration has proved challenging, however, because the shorelines that Paleocoastal people would have followed have long since been submerged by rising seas.

But in recent years, surveys of California’s modern-day coasts and islands have turned up several prehistoric sites that are still on dry land, farther inland from the now-submerged shores.

It was while studying some of these sites on San Miguel Island — another of the Channel Islands — that Rick and his colleagues made a key observation: They noted that Paleocoastal settlements tended to have certain traits in common that made them more suitable than sites right on the water.

The earliest sites tended to be 1 to 7 kilometers from where the shoreline used to be, for example, in elevated areas that offered commanding views of the coast and often the island’s interior. Optimal locations were also near sources of useful raw materials, like chert for making tools, as well as fresh water and rockshelters or caves for refuge.

With these factors in mind, Rick’s team turned to Santa Rosa Island to survey its previously unexplored southwestern coast. [Find out what similar predictive models have recently found: “13 Ancient Villages Discovered in Wyoming Mountains May Redraw Map of Tribal Migrations“]

The island was already famous as the home of Arlington Man, perhaps the oldest human remains ever found in North America, discovered in 1959 and dated to 13,000 years ago.

But the southwestern portion of the island had received little scientific attention, and it fit many of the criteria Rick’s team had identified. What’s more, it was the steepest part of the island, which meant it had been less susceptible to rising sea levels and more likely to still hold evidence of early sites.

Upon surveying the area, the team found 19 sites that showed signs of human occupation, mostly middens, or piles of detritus left over from generations of tool making and food preparation.

Although they were essentially prehistoric trash piles, these middens offered a wealth of useful archaeological clues, some deposits covering more than 75,000 square meters (over 18 acres).

Nine of the these sites contained the distinctive Channel Island barbed stone points that are indicative of Paleocoastal culture from the late Ice Age, Rick reported, and several also contained caches of shells from red abalone — a staple food of Paleocoastal Indians.

“They probably used boats since they had to get to the island, and they hunted a variety of marine birds, seals and sea lions and collected shellfish,” Rick said. “These are all early clues to human life ways at the [late] Pleistocene.”

The large amounts of shells, found with stone tools several kilometers from the ancient shoreline, suggest that the shellfish were carried inland to be processed, Rick said.

And even more important, the shells — unlike stone — can be radiocarbon dated. All four of the abalone shell middens returned dates from similar ranges, from 10,900 to 12,100 years ago.

Santa Rosa’s ancient inhabitants also left behind a number of unusual crescent-shaped tools made of chipped stone, artifacts similar to those found throughout the Great Basin, typically near water, but whose exact purpose, Rick said, was “a topic of debate.”

“People have speculated from everything like hunting to even brain surgery — a bizarre 100-year-old idea,” he said. “Today, we think they … would have been used to hunt aquatic birds and possibly other fauna.” [Read about another unusual find in California: “Mass Grave of ‘Prodigal Sons’ in California Poses Prehistoric Mystery“]

One sign absent from any of the newly discovered sites, however, was evidence of construction.

“Unfortunately, there are no clear signs of a structure,” Rick said. “We are hoping to go back soon to one of these sites to excavate a larger area, and we hope to find some of this evidence.”

The discoveries on Santa Rosa will likely bring more attention to the Channel Islands and Southern California for future research into the early coastal settlement of the Americas, Rick said.

But more research at these and other sites is still needed to help clarify the breadth and depth of the first Americans’ occupation on the Pacific Coast, he noted.

“Now the important thing to do is excavate some of these sites in detail to see what more we can learn about ancient cultural practices, environmental changes, and other variables,” he said. “We of course, want to do other surveys like this to find other contemporary sites.

“As excited about these finds as we are, to us they inspire more work.”

Rick and his team report their findings in the Journal of Field Archaeology.

Torben C. Rick, Jon M. Erlandson, Nicholas P. Jew, & Leslie A. Reeder-Myers (2013). Archaeological survey, paleogeography, and the search for Late Pleistocene Paleocoastal peoples of Santa Rosa Island, California Journal of Field Archaeology DOI: 10.1179/0093469013Z.00000000065