In The Smithsonian, Guy Gugliotta has an article taking a look at archeological evidence for very early human habitation in the Western Hemisphere, predating the Clovis tool-making culture which existed between 13 and 13.5 thousand years ago. It's a good overview of the state of affairs.
The discovery of numerous artifacts that pre-date Clovis has, over the years, required scholars to come up with different ideas about not only when people arrived in the Americas but how they got here. For instance, if they were already established 14,800 years ago, they must not have used the famed ice-free corridor through North America: Researchers say that it would not appear for another 1,000 years.
Maybe the first Americans didn’t walk here but came in small boats and followed the coastline, some researchers say. That possibility was first suggested in the 1950s with the discovery of Clovis-era human bones—but no artifacts—on Santa Rosa Island in the Santa Barbara Channel off the California coast. Over the past decade, though, a joint University of Oregon-Smithsonian team of archaeologists unearthed dozens of stemmed and barbed projectile points from Santa Rosa and other Channel Islands, along with the remains of fish, shellfish, seabirds and seals. Radiocarbon dates showed much of the organic material was about 12,000 years old, roughly within the Clovis time frame.
The findings do not prove that the continent’s first settlers came by sea, of course. The islands were only about four miles offshore at the time, and could have been visited by people who’d settled on the mainland. Still, the sites establish that these island dwellers were seafarers of a sort and accustomed to a seafood diet.
Jon Erlandson, a University of Oregon archaeologist, and Torben Rick, an anthropologist at the Smithsonian National Museum of Natural History, propose a pre-Clovis “kelp highway” for coast-hugging seamen skirting the southern edge of the Bering land bridge on their way from northeast Asia to the New World. “People came between 15,000 and 16,000 years ago” by sea, and “could eat the same seaweed and seafood as they moved along the coastline in boats,” Erlandson said. “It seems logical.” The notion that ancient people could travel great distances by boat isn’t far-fetched; many anthropologists believe that humans voyaged from the Asian mainland to Australia 45,000 years ago.
Though Erlandson said he’s convinced that the Clovis people were not the first in the Americas, he acknowledged that definitive proof of a pre-Clovis coastal route may never be found: Whatever beach settlements existed in those days of especially low sea level were long ago submerged or swept away by Pacific tides.
Moreover, the Channel Islands projectiles have nothing in common with Clovis points, as Erlandson pointed out. They appear to be related to a different toolmaking approach called the western stemmed tradition; featuring stems of different shapes that attach the projectile points to spears or darts, they were prevalent in the Pacific Northwest and the Great Basin. And they do not have the fluting characteristic of Clovis. Those observations strengthen the view that other tool-making human cultures were present in the Americas at the same time as the Clovis people, and in all likelihood beforehand as well.