> For someone to cross from Alaska to the southern part of the continent, they would have to sail over 2000km without access to anything but deep ocean and flo

> For someone to cross from Alaska to the southern part of the continent, they would have to sail over 2000km without access to anything but deep ocean and floating ice. Not exactly. The leading explanation is the "Kelp Highway" theory - the complex coastal geography of SE Alaska and ocean currents could have created a small pockets of ice-free land and given access to rich marine resources. You could effectively island hope down the inland passage until you hit the Columbia river. Having made it to Australia 50kya, we know at least some prehistoric humans had the maritime technology to pull it off. My take is, it seems likely that at least some of the Beringians made it south of the ice wall during the glacial maxima. If the ice wall was indeed impenetrable 25kya (ie the kelp highway theory is wrong) , the White Sands footprints must have come from people who migrated even earlier.