If these beasts are still around in 13,000 BC, there's a good chance human beings are encountering them-- at least on rare occasions.
-- Terror, Take Two
Discover Magazine, found on or about 9-1-99
Apparent remains of "...large flightless predaceous birds..." from as recently as 8,000 BC have been discovered by Texas A&M University-Kingsville researchers.
-- TAMUK Scientists Recover Mastodon, Other Species that Once Roamed the Lower Nueces River Valley, found on or about 9-1-99 (TAMUK stands for Texas A&M University-Kingsville); An original phone number related to this report included the Connor Museum at (512) 595-2810.
It appears the earliest migrations into North America via the Bering land bridge (before 13,000 BC) take place along the western coast rather than inland. At this time glaciers cover much of North America, with inland routes perhaps almost as difficult to traverse as crossings of the North Pole. The coasts by contrast offer some respites from the ice for travelers.
Around 13,000 BC to 12,000 BC the continental glaciers have sufficiently retreated to offer more hospitable inland routes south through the continent than before. This allows perhaps larger mass migrations to proceed inland compared to the traveling possible along the coast, for both people and animals.
There appears to be frequent and intense conflict occurring among these early migratory peoples.
-- Mystery of the First Americans, NOVA, PBS, 2-15-2000
The possibly large numbers of people which have piled up in the northwestern Yukon due to migrations from Asia, plentiful food and game, and the obstacles of ice sheets and bears blocking their way south and east up to now, are under growing pressures to expand beyond this region, into the rest of the continent.
The ice sheets are disappearing. Food is getting harder to come by locally. More people keep coming across the bridge from Asia. Things are getting crowded, and people are getting testy. More and more fighting is breaking out. Better quality weapons only add to the carnage. The improved weapons, combined with hardwon experience regarding the enormous bears of the continent, are building up men's confidence to push forward.
Much of the population may soon be on the move.