Approximately 400,000 BC: The Earth warms up considerably and stays that way for at least 30,000 years; Beringia is definitely closed
Global sea levels stabilize at 7.5 meters above 1999 AD levels for millennia, then rapidly rise to about 20 meters higher than 1999 levels, remain there for thousands of years, and finally decline back to 1999 levels over a period of a couple millennia.
-- "In Ancient Ice Ages, Clues to Climate" By WILLIAM K. STEVENS, 2-16-99, The New York Times
-- "The big thaw"by Jeff Hecht, Boston, From New Scientist, 17 April 1999
From 128,000 BC - 108,000 BC global sea levels are likely near the same as circa 1999 AD (or slightly higher), thereby insuring that land links such as the Bering landbridge (and many islands) are submerged.
-- "In Ancient Ice Ages, Clues to Climate" By WILLIAM K. STEVENS, 2-16-99, The New York Times and Essay: Climate Future Told Through Mud ["http://www.discovery.com/news/briefs/20000214/weather_interglacial.html"] By Larry O'Hanlon, Discovery.com News, February 15, 2000
MAJOR ENVIRONMENTAL FACTOR: 118,000 BC to 8,000 BC: the last Ice Age; a third of Earth's surface is sheathed in ice for much of this period-- and world geography undergoes drastic changes
For the 15,000 years preceding this time, world sea levels never rose more than two meters above that of the sea level circa 1999 AD. But now sea levels worldwide drop to 1999 AD levels and significantly below between 118,000 and 8000 BC.
-- "Surprise: Geologists Find Glaciers Can Suppress Volcanic Eruptions", 12-8-98, University of North Carolina at Chapel Hill
-- "The big thaw" by Jeff Hecht, Boston, From New Scientist, 17 April 1999
Sudden large climate changes worldwide occurred during the last Ice Age. Apparently there were six events where immense numbers of ice bergs were created in Canada, which then flowed into the Atlantic Ocean. These spurred global climate changes.
-- SIX IMMENSE ARMADAS OF ICEBERGS INVADED THE NORTH ATLANTIC From Science Frontiers Digest of Scientific Anomalies ["http://www.knowledge.co.uk/frontiers/"] #98, MAR-APR 1995 by William R. Corliss, citing Wallace S. Broecker; "Massive Iceberg Discharges as Triggers for Global Climate Changes," Nature, 372:421, 1994
A couple of items to note here:
Humanity's numbers may have dwindled to almost nothing by 69,000 BC and before, due to one or several calamities-- ergo, the reason why there's little evidence of migrations across the land bridge until much later. However, there remains the possibility for a few human predecessors (or other primates) to enter the Americas at this very early date via the bridge. It is likely many animal species did so.
There seems to be significant evidence of humans crossing the bridge in relatively large numbers around 38,000 BC-- which appears to be almost a mad dash across the bridge in the years it may have been re-submerging again, after spending millennia as dry land. Were these first bridge crossers fleeing something? Or is it only coincidence that they crossed just prior to the bridge submerging again?
Mitochondrial DNA analysis indicates most Amerinds (majority of native north americans) arrived in one wave around 38,000 BC- 18,000 BC.
Some early migrations into the Americas possibly came across the Arctic, as well as the Pacific and Atlantic. Artifacts suggesting connections to Iberia and France have been found. The genetic diversity in native Americans strongly implies at minimum 30,000 years of development on the continents (under some reasonably plausible scenarios). The 140 language families of native Americans suggests 40,000+ years of development. The entire Pacific Rim shares ancient commonalities in native languages.
-- ANTHROPOLOGY UNBOUND From Science Frontiers #119, SEP-OCT 1998 by William R. Corliss, citing David Lore; "Bering Strait May Not Have Been Only Route to Americas," Columbus Dispatch, February 17, 1998, and Ann Gibbons; "Mother Tongues Trace Steps of Earliest Americans," Science, 279:1306, 1998
Scientists estimate it should have required 7000 years of progressive migrations for people to reach and settle the vicinity of Monte Verde Chile, if they first entered the Americas by way of the Bering land bridge some 9000 miles away. (If Monte Verde was settled by 31,000 BC, which it appears it was, then allowing 7000 years to reach it from Beringia results in a date of 38,000 BC for crossing the land bridge)
-- The Diffusionists Have Landed by Marc K. Stengel, The Atlantic Monthly, J A N U A R Y 2 0 0 0, http://www.theatlantic.com
Of course, the 7,000 year estimate for arrival cited above may be fatally flawed by way of its assumption that the Monte Verde people came across the land bridge at all; there's mounting evidence that South America received migrations from directly across both the Pacific and Atlantic Oceans as well as through Central America from the north, at perhaps even earlier dates than migrations across Beringia itself.
Humans seem to be living in New Mexico between 38,000 BC and 28,000 BC
-- LONG BEFORE THE VIKINGS AND POLYNESIANS From Science Frontiers Digest of Scientific Anomalies ["http://www.knowledge.co.uk/frontiers/"] #81, MAY-JUN 1992 by William R. Corliss, citing Tim Appenzeller; " A High Five from the First New World Settlers?" Science, 255:920, 1992 and Roger Lewin; "Mitochondria Tell the Tale of Migrations to America," New Scientist, p. 16, February 22, 1992