13,000 BC

When I tried to actually research the comet impact that my wife was talking about online that supposedly happened in 7021 BC I wasn't able to find it. It is possible that this research is somewhere else, I don't know but the event I was able to find scientific coroboration for with three addresses

http://archaeologica.boardbot.com/viewtopic.php?t=2107&sid=1ac2175cac76f89c4d52bfc73bf3df77

http://news.softpedia.com/news/Ancient-Comet-Exploded-Over-Earth-Nearly-Wiped-Out-Life-In-Northern-Hemisphere-55197.shtml

http://archives.chicagotribune.com/2009/jan/02/local/chi-comet-webjan02

begin quote:

First an explosion as powerful as thousands of megatons of TNT rained meteorites down on North America. Then forest fires broke out across the continent, sending up a thick layer of soot and dust that blocked out the sun. A sudden ice age ensued, and some of the Earth’s largest animals went extinct in a blink of geological time.

It’s well-known that a meteorite colliding with Earth is considered the most likely reason why dinosaurs died off 65 million years ago. Now a team of scientists says it has found new evidence that a comet triggered a similar extinction much more recently: 13,000 years ago, when humans were around to witness the event and suffer its terrible consequences.

The researchers also think that when the comet exploded above the planet’s surface—ultimately killing off mammoths, saber-toothed tigers and other large mammals that roamed North America—Chicago wasn’t far from ground zero.

“If you’d been in Chicago back in that time, it would’ve been one very bad day,” said Allen West, an Arizona geophysicist and one of the authors of a paper appearing Friday in the journal Science. end quote.

If this is true then any developed civilization in North America 13,000 years ago would have likely been wiped out at that time along with most large animals and humans unsheltered by caves or underground tunnels. Also, the oxygen might have been temporarily removed from extreme combustion so there might have been no oxygen temporarily at that time as well(at least on the surface for humans or other creatures exposed to outside air).