we face extinction everyday. due to multiple cosmic events as well as the human race's own stupidity.
the earth has nere miss' with astorids at liest 5 times a year each one would be an "extinction level event" if it was to hit the earth.
as for the sun its due to expand and go red giant in aproximitly 50,000,000,000 years.
however by that time due to human nature we will more than likely be either on multiple planets or extinct.
the most likely cause of the human race to go extinct is our own stupidity.
my theory, due to human nature when we eventually try to colonize on a different planet, we are bound to piss some "alien" species off far more powerful than we are and get our heads kicked in. its human nature to "negotiate" with other cultures with force. our own history more than proves that.
For us being the most Dominant sentient species on the planet, we are by far one of the dumbest in terms of survival, the only reason we are still around is our ingenuity, resourcefulness and destructive tendency's.
and finally in regards to the myan calender.. they had to of got bored at some point and stopped adding dates to it.