The answer, I suppose, largely depends on the meaning of the question.
Does it mean when the earth ceases to exist? Or does it mean when life as we know it ceases to exist?
For the latter, I would fancy humans destroying themselves before aliens had half a chance.
For the former, I'd subscribe to "the universe doesn't have the gravitational force to continue expanding and will eventually implode into the singularity from whence it came" theory (which other, more notable, people have more eloquently put than I just did
)
By the time any of this happens, I should have posted my tax return. Phew.