To quote a guy called Elon Musk:

"Things have happened quickly. It took us millions of years to evolve into what we are, but in the last sixty years, with atomic weaponry, we've created the potential to extinguish ourselves. And if it's not us, it will inevitably be something else. If not a meteorite in the relative short term, then the expansion of the Sun's corona. It will happen."

He has also said that there are a number of other threats with the ability to extinct life on Earth. They include an engineered virus unleashed by terrorists, an inadvertent creation of a micro black hole, or a yet unknown technology.