Alex Davies contextualizes Elon's declaration at the D:

Colonization of new planets, starting with Mars, is crucial to the survival of the human species, SpaceX CEO Elon Musk said Wednesday night at the Wall Street Journal's D: All Things Digital Conference.

That's why SpaceX, the space transport company he runs (in addition to electric automaker Tesla Motors), is working toward putting humans on Earth's neighbor. 

"Either we spread earth to other planets, or we risk going extinct. An extinction event is inevitable and we’re increasingly doing ourselves in."

At SpaceX, "the goal is to improve rocket technology and space technology until we can send people to Mars and establish life on Mars. I agree this is an unlikely outcome, but if we don’t keep improving technology every year, we won’t get there."

SpaceX currently has a contract with NASA to deliver cargo to the International Space Station, via its reusable rockets. 

And while Mars isn't nearly as suitable for human life as Earth is, it's our best bet. "Mars is a fixerupper of a planet," Musk said. "But we could make it work."