Why is Earth still habitable after billions of years? In part, we're just lucky.

When you look around you, almost anywhere on Earth, you see life. Earth seems exquisitely supportive of life: We see it in the air, in the water, in the land, and even deep underground.

But was that inevitable? We know that there have been mass extinction events in the past, some taking out the majority of life on Earth. However, since life got started and spread around on Earth there hasn't been any event that completely eradicated life. Of course! Else we wouldn't be here to ponder it.

Still...