Our world has been in almost never-ending conflict. For the past 3,400 years, humans have been at peace for only 268. We killed 108 million of our own in the 20th century alone, and the estimates for the rest vary wildly from 150 million to a staggering 1 billion. 

We may have amassed great knowledge over the centuries, but hatred for our fellow humans remains on the same primal level it has been since we first walked the Earth. Then there's pollution and global warming that raise concerns over sustainability of our way of life. Whether it's purely cyclical, or human-induced (or, most likely, both), many bad things await us as atmospheric temperatures slowly rise and ice caps melt. 

And what about comets? Our solar system is full of chunks of rock larger than a few miles in diameter that could destroy all life on our planet. A 10-mile comet or asteroid that struck the Yukon peninsula in Mexico 65 million years ago is thought to have wiped out most of the dinosaurs, so if a 60-mile comet hits us, it is unlikely that anything would survive.