"NEIL deGRASSE TYSON: In its infancy, Earth was a primeval hell, a lifeless planet bombarded by massive asteroids and comets. The moon, much closer to Earth, loomed large in the sky. Instead of water, red hot lava streamed across the surface of our planet. Volcanoes spewed noxious gases into the primitive atmosphere. Scorched and battered, Earth was a planet under siege. Yet somehow, the world we call home emerged from these violent origins.

So how did Earth make such an astonishing transformation? How did it change from a raging inferno like this, to a place we all know and love, with firm ground under our feet, air we can breathe, and water covering nearly three quarters of its surface? A place where life could take hold and evolve into complex organisms like you and me?

Well, it turns out, Earth became a habitable planet only after a series of devastating disasters in its early years. And to see how this happened, let's imagine all of Earth's four-and-a-half-billion-year history condensed into a single day, just 24 hours on an ordinary clock or watch like this.

If we start right now, then the first humans walked the Earth only 30 seconds ago. Dinosaurs began roaming the planet just before 11 p.m. The first multi-celled animals evolved at 9:05. Before that, mostly single-celled organisms existed, and we think the first of those appeared around 4 o'clock on the morning.

Earth was born at midnight on this 24-hour clock, 4.5 billion years ago, but its violent history began well before that, when huge ancient stars that had reached the ends of their lives exploded. These supernovas cooked up all the chemical elements we know today including iron, carbon, gold and even radioactive elements like uranium. Over time, gravity took hold, and this cloud of stardust collapsed into an enormous rotating disk: the solar nebula.

In the center of this disk, temperature and pressure rose, and a star, our sun, was born. Eventually, gases like hydrogen and helium would be swept to the far reaches of the disk, but closer to the sun were dust grains made of the heavier elements."
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PBS NOVA screen shot from 'A Brief History of Life' slide show

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