Gaia - Theia

Few people realize this, but our world is, in fact, two.
Four and a half billion years ago, when the solar system was taking shape, many of the planets were out of alignment. Some were born sharing the same orbit around our young star.
That was the start of Earth's true twin, Theia. Roughly the size of mars, it straddled about sixty degrees from Earth and shared many of the same traits. This included all of the same problems, though. Neither planet had a consistent length of day, and both were lakes of fluid magma. Life was impossible for either planet.
Over the next fifty million years, though, gravitational pulls from Venus warped the path of the smaller planet, bringing it ever closer towards its twin. Then, in an instant, the planetary relationship was over. Theia burst through the atmosphere travelling over fourteen thousand kilometers per hour. It hit at a low angle, sending trillions of tons of debris into space. What was left of that planet sunk into ours, and the exoplanet was no more.
Some good came from Theia's death, though. The angle at which it hit set Earth on a more regular spin cycle, and the molten debris collapsed into what would eventually become our moon. This gave our world a chance to stabilize, cool off, and hold liquid water on its surface. Over the next four billion years, this sacrifice would lead to the formation of every single life-form Earth has record of.
Just something to think about.

Rendered in Photoshop.