In 2033, amateur astronomers discovered a chunk of solar debris whose impact would sterilize the Earth in 2534. Faced with virtually no hope for the future, society began to fall apart.
In 2035, an ambitious inventor and all around jerk managed to craft the first stable wormhole between two particles. This was followed shortly after by detached free-floating wormhole mouths that could act as cameras and microphones. The inventor's family reasoned that if these are holes in the fabric of space-time, if these could reach into the past as well. As we were denied our future, we were granted our past.
The wormhole technology continued to miniaturize, to the point where a wormhole generator could fit inside a human body. By the 2080's a hive-mind was growing that was smart enough to conceive ways to manipulate weather patterns, transforming the Sahara into fertile farmland. Wormholes were by this point stable enough to gather energy from the Earth's mantle, or even directly from the Sun. With the energy/food crises solved, humanity could move on to its bigger enemies.
By 2097, destroying the rock hurtling towards them was simple as thinking of flipping a switch.
Then, in 2107, an ordinary member of the newly joined humanity made a very interesting discovery when he turned his mind to the arid rocks of Promethei Planum on Mars...