Great Breakout

The Great Breakout was a period in history, around the year 5000, in which humanity spread out to the stars. This also meant the solar system became more busy, as large amounts of frontiersmen and pioneers got concentrated in the asteroid belt while waiting to spread out in the Galaxy.

The Doctor noted that the spread of humanity at this time was like a tidal wave...or a disease. He added that when humans get together in large numbers other lifeforms sometimes suffer. (DW: The Invisible Enemy)

This event apparently took place several hundred years after the Revolution of the Ood, at which time the Second Great and Bountiful Human Empire already controlled three galaxies. (DW: Planet of the Ood) It is possible that the nature of the Great Breakout was a leap forward in human space technology, or possibly the end of a "dark age" in human space exploration (perhaps following the Second Great and Bountiful Human Empire's collapse). Notably, around the same time, World War VI broke out between regional superpower blocs on Earth, although it was resolved swiftly by the collapse of the heavy-handed Supreme Alliance. (DW: The Talons of Weng-Chiang)