97,445 BCE

In 100,000 BCE, (approx.) the Forerunners fired the Halo Array. The activation is a last resort intended to end the Forerunner-Flood War, killing all sentient life within three radii of the galactic center, including any Forerunners not located on The Ark or within Shield Worlds.

The Forerunner AI known as Mendicant Bias was designed to battle the Flood but he was seduced by Gravemind and descended into rampancy. He allowed the Flood into the Maginot Sphere, which gave them the opportunity to attack the Forerunners' inner colonies. This forced the Forerunners to fire the rings and effectively commit "mass suicide." Part of his personality construct was placed in a Keyship which eventually ended up on the San 'Shyuum homeworld.

Some time after every sentient species, carefully indexed in the Ark, is returned to their homeworlds as a part of the Library Project, the Homo sapiens species begins migrating out of Africa.

Trivia

 * 343 Guilty Spark put the date of the firing of the Array as the year 97,448 BCE, though it should be noted that he measured it on Installation 04's year, not the 365.25 day Earth calendar.