97,445 BCE
From Halopedia, the Halo wiki
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The date 100,000 B.C.E. is the approximate time when The Forerunners fired the Halos[1], what the Covenant refer to as The Great Journey. The activation killed all sentient life in three radii of the galactic center including the Forerunners themselves. The date, 100,000 years ago, is around the time our Homo sapiens ancestors began migrating out of Africa. Also the time period when the Halo 3: The Cradle of Life mini comics takes place. These comics tell us that human beings saw something (the portal to the Ark) being built by "Gods" (Forerunner machines) in Africa.
