Apparently, after defeating Flavius in Assassin’s Creed Origins and taking his Apple of Eden, you can return to the vault beneath the Temple of Amun in Siwa (where Hepzefa was found), reopen it and listen to a secret Isu message be a new First Civilization member. The message states:
Here they are, awakened from their slumber. One by one. So many. A library. A Repository. We extinguished the flame. Knowledge will return. Eventually. The Eye. The Temples. The Vaults and the Apples. Until then, how much benighted use will this power serve? It may take ages until your Kind becomes Enlightened. Who stands in this chamber? At long last, a light has been cast. But I fear your blood is the thickest of shadows.
— Unknown Isu (Assassin’s Creed Origins)
In this message, the Isu member clearly refers to the Vaults and Temples pin-pointed on the holographic globe within the vault which span the entire world (full analysis of the locations, here), some of which are where the Isu worked to prevent the Toba Catastrophe. The fall of the Isu resulted in their knowledge disappearing, which humanity might eventually regain. The Eye is a divination device to read through the calculations and predict potential futures which Minerva created. In the end, the narrator stats that he’s aware that Bayek is listening and laments the possibility that we may never understand this message due the our lack of their sixth sense.
Many thanks to Sparked Synapse who provided me with the video as well as Taranis and NightmareT12 for the discovery. You can watch the footage in the player above.