From the end of the Ice Age 11,500 years ago to Ancient Egypt, the Sahara was not the absolute dry and hostile place. The Sahara was strewn by oases, megalakes, swamps, large rivers. Giraffes, crocodiles and lions lived there.
The origin of Green Sahara ? higher obliquity of Earth's spin axis, warmer ocean, thus induced more precipitations.

Here the evolution of Saharan environment every year from the last Glacial Maximum 21,000 years ago to nowaday.

DISCLAIMER : This video is based on numeric simulations, this may not be fully accurate regarding how was the reality. In my opinion, these simulations underestimate past precipitations over Sahara during the African Humid Period, regarding pollen-based paleoenvironmental reconstructions and prehistoric fauna rock art found everywhere in north Africa.

------------- REFERENCES---------------------
Climate zones displayed in the video is based on the Köppen-Geiger classification :
https://en.wikipedia.org/wiki/K%C3%B6...

Simulated climate data (temperature, precipitation): PaleoView Software
https://onlinelibrary.wiley.com/doi/f...

These data were downscaled by using MACHISPLIN and SpatialEco packages (R code)

Sea-level variation data : https://www.nature.com/articles/natur...

Megalakes in the Sahara and paleorivers : https://www.cambridge.org/core/journa...

Megalake Chad paleo level : https://www.pnas.org/content/112/28/8....

Evidences of an giant paleo river in N-W Africa, the Tamanrasset river :
https://www.ncbi.nlm.nih.gov/pmc/arti...

About Saharan rock art : https://en.wikipedia.org/wiki/Saharan...

----------------- SOUNDTRACK -------------------

Starcraft II soundtrack - Prophecy
Andrea Krux - Sinbad Tales
World of Warcraft Burning Crusade - Zangarmarsh theme
World of Warcraft Cataclysm - Stranglethorn Vale theme
Immediate Music - Ra
Epic Middle Eastern War Music- Escape From The Hell