New research indicates that Homo erectus likely capitalized on a “greener” corridor through the Sahara Desert in northeastern Africa, which was wetter and more vegetated than it is today, during their migration out of Africa. Climate cycles aligned to create this green passage, facilitating their journey.
Apparently the desertification of the Sahara is cyclic.
Approximately every 20,000 years, the Sahara transforms into a savannah covered with lush grasses due to the angle of the Earth’s axis changing. This axis change causes the position of the North African monsoon to shift, a monsoon that could revive the Saharan region. (source)
Here’s a graphic on the timings of early human migration. They list two migrations northeast, one occurring 120k years ago and another 100-90k years ago.