Early humans would have had several rainy opportunities to move out of Africa and into the normally arid and challenging Arabian peninsula (pictured).

Credit: SeaWiFS Project, NASA/GSFC; ORBIMAGE

Ash Parton of the University of Oxford, UK, and his colleagues discovered layers of sediments laid down by ancient rivers in southeast Arabia, which flowed for several long periods during the past 160,000 years. Those wet spells could have enabled humans to push into the Arabian interior much earlier than some theories have suggested. Since at least 160,000 years ago, monsoon rains would have provided enough fresh water and plants to sustain human migration approximately every 23,000 years.

Geology http://doi.org/2f8 (2015)