A cloud of dust escapes from an excavation site in the sand of Chad's arid north, where scientists are looking for signs of human habitation in an area once humid and called the "Green Sahara".
Kneeling, armed with a brush and trowel inside the largest rock shelter at the Gaora Hallagana site in the Ennedi West province, Djimet Guemona, 35, meticulously removed every layer of sand.
