France, Aug. 5 -- In Gabon's sprawling forest, archaeologists dig for ancient clues that could unlock the secrets of how prehistoric humans lived and interacted in the changing landscape of central Africa.

Two billion years ago, the eastern Gabonese region of Lastourville was covered by a vast ocean.

But that has long given way to dense forest and dolomite cliffs dotted with caves, within which scientists have unearthed traces of human life dating back to 25,000 years BC.

Off the beaten track even for archaeologists, the Youmbidi rock shelter, a cavern typical of those chosen by prehistoric humans to set up their homes, is the focus for French geoarchaeologist Richard Oslisly's team.

Among their finds: a stone tool which could have be...