Stone Age tools uncovered in Yemen point to humans leaving Africa and inhabiting Arabia perhaps as far back as 63,000 years ago, archaeologists report.

"The expansion of modern humans out of Africa and into Eurasia via the Arabian Peninsula is currently one of the most debated questions in prehistory," begins an upcomingJournal of Human Evolution report led by Anne Delagnes of France's Université Bordeaux. The archaeologists report from the site of Shi'bat Dihya located in a wadi, or gully, that connects Yemen's highlands to the coastal plains of the Red Sea.

The age of the site puts it squarely at a time when early modern humans were thought to be first emigrating from Eastern Africa to the rest of the world. "The Arabian Peninsula is routinely considered as the corridor where migrating East African populations would have passed during a single or multiple dispersal events," says the study.

"It has also been suggested that the groups who colonized South Asia rapidly expanded from South and East Africa along the Arabian coastlines around 60 ka BP (60,000 years ago), bringing with them a modern behavioral package including microlithic (stone) backed tools, ostrich-eggshell beads or engraved fragments. However, this scenario is not supported by any 'hard' archaeological evidence from the Arabian Peninsula. Up until recently, the absence of stratified contexts (archaeological sites) from the entirety of the region has rendered issues concerning the timing and trajectories of the earliest expansions of modern humans into the region largely theoretical."

One new site is the study's subject, Shi'bat Dihya, located along the Wadi Sudud (see map below). Excavating down to a level dating to perhaps 63,000 years ago, when the region was quite arid, the team found some "5,488 artifacts" -- Stone Age blades, pointed blades and pointed flakes, nearly an inch long or longer, as well as the bones of 97 animals, mostly cows, horses, pigs and porcupines.

Finding tool-makers so far inland, nearly 75 miles from the coast, surprised the study team, as most models of human expansion picture our ancestors migrating along the coasts on their way to Europe and Asia. "The adaptation of the occupants of Wadi Sudud to an arid environment significantly nuances the environmental determinism inherent in nearly all models concerning the peopling of southern Arabia," says the study.

"The bioenvironmental setting of the Wadi Surdud basin certainly accounts for the attractiveness of the region, even during arid periods. In the context of the Saharo-Arabian arid belt, the medium altitude foothills form ecological niches with long-lasting and predictable sources of water and herbivores that provided ideal conditions for human settlement."

Most intriguing, the stone tools found at the site fall into the tradition of older Stone Age tools, rather than ones associated with the early modern humans thought to have left Africa roughly 60,000 years ago. They might have belonged to descendants of earlier modern human migrants from Africa who established themselves in Arabia despite its desert conditions. Or maybe they belonged to a sister human species, our Neanderthal cousins, suggest the researchers: