A cache of stone tools found on the east coast of the Arabian Peninsula has reopened the critical question of when and how modern humans escaped from their ancestral homeland in eastern Africa. Artifacts unearthed in the United Arab Emirates dating back more than 100,000 years suggest that modern humans first left Africa much earlier than scientists originally believed. The finding, reported in Thursday’s issue of Science, points to the importance of Arabia in understanding the human story. Other questions are whether some social or cultural advance, possibly an evolutionary one, was required for modern humans to escape from Africa and whether modern humans could have interbred with Neanderthals,