The geographical origin of modern humans is the subject of ongoing scientific debate. The `multiregional evolution' hypothesis argues that modern humans evolved semi-independently in Europe, Asia and Africa between 100,000 and 40,000 years ago, whereas the `out of Africa' hypothesis contends that modern humans evolved in Africa between 200 and 100 kyr ago, migrating to Eurasia at some later time. Direct palaeontological, archaeological and biological evidence is necessary to resolve this debate. Here we report the discovery of early Middle Stone Age artefacts in an emerged reef terrace on the Red Sea coast of Eritrea, which we date to the last interglacial (about 125 kyr ago) using U-Th mass spectrometry techniques on fossil corals. The geological setting of these artefacts shows that early humans occupied coastal areas and exploited near-shore marine food resources in East Africa by this time. Together with similar, tentatively dated discoveries from South Africa this is the earliest well-dated evidence for human adaptation to a coastal marine environment, heralding an expansion in the range and complexity of human behaviour from one end of Africa to the other. This new, widespread adaptive strategy may, in part, signal the onset of modern human behaviour, which supports an African origin for modern humans by 125 kyr ago.