Abstract(#br)East Africa, the region where Merrick Posnansky started his professional career, has long been accepted as the major centre for the study of the origins of hominids and their technological systems. Recently, human geneticists and some palaeoanthropologists have also proposed an African origin for anatomically modern humans ( Homo sapiens sapiens) , although much of the proof cited is from non-African sources — mainly Europe and the Middle East. Fortunately these models have stimulated reassessment of the sub-Saharan Middle Stone Age (MSA), the archaeological phase between 200,000 and 30,000 years ago which represents the beginnings of regional variation in technology and cultural adaptation as well as the period in which modern humans appeared. Long ignored by East African... archaeologists in favour of the earliest cultural record, or, at the other extreme, Neolithic and Iron Age research, the MSA is critical to the evaluation of models of the emergence of our own species. This paper reviews the current arguments concerning the origin and dispersal of modern humans, and the importance of the MSA for the resolution of the problem. It also describes the results from a 1990 survey for MSA sites conducted in southwestern Tanzania within the framework of these current models.