The Later Stone Age (or LSA) is a period in African prehistory which follows the Early Stone Age and Middle Stone Age. The Later Stone Age along with the Early Stone Age and Middle Stone Age are often confused with the Lower Paleolithic, Middle Paleolithic, and Upper Paleolithic. In the 1920s, it became clear to archaeologists that the existing chronological system of Upper, Middle and Lower Paleolithic were not a suitable correlate to the prehistoric past in Africa. The terms Early, Middle, and Later Stone Age were developed to address this issue. Some scholars, however, still view these two chronologies as parallel, arguing that they both represent the development of behavioral modernity. The Later Stone Age is associated with the advent of modern human behavior in Africa, although definitions of this concept and means of studying it are up for debate. The transition from the Middle Stone Age to the Later Stone Age is thought to have occurred first in eastern Africa between 50,000 and 39,000 years ago. It is also thought that Later Stone Age peoples and/or their technologies spread out of Africa over the next several thousand years.