The Middle Stone Age (or MSA) was a period of African prehistory between the Early Stone Age and the Late Stone Age. It is generally considered to have begun around 280,000 years ago and ended around 50–25,000 years ago. The beginnings of particular MSA stone tools have their origins as far back as 550–500,000 years ago and as such some researchers consider this to be the beginnings of the MSA. The MSA is often mistakenly understood to be synonymous with the Middle Paleolithic of Europe, especially due to their roughly contemporaneous time span; however, the Middle Paleolithic of Europe represents an entirely different hominin population, Homo neanderthalensis, than the MSA of Africa, which did not have Neanderthal populations. Additionally, current archaeological research in Africa has yielded much evidence to suggest that modern human behavior and cognition was beginning to develop much earlier in Africa during the MSA than it was in Europe during the Middle Paleolithic. The MSA is associated with both anatomically modern humans (Homo sapiens) as well as archaic Homo sapiens, sometimes referred to as Homo helmei. Early physical evidence comes from the Gademotta Formation in Ethiopia, the Kapthurin Formation in Kenya and Kathu Pan in South Africa. There are MSA archaeological sites from across the African continent, conventionally divided into five regions: northern Africa, comprising parts of the modern countries of Morocco, Algeria, Tunisia, and Libya; eastern Africa, stretching roughly from the highlands of Ethiopia to the southern part of Kenya; central Africa, stretching from the borders of Tanzania and Kenya to include Angola; southern Africa, which includes the numerous cave sites of South Africa; and western Africa. In northern and western Africa, the wet-dry cycles of the modern Sahara desert has led to fruitful archaeological sites followed by completely barren soil and vice versa. Preservation in these two regions can vary, yet the sites that have been uncovered document the adaptive nature of early humans to climatically unstable environments.