the Mediterranean Sea. The Saharan pump theory posits that this Middle Eastern land bridge was extremely important to the modern distribution of Old World flora and fauna, including the spread of humanity. The area has borne the brunt of the tectonic divergence between the African and Arabian plates and the converging Arabian and Eurasian plates, which has made the region a very diverse zone of high snow-covered mountains. The Fertile Crescent had many diverse climates, and major climatic changes encouraged the evolution of many "r" type annual plants, which produce more edible seeds than "K" type perennial plants. The region's dramatic variety in elevation gave rise to many species of edible plants for early experiments in cultivation. Most importantly, the Fertile Crescent was home to the eight Neolithic founder crops important in early agriculture (i.e., wild progenitors to emmer wheat, einkorn, barley, flax, chick pea, pea, lentil, bitter vetch), and four of the five most important species of domesticated animals—cows, goats, sheep, and pigs; the fifth species, the horse, lived nearby. The Fertile Crescent flora comprises a high percentage of plants that can self-pollinate, but may also be cross-pollinated. These plants, called "selfers", were one of the geographical advantages of the area because they did not depend on other plants for reproduction. History As well as possessing many sites with the skeletal and cultural remains of both pre-modern and early modern humans (e.g., at Tabun and Es Skhul caves in Israel), later Pleistocene hunter-gatherers, and Epipalaeolithic semi-sedentary hunter-gatherers (the Natufians); the Fertile Crescent is most famous for its sites related to the origins of agriculture. The western zone around the Jordan and upper Euphrates rivers gave rise to the first known Neolithic farming settlements (referred to as Pre-Pottery Neolithic A (PPNA)), which date to around 9,000 BCE and includes very ancient sites such as Göbekli Tepe, Chogha Golan, and Jericho (Tell es-Sultan). This region, alongside Mesopotamia (Greek for "between rivers", between the rivers Tigris and Euphrates, which lies in the east of the Fertile Crescent), also saw the emergence of early complex societies during the succeeding Bronze Age. There is also early evidence from the region for writing and the formation of hierarchical state level societies. This has earned the region the nickname "The cradle of civilization". It is in this region where the first libraries appeared about 4,500 years ago. The oldest known libraries are found in Nippur (in