The kingdom is also a refuge to thousands of Iraqi Christians fleeing persecution by ISIL. While Jordan continues to accept refugees, the recent large influx from Syria placed substantial strain on national resources and infrastructure. Jordan is classified as a country of "high human development" with an "upper middle income" economy. The Jordanian economy, one of the smallest economies in the region, is attractive to foreign investors based upon a skilled workforce. The country is a major tourist destination, also attracting medical tourism due to its well developed health sector. Nonetheless, a lack of natural resources, large flow of refugees and regional turmoil have hampered economic growth. Jordan takes its name from the Jordan River which forms much of the country's northwestern border. While several theories for the origin of the river's name have been proposed, it is most plausible that it derives from the Semitic word "Yarad", meaning "the descender", reflecting the river's declivity. Much of the area that makes up modern Jordan was historically called "Transjordan", meaning "across the Jordan", used to denote the lands east of the river. The Old Testament refers to the area as "the other side of the Jordan". Early Arab chronicles referred to the river as "Al-Urdunn", corresponding to the Semitic "Yarden". "Jund Al-Urdunn" was a military district around the river in the early Islamic era. Later, during the Crusades in the beginning of the second millennium, a lordship was established in the area under the name of "Oultrejordain". The oldest evidence of hominid habitation in Jordan dates back at least 200,000 years. Jordan is rich in Paleolithic (up to 20,000 years ago) remains due to its location within the Levant where expansions of hominids out of Africa converged. Past lakeshore environments attracted different hominids, and several remains of tools have been found from this period. The world's oldest evidence of bread-making was found in a 14,500 years old Natufian site in Jordan's northeastern desert. The transition from hunter-gatherer to establishing populous agricultural villages occurred during the Neolithic period (10,000–4,500 BC). 'Ain Ghazal, one such village located in today's eastern Amman, is one of the largest known prehistoric settlements in the Near East. Dozens of plaster statues of the human form dating to 7250 BC or earlier were uncovered there and they are among the oldest ever found. Other than the usual Chalcolithic (4500–3600 BC) villages such as Tulaylet Ghassul in the Jordan Valley, a series of circular stone enclosures in the eastern basalt desert−whose purpose remains uncertain–have baffled archaeologists.