Evidence of slavery predates written records, and has existed in many cultures.[3] Prehistoric graves from about 8000 BC in Lower Egypt suggest that a Libyan people enslaved a San[disambiguation needed ]-like tribe.[16] Slavery is rare among hunter–gatherer populations, as slavery is a system of social stratification. Mass slavery also requires economic surpluses and a high population density to be viable. Due to these factors, the practice of slavery would have only proliferated after the invention of agriculture during the Neolithic Revolution about 11,000 years ago.[17] The earliest records of slavery can be traced to the oldest known records, which treat it as an established institution, not one newly instituted. The Code of Hammurabi (ca. 1760 BC), for example, stated that death was prescribed for anyone who helped a slave to escape, as well as for anyone who sheltered a fugitive.[18] The Bible refers to slavery as an established institution.[3] Slavery was known in civilizations as old as Sumer, as well as almost every other ancient civilization, including Ancient Egypt, Ancient China, the Akkadian Empire, Assyria, Ancient India, Ancient Greece, the Roman Empire, the Islamic Caliphate, and the pre-Columbian civilizations of the Americas.[3] Such institutions were a mixture of debt-slavery, punishment for crime, the enslavement of prisoners of war, child abandonment, and the birth of slave children to slaves.