The Roman Empire depended heavily on maritime trade, which was a much more efficient way of importing grain from Egypt, the absolute staple for maintaining a quiet population, and the alternative transport by land was unacceptable (beyond a certain distance, approximately 25 mils, oxen and horses eat more grain than they can carry). As a result, maritime trade developed exponentially with the prosperity of Rome; and with it an embryonic form of marine insurance. The risks inherent in shipping were naturally grave, particularly from heavy weather, piracy and dishonesty (not much different from today, then), as a result of which a system of financial protection was developed, involving a policy of insurance in the form of a loan. Naturally it was essential to prove that the trader had delivered the cargo into the Carrier's care, so a receipt was produced as evidence, some of which have survived today in the form of clay or wooden tablets and, remarkably, share much the same character as today's Bill of Lading.