The earliest known use of personal lubricant dates back to the ancient Greeks and Romans who used olive oil as a sexual aid. These cultures had developed beyond their hunter-gatherer ancestors and had time to focus on things like agriculture, philosophy, and even sex. As a result, olive oil (and other vegetable oils) were widely available and used for many things. Ancient Greeks were described as somewhat sex-positive at least with respect to male homosexuality. When sex is focused in areas that don’t naturally provide lubrication, you need to add some artificial lubricant to make things more comfortable. The Ancient Greeks solution was olive oil, the original lube.
In his book, The History of Animals, written in 350 BCE, Aristotle gets everything wrong when he suggests using oil as a form of birth control, but he does make it very clear that, during that time period, people were using lube.
He says:"It is a sign of conception in women when the place is dry immediately after intercourse. If the lips of the orifice be smooth conception is difficult, for the matter slips off…Wherefore, since if the parts be smooth conception is prevented, some anoint that part of the womb on which the seed falls with oil of cedar, or with ointment of lead or with frankincense, commingled with olive oil."