With all the concern about hand-washing to prevent flu, I am somewhat more conscious of soap these days. Arguably soap is a rather important blessing, but it is one of those things that is just "there," not considered. It is actually a very curious substance. Every soap molecule has two ends: one end sticks to oil molecules, the other to water molecules. So when we rub soap on our hands, the little bits of oil on our skin become surrounded by soap molecules,with their oil-attracting ends stuck. This is called emulsification. Then along comes the water, grabbing the little porcupine of oil and soap by the water-attracting ends and whooshing it away. What an amazing process!
According to wikipedia, the earliest mention of soap-like materials is around 2800 BC and a Babylonian clay tablet exists from 2200 BC giving a method for making soap. Eventually soap-making became one of the precious skills of the homemaker, although fancy soaps were made by skilled artisans. It wasn't until the late 18th century that commercially manufactured soap became more common, along with increasing public awareness of the role of hygiene in health.
If you wanted to make simple soap, you would need to get some lye - nasty alkaline stuff. It was obtained by leaching water through ashes. Then you would boil the lye with any kind of oil or fat until the mixture saponified - a sort of curdling that binds together the fat and alkali into soap molecules. If you used enough fat, the lye would be all bound up and wouldn't eat into your hands when you used your soap. Soap can also be made without boiling, if you follow an exact recipe and know the strength of your lye. I can imagine scenarios involving a campfire, rain and spilled grease that might have led to the discovery of soap.
Now we have every sort of soap from artificial detergents for every purpose to elegant hand-made bath soaps scented with cinnamon or sage or whatever. But I am enjoying thinking about a determined colonial home-maker, well-aproned against lye splashes, carefully stirring her pot of soap over the fire.