Legend says that soap was first discovered on Sappo Hill in Rome when a group of Roman women were washing their clothes in the River Tiber at the base of a hill, below which animal fats from the sacrifices ran down into the river and created soapy clay mixture. They soon found that using this same cleansing substance the clothes were coming clear easier. Since that time we know soap as soap.
However, the ancient Babylonians were the ones who invented soap and evidence for this are Babylonian clay containers dated at 2800 B.C. Inscriptions on the containers present the earliest known written soap recipe and they state that the product was made from fats combined with wood ash and water. These early references to soap and soap making were for the use of soap to wash wool and cotton in preparation for weaving into cloth, soap was not necessarily used to wash the body.
The Ebers papyrus (Egypt, 1550 BC) reveals that ancient Egyptians combined both animal and vegetable oils with alkaline salts to produce a soap-like substance. They used this mixture for treating sores, skin diseases as well as washing.
According to the Pliny the Elder, the Phoenicians made soap from goat's tallow and wood ashes in 600 BC.
The ancient Greeks were said to have combined lye and ashes as a cleanser for pots and the statues of their gods.
Early Romans used urine to make soap like substance in the first century A.D. Later, they combined goat's tallow and the ashes of the beech tree to make both hard and soft soap products. The discovery of an entire soap factory in the ruins of Pompeii, one of the cities destroyed by the volcanic eruption of Mt. Vesuvius in 79 A.D suggest that the industry was established and that soap was widely known in the Roman Empire. During the early century of the Common Era, although the Romans are well known for their public baths, generally soap was not used for personal cleaning; it was used by physicians in the treatment of disease. Soap for personal cleaning and hygiene became popular during the later centuries of the Roman era.
The Celts, who used animal fats and plant ashes to make their soap, named the product saipo, from which the word soap is derived.
The Arabs produced the soap from vegetable oil as olive oil or some aromatic oils such as thyme oil. Sodium Lye NaOH formula was used for the first time and it hasn't changed from the current soap sold in the market. Arabian soap was perfumed and colored, and they made both liquid and hard soaps.