The use of plants as medicines has been observed in the behaviour of animals, which may seek a particular plant when they feel unwell. So it is possible that our pre-human antecedants used them too. Archeologists have found plenty of indirect evidence for their use by prehistoric peoples.
There is extensive documentation of the medicinal use of plants from all the ancient centres of civilisation, and all have references to using oils and burning plants to make smoke and smells as part of their cures.
Although relating to aromatherapy, these approaches probably did not use essential oils - which are produced by methods of distillation. Instead plant pieces were crushed and left to dissolve in oils such as olive or sesame.
The discovery of distillation is often credited to Arab physicians of the Middle Ages, but they invented a much improved apparatus.
The earliest evidence so far is a distillation apparatus and perfume containers recently identified in the Indus Valley and dating from around 3000 B.C.
Speculation has linked some Egyptian illustrations with distillation, but the first firm documentary evidence is Herodotus' record of the method of distilling turpentine from 425 B.C.
The most significant development in the history of distillation was the invention of the refrigerated coil by Persian philosopher-scientists. This is usually credited to Abn 'Ali Al-Husayn Ibn 'Abd Allah Ibn Sina (shortened to Ibn Sina and usually referred to as Avicenna in the West), who lived from 980 to 1037 A.D.
The Arabs used their new technique to distill ethyl alcohol (ethanol) from fermented sugar, providing a new solvent for the extraction of plant oils in place of the fatty oils that had been used for millennia.
Knowledge of distillation gradually spread around Europe through trading and crusading until essential oils had become a speciality of mediaeval pharmacies by the 16th century.
The growth of organic chemistry tied in with the industrial revolution lead to the first synthetic fragrance (coumarin) being produced in 1868, the year before Otto Wallach received his doctorate in chemistry. Wallach went on to analyse the constituents of essential oils, discovering a new chemical family (isoprenoids or terpenes) and receiving the Nobel prize in 1910. By this time, thousands of synthetic flavours and fragrances were being produced.
Essential oils were so-called because they were thought to represent the very essence of odour and flavour. Their extraction was researched and developed by alchemists in their search for the philosophers' stone that would turn common metals into gold. The knowledge of distillation passed from the Egyptians to the Gnostic Christians and the Arabs, and from there to Europe - the same route traced for magical and mystical knowledge.