The land and waters of the earth sustain a vast assemblage of plants upon which all other living forms are directly or indirectly dependent. These autotrophs have the remarkable property of capturing the inexhaustible energy of the sun to synthesise organic compounds which are vital for the existence of all life on earth. Organic deposits, such as coal, lignite, peat and petroleum are evidence of the photosynthetic activity of plants in the geological past. In addition, plants stabilise soil, conserve moisture and preserve an equable climate. After violent disturbances of the earth such as volcanic eruptions and upheaval of mountains, plants cover the denuded ground with a carpet which protects the surface from being washed away.

Before man domesticated animals and learned how to cultivate plants, he was unable to form settlements because his entire time was occupied with wandering in search of food. Presumably, prehistoric man lived on berries, succulent herbage and wild game which he could catch by primitive methods. These people lived in small groups and had a fierce struggle for existence against the carnivorous animals of that time and the vagaries of nature. Undoubtedly, their life was a hard one — a life in which only the hardiest could survive.

During the earliest and longest period of human history often called the Palaeolithic or ‘Old Stone Age’, which began one and three quarter million years ago, the concept of farming and domestic animals as such did not exist. The people of this Age were able to use fire and chipped pieces of hard stones (flint) into rough implements (such as crude hand axes and scrapers), which were used to remove flesh from animal hides.

During this period, the population was restricted to Africa, with a density of probably only 0.00425 persons per square kilometre and a total population of about 125 000 only. At present, there are 16.4 persons per square kilometre of the earth's total land surface.

Presumably, agriculture began in the Mesolithic or ‘Middle Stone Age’ (12 000 -6000 BC) when man lived by the spear, the bow and the fishing net.This shift from a food gatherer to a food producer is assumed to develop independently at different times in different parts of the world. The Neolithic or ‘New Stone Age’ began about 6000 BC when ground and polished stone tools became the rule, and agriculture continued to develop.