The first thing on earth that could properly be termed 'alive' was a molecule with the unique property of reproducing itself. To do this it must have been able to break down complex molecules such as polysaccharides and use their constituent parts to build a mirror image of itself. Any characteristic of the basic molecule that helped it in this task would have enhanced its chances of survival and that characteristic would have been perpetuated in the replication process. Any feature that hindered it would have led to that molecule's extinction. Evolution had begun.
This activity continued until all the original polysacchandes present in the primeval 'soup' had been used up. The proto-organisms would have then run out of food had they not evolved the ability to synthesize their own from inorganic substances using the sun's energy. This process, known as photosynthesis, was made possible by the presence of the chlorophyll molecules.
Eventually more than one complex molecule became involved in each replicating body and there appeared the compact organic unit known as the cell. Some of the most primitive cells lacked a central nucleus, the site of the cells' reproductive machinery, and this function was instead spread throughout the cytoplasm. It was the cells with nuclei, however, that were to go on to greater things, and in the course of evolution smaller cells became incorporated into larger ones, remaining there to perform certain vital intercellular functions. Eventually complex structures arose consisting of more than one cell, each cell having its own particular role to play in keeping the whole unit alive. The organism had evolved.
The evolution of the first multi-celled organisms may have come about in one of two possible ways. Either by free-living cells of different types coming together as a single unit, or by cells failing to separate completely during subdivision and remaining together as a complex entity. Regardless of their formation, these multiple-celled organisms must as whole units have been more successful than the sum of their parts or they would not have survived.
The cells of multi-celled creatures are not identical and have quite different functions depending on the tissues or organs they constitute. In the higher forms of life, some are structural elements such as bone cells, others such as blood cells provide defence against disease and transport food, whereas others such as nerve cells form the organism's sensory and communications system. Cell differenti�ation in most cases occurs at the embrionic stage. To begin with an embryo's cells are all identical. The initial fertilized cell divides into two daughter cells which divide into four cells and so on until several hundreds, of identical cells have been produced. However at a particular point in the embryo's development this stage ceases and specific cells are produced that are designed to fulfil definite roles. It is unclear how this cell differentiation occurs. All cell nuclei contain the same genetic information, but only part of it is used in the production of a new cell. Some agent within the cell, most likely within the nucleus itself, must determine which piece of the genetic code is used to produce the new cell so that it can fulfil the function allotted to it.