Common origin and phylogenetic diversification of animal hormonal systems

The comparative view leads to the following main conclusions: Hormones are intercellular messengers in multicellular organisms. However, the receptor mechanism, the ability of forming receptors to substances in the cell's environment and the ability to synthesize most of the substances which serve as hormones in metazoans, are present in unicellular organisms, too. The main achievement of multicellular organisms in evolving hormonal mechanisms is due to their ability of differentiation. Though the whole genome and the ability to synthesize certain substances is, in principle, common to all body cells, the forming of certain substances and the ability to react to them in a certain way is delegated to certain cell groups only. This may be common to many intercellular messenger substances such as chalons, prostaglandins, morphogenetic and tissue-specific growth substances. A special feature in hormonal systems is that the two sites of release and reaction are distinct and are located at a distance from each other. Possibly this is the main or even the only difference from other intercellular messengers. However, it is of great functional importance because it enables hormones to control the temporal coordination of entirely different processes in different tissues located at a distance from each other or distributed all over the body. This feature is common to all compounds presently known as hormones. While the localization of receptors in target cells, as well as the nature of the releasing tissue or the mode of transport through the blood, may be too marrow borders for a definition (for instance in animal groups without a closed blood circulation system) the fact that there is transport over a distance is not. The fact that release and reaction sites are located at a distance from each other within the multicellular body may serve as a definition of "hormones' based on a common phylogenetic root and functional importance. On this common base different animal phyla have evolved different hormonal systems in relation to the particular problems of long distance temporal control of physiological processes posed by their special type of organization.