What comparative endocrinology tells us about the original function of the insulin superfamily.

Comparative endocrinology is a research subfield in endocrinology that delves into deeper understanding of the endocrine system from an evolutionary or phylogenetic perspective. To date, this approach has contributed significantly to the development of endocrinology by elucidating the evolutionary history of hormone molecules and their functions from invertebrates to vertebrates. In this review, the author initially introduces how the comparative approach has expanded and enlightened the view in endocrinology using the concept of hormones as an example. The expansion of the hormone concept blurs boundaries between signaling molecules of the three homeostatic systems, namely, the endocrine, nervous, and immune systems. Subsequently, the evolutionary history of the endocrine system is introduced in terms of both molecules and functions using the insulin superfamily as a model. This hormone family is one of the most ancient hormonal systems in animal (metazoan) phylogeny and the homologous hormones are identified in the most ancient metazoans such as sponges and hydra. In addition, this hormonal system was chosen as a topic of this review, because insulin is one of the most focused research topics in modern medicine in relation to insulin resistance and metabolic syndrome. Finally, the ancestral molecule of the insulin superfamily and its original or essential function will be discussed with some speculations to illustrate the value and joy of comparative studies that can create an original concept of the endocrine system from the evolutionary viewpoint. The comparative approach certainly helps deeper understanding of the insulin superfamily of humans.