Among multicellular animals, collagen is probably one of the most constant extracellular proteins. Collagen fibrils are encountered in sponges. The organization of their genes suggests that they have not varied to a great extent during evolution. However, they are not present in all animals. Basement membrane collagen is certainly the only ubiquitous collagen.
The comparisons of collagen in lower animals suggest similarities with several collagen types of vertebrates. It is interesting to try to imagine the fundamental problems for which solutions based on collagen have been adopted. The first one isadhesion. Animals with poorly packed cells need a cement to be attached to the bottom of the sea. In sponges, this function has been fulfilled by microfibrils of spongin, a short-chain collagen. Later on, other organisms, such as mussels, have used a chimeric elastin-collagen protein to obtain both elasticity and strong attachment. The second important problem has been mechanical support, that is askeleton. Here again, spongin has been involved in sponges. The best is the solution combining vertebrate collagen fibrils and calcium phosphate. A third vital problem has beenprotection. It is achieved by different kinds of collagens in invertebrates and lower vertebrates. It could be suggested that in vertebrates, some of the non-fibrillar collagen types are involved in tissue micro- or macro-specialization.