600-555 million years ago


Although invertebrate coelomates lack bone, cartilage, dentine, and enamel, they do possess homologs of many of the proteins essential for these tissues in addition to "cartilage-like" tissues.


Cartilage-like tissue is actually known from a variety of organisms including vertebrates, jellyfish, annelids (sabellid worms), snails, cephalopods, horseshoe crabs, and the imagos of locusts. Chondromucoid, in the matrix of cartilage, is similar to molecules found in certain sensory receptors. The cartilage proteins type II collagen and aggregan are also found in the nervous system. Molecules once thought to be specific to cartilage (such as the chondroitin sulfate proteoglycan) have been found in the nervous system and proteins thought to be specific to the nervous system (such as S-100 acidic protein) have been found in cartilage. (Hall, 1999). Cartilage/cartilage-like tissue is known in cnidarians, annelids, arthropods, and mollusks (Robson, 1999). The cartilage of squid does possess collagen and resembles the hyaline cartilage of vertebrates, although it is not the collagen II found in vertebrate hyaline cartilage. A variety of collagenous and non-collagenous proteins are known from invertebrate cartilages (excluding type II collagen) which seem to have evolved separately in different lineages (Robson, 1999).

The secretory calcium-binding phosphoprotein family (SCPP) includes three proteins in enamel matrix (amelogenin, enamelin, and ameloblastin), five proteins involved in the formation of dentin and bone (dentin, sialophosphoprotein, dentin matrix acidic phosphoprotein 1, integrin-binding sialoprotein, matrix extracelullar phophoglycoprotein, and secreted phosphoprotein 1, caseins, and several salivary proteins. Most of these genes are located on a cluster on chromosome 4q13 in humans. This gene family seems to have arisen from the SPARC gene. SPARC, which is expressed in fish bone and scales, may have been the first gene expressed in vertebrate mineralized tissue. SPARC is expressed where the epithelium meets the connective tissue beneath in invertebrates and jawless fish (Kawasaki, 2004).