The first evidence of enamel in the fossil record appears in early placoderms, an extinct group of armoured fishes. These fishes had small, tooth-like structures called dermal denticles that were covered in a thin layer of enamel.

Acanthodians

Acanthodians were another group of early fishes that had enamel-like tissues on their teeth. These tissues were composed of a mineralized material called dentine, which is similar in structure to enamel but not as hard.

Osteichthyes

The evolution of enamel reached its peak in osteichthyes, the bony fishes. These fishes have hard, enamel-covered teeth that are used for capturing and processing food. The enamel in osteichthyes is composed of hydroxyapatite, a mineral that is also found in human teeth.

Mammals

Mammals also have enamel-covered teeth, but the structure of their enamel is different from that of fish teeth. Mammalian enamel is composed of a combination of hydroxyapatite and other minerals, and it is harder than fish enamel.

The Evolution of Enamel Function

In addition to changing in structure, the function of enamel has also evolved over time. In early fishes, enamel was primarily used for protection. However, in modern mammals, enamel also plays a role in food processing. The hard enamel of mammalian teeth helps to grind down food, making it easier to digest.

The Evolution of Enamel Genes

The evolution of enamel is also reflected in the evolution of the genes that encode enamel proteins. These genes have been highly conserved throughout evolution, suggesting that enamel is an important tissue for survival.

Summary

The evolution of tooth enamel tissue is a complex process that has spanned hundreds of millions of years. Enamel has evolved from a thin layer of mineralized tissue in early placoderms to the hard, protective covering that we see in modern mammals. The evolution of enamel has been driven by a number of factors, including changes in diet and the need for more efficient food processing.