Coral reefs are many corals that come together and become a big group and life symbiotically with photosynthetic algae. Coral reefs have passed a long time of evolution, around 500 million years ago until now, They have passed a countless natural selection, and they still exist until now, but now; they’re face a new problem that might lead them to the extinction of coral. Not only because of the nature or predators, but also human acivities that give them a big impact. Now, many people try to rescue coral reef, to keep them alive and also to keep the ocean ecosystem and the biodiversity in the ocean, because coral reefs are one of the most important part of ocean ecosystem. “According to the fossils that found in China and Scotland, scientists estimated that the evolution of Coral has divided into two eras : Mesozoic and Cenozoic. Since the first organisms that has been estimated around 500 million years ago, in Cambrian period, Paleozoic era, called Scleractinians. The clearly ancestral Scleractinia lived in Middle Triassic, estimated around 247 – 237 million years ago, consisted of at least seven, possibly nine. The coral at that time did not build reefs, they just separate and make a small groups of coral or Phaleroid organisms of the shallow Tethys of Southern Europe and Indo – China. The Mesozoic era started during the Middle and Late Triassic, corals became widespread over the Tethys region, now their fossils are found around equatorial Panthalassa Ocean rim. Between the earliest Triassic corals and earliest widespread coral reefs, there was a time interval, around 20 – 25 million years. In the end of Triassic era, coral reef has a development collapse and after 5 – 8 million years of hiatus, they start to exist again at Jurassic era ( Jurassic reefs ). In Jurassic era, many new Scleractinian families are formed. The Late Jurassic was probably the all – time global maximum of Mesozoic coral diversity with at leas 150 genera recorded. The origin of the families of extant Scleractinia also have a high proportion in the Middle to Late Jurassic. Mesozoic era was end after the early Cretaceous. There many similarities between the coral in Late Jurassic and Early Cretaceous. In the middle of Cretaceous the reefs has been dominated. Rudist bivalves and enviromental pertubations are affected the reef development. And at the very late Cretaceous rudist bivalves did not affect the enviromental reef development anymore, it’s because a total extinction of rudists that can’t be explain and it’s make corals returned to a position of dominance.” ( http://coral.aims.gov.au/info/evolution.jsp ) Cenozoic era was start when One-third of all families and over 70% of all genera became completely extinct at or near the end-Cretaceous boundary. In Cenozoic era, there was two families that also the major component of Mesozoic era that still exist in Cenozoic era, they’re the Faviidae and Caryophiillidae. The evolutionary of modern corals is divided into three geological interval : (1) the Paleogene, at this time, the survivors of end Cretaceous and Late Paleocene extintions proliferated into a diverse cosmopolitan fauna, (2) the Miocene, at this time , coral was subdivided into the broad biogeographic provinces anf pre-cursors of most extant species evolved , and (3) the Plio-Pleistocene to present , the world went into full glacial mode and modern distribution pattern emerged , at this time , coral reefs have passed many natural selection and the remain of the corals have evolved and adapted with the earth’s condition. Corals are related to sea anemones and they also have a same simple structure, the polyp. The polyp use to capture small animal that swim near to him. It’s like a can that can open at just one end with a ring of tentacles surrounding him. The biggest component of coral reef is calcium carbonate, or limestone, they also the main component to build a new reef. After the reef dead, they will turn to coral skeleton and the living reef will build over the top of this, to build the new reef, the polyp will add new limestone to the massive base structure. The polyps make skeleton calcium carbonate surrounding themselves. Reef have many colourful coral, but how they can make many colourful coral ? The colour is come from the tentacles and coral polyps and also from the zooxanthellae algae that live in many species’s tissue. The family of corals is complicated, many corals look similar to each other. All but the fire corals are anthozoans, and they’re divided into two main groups. The first one is Hexacorals ( including the true stony coral and black corals, as well ad the sea anemones ), they have smooth tentacles, and they often have tentacles that multiples by six, and the Octocorals ( soft corals, seafans, organpipe corals and blue corals ) have eight tentacles, and every tentacle has tiny branches running along the sides. All coral are in the Phylum Cnidaria, the same as jellyfish. Coral reefs play an important role in ocean ecosystem, they symbolize the diversity of ocean, they also keep the balance of the ecosystem in the ocean. They are a home for countless species of fish, crustaceans, molluscs, echinoderms, sponge, worms and any other life forms. Coral reefs are very suitable for a lot animals to life because it’s a safe place for many animals to hide from the predator. Even though there is the biggest communities in the ocean ecosystem, but they still have some requirements to survive, they can’t life in every water condition. Coral reefs need warm ( but not too warm ) water, and they can’t survive in a water that have a high nutrient concentrations or runoff from the land create dense layers of algae, fungi, or sediment.