A calcium carbonate exoskeleton provides an excellent defence (at least until parrotfish evolve), but limits the size of the individual polyp. Many tiny polyps living close together tend to make lots of calcium carbonate which forms into the massive chunks we call reefs.
These can grow and grow and grow, very slowly but inexorably, so that the longest reef today is the Great Barrier Reef, which is two and a half thousand kilometres long. The biggest reef area is the South China Sea Reef Bank, nearly nine thousand square kilometers (half the size of Wales).
Other species of coral create smaller, distinctly structured shapes, such as Brain Coral or Fan Coral. Some have colonised the deep ocean, hundreds of meters down, where they may have developed other energy gathering approaches such as methane collection.
In fact, the only thing in two billion years to trouble the coral family is the rising water temperature, which makes calcium carbonate more soluable and may eventually dissolve their entire ecosystem.
Evolution starts with the Last Universal Ancestor.
You can find out what happened before evolution here.
You can also short-cut to Land plants, or Animals
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