Where did it all begin?   Reefs have experienced a lot of periods of change to become the complex structures we see today. Over a long geological time scale, reefs have undergone periods of growth and decline, brought about by events such as the movement of the earths continents (plate tectonics), the rise and fall of the sea and interruptive events, such as meteor impacts and global ice ages. The structures which we recognise as reefs today, barely resemble what the reefs of the past would have looked like. A whole different group of animals, plants, bacteria and environmental conditions helped set the stage for a dynamic transition from the reefs of 2000 million years ago, to the modern reefs of today.	Tethys sea in the Eocene epoch
	Geological History
	Various types of reef structures and communities have existed over the last 2000 million years. These communities have arisen, flourished and become extinct in turn. Algae of some type have always been involved in building the reefs, whether alone or in conjunction with an invertebrate reef-builder. The first reefs were not made of coral, but of cyanobacteria which trapped sediment and secreted calcium carbonate, forming large structures (up to 450m thick, but more usually less than 5m thick) known as stromatolites. These stromatolites formed in an early atmosphere that had very little oxygen. The cyanobacteria probably released oxygen into the atmosphere and thus contributed to the boom in invertebrate evolution which occurred about 570 million years ago (mya). Around 560 mya, in the very early evolution of animals, sponge-like creatures called archaeocyathids invaded the stromatolitic reefs and formed calcareous thickets in areas of shallow water. The archaeocyathids became extinct around 520 mya (middle Cambrian), and the stromatolites were dominant until Ordivician times (~ 500 mya) when reefs of well established algal and invertebrate reefs started to appear. These reefs were made up of Rugose and Tabulate corals, red algae and stromatoporoids (calcium-carbonate secreting relatives of sponges), and were home to many other invertebrates including stony bryozoans, brachiopods and trilobites. Rugose and tabulate corals differed from scleractinian corals of today mainly in skeletal structure. The reefs the rugose corals formed would have been very similar to the reefs of today in structure, although the individual corals and other reef-builders were different from those of today. These reefs hung in there for a while (about 125 million years!), but for some reason they disappeared about 375 mya (the late Devonian), although rugose and tabulate corals could still be found in the fossil record until the late Permian (about 250 -245 mya). The reefs of the Carboniferous and Permian periods were again algal in nature, with many stromatolites. These reefs disappeared as most of the warm, shallow Permian seas shrunk and dried up, and there were no more reefs until the emergence of the scleractinians in the Triassic. The first scleractinians, or modern day hard corals, turned up during the Triassic period about 230 mya. The success of scleractinian corals may have been associated with the development of a symbiosis with zooxanthellae, and also major extinctions at the end of the Permian and the late Cretaceous of previously dominant animals and plants. Coral reefs were almost replaced by bivalve reefs in the early Cretaceous period - rudist bivalve reefs dominated corals for around 30 million years. When these rudists became extinct corals again became dominant, and late Cretaceous coral reefs were quite similar in appearance to those of today - all the major families of corals alive today were established by this period.
	Mass Extinctions, or Interruptive events.
	"Mass extinction" or interruptive events have played an important role in shaping the evolution of marine invertebrates. Important extinction events in reef evolution occurred at the end of the Permian, Triassic and Cretaceous periods, also the Eocene epoch. At the end of the Permian (245 mya), 57% of marine invertebrate families (and up to 95% of all species) on the earth became extinct. The primary factors involved in the event were a reduction in global temperature (with extensive ice formation at the poles) and a large fall in sea level, reducing the area of warm shallow seas on earth for reefs to grow in. It is likely that other associated factors were involved over a long time (in human terms) - about 10 million years. The end-Triassic extinction was due to climate change, and resulted in the disappearance of 49 out of 67 genera of corals, and a pause in reef development of 4 - 10 million years. The end-Cretaceous extinction was the one that wiped out the dinosaurs (and 75% of all species) and was probably caused by a huge meteor impact causing major, rapid climate change. The end result of mass extinctions was often the emergence of a substantially new fauna.
	Evolution of modern day scleractinian corals - the last 230 million years.
	Modern day corals (the scleractinian corals) probably developed from a group of soft-bodied anemone-like animals about 230 mya. The development of a symbiosis with a dinoflagellate algae (known as zooxanthellae) was important to enable the corals to produce calcium carbonate skeletons, and the extra supply of energy from the alga to the coral gave the corals an advantage over many other marine invertebrates. Symbiosis with zooxanthellae evolved independently several times in the Triassic (ie the same events happened that led to the symbiosis being formed with different corals at different times). The first scleractinian corals did not form reefs, and were solitary animals. The first true coral reefs date from the late Triassic (~220 mya). The Scleractinia arose in the warm waters of the Tethys Sea. This was the large ancient sea that existed between the Northern Europe and the Asian land masses and the Southern African and Indian continents (see figure at top of the page). In Jurassic times, there was only one land mass on the planet - all the continents were combined. The Tethys sea at that time ran nearly through the middle of all the major continents, and most corals had a world-wide distribution. As the Americas split away from Africa and Asia to form the Atlantic Ocean, two major centres of coral reef development were created - the Caribbean region and the Indo -West Pacific region (from southern Africa to east Australia). These two areas were completely separated when the gap between North and South America (at Panama) was closed in the Pliocene (3.3 mya), cutting off the link between the Caribbean and the Pacific Ocean. This long time of separate evolution has resulted in the development of many different coral reef animals and plants in the two regions - the Caribbean and the Great Barrier Reef do have many coral species in common, but there are also many that are unique to each region. The Indo-Pacific region has a much higher diversity of corals (80 genera (a large grouping of species)and 500 species) compared to the Caribbean region (20 genera and 65 species). A lot of the coral genera in the Indo-Pacific are younger than those in the Caribbean (ie they evolved or "branched-off" more recently in the Indo-Pacific) - around 30 million years old compared to 60 million years old in the Caribbean. The Tethys sea was gradually closed during the Miocene (5-25 mya) by the movement of the southern continents northward, due to continental drift. As the Tethys Sea closed, corals started to spread eastward into the Western Pacific, to meet the Australian continent which was moving northwards away from the icy polar regions in the south and towards the warmer tropical waters. The Great Barrier Reef formed on the north-east continental shelf of the Australian land mass when the continent came closer to the region of coral reef development in the western Pacific, and is a relatively young coral reef, at the age of around 18 million years.
	The Rise and Fall of the Sea
	Modern corals are restricted to growing in warm, clear, well lighted and shallow water. Since coral reefs grow best from low tide to about 20m depth, relatively small changes in sea level can have dramatic effects on coral reef growth. The position of fossil reefs in geological strata are an accurate indicator of sea level at the time of formation of the reef, because reefs will always grow to the level of low tide. Sea level change has been going on continuously during the evolution of corals and reef organisms. There have been 17 cycles of sea level rise and fall in the last 2 million years, and these fluctuations have been of major importance in determining the form, distribution and organisms of reefs. Most of the changes in sea level over this time have proceeded at the rate of between 10-12 m per 1000 years, and coral reef growth is usually quick enough to keep up with this sort of change. There have been periods when reefs have drowned or left high and dry, but the most important effect of sea level change is changing the amount of suitable substrate for corals to grow on, especially when global sea level is very low. Accompanying sea level change there are also important changes in oceanic currents (affecting dispersal of invertebrate larvae) and sedimentation patterns (corals can't grow in regions of high sedimentation) which affect the distribution of coral reefs. Very low global sea level (or sea regression) has been an important factor in many of the "mass extinction" events through geological time. Sea level rise and fall may be caused mainly by (1) movements of the earth's crust, such as up and down movements of the continental plates and sea floor spreading; and (2) changes in the amount of water locked up in the polar ice caps (there are many complicated factors contributing to the changes in global temperatures which in turn cause the melting or freezing of the ice-caps).
	Highs and Lows
	The last great polar melt (period of high sea level) was 130 to 75 thousand years ago, when sea level was about 6m higher than today. The sea level dropped over the ages, with a few ups and downs, reaching a low at about 18,000 years ago. At this time, sea level was about 120-135m below that of today, and the ice-caps extended much farther toward the equator (the northern ice cap extended to northern USA). In the Indo-Pacific region, there was very little suitable habitat remaining for reefs to flourish in, and reefs were only found in the area east of Indonesia. This situation was important for the development of corals in the region, as when the sea level rose again, the small area east of Indonesia had to re-populate the entire region. The Great Barrier Reef was close to this area, and this (good supply) may partly explain why the GBR has the highest diversity of corals in the world. The polar ice-caps then started to melt, and sea levels to rise (at about 10m/1000yrs). In between times there have been 3 or 4 "Little Ice Ages", where the polar ice caps have expanded for short periods, lowering sea level. The last of these was about 5 or 6000 years ago. When these little ice ages occurred, they exposed the reefs and set back coral growth to a lower level. In this light, the growth of modern reefs has been proceeding from that last low water level for the last 6000 years (don't confuse this with the colonisation of the region by the corals that make up the reef 18 million years ago! - see above). The rate of sea level rise slowed down over the ages, and sea level has been at a constant level now for 2-3000 years. We are now facing an extremely rapid rise in sea level, due to the effects of man's activities on the global climate raising global temperatures. These changes could bring about the highest sea levels seen on earth in 30,000 years, but at a rate much faster than ever before. We can only hope that the reefs can keep up!
	Plate Tectonics - a-movin' and a-shakin', a-rockin' and a-rollin'........(yeesh)
	Plate tectonics is a theory suggesting that the outside edge or the crust of the Earth is a jigsaw of moving plates. Throughout time these plates have been moving against, underneath and into each other. Plate tectonics is the process responsible for continental drift and the changing position of the landmasses compared to today. In the Triassic period, 240 mya, there was only one giant continent on the earth, known as Pangea. Over the aeons, the tectonic plates making up Pangea split apart, and kept drifting to form the world we know today. The plates are still moving today, and the world may look very different in 50 million years! The interaction of the plates causes the formation of mountain ranges or deep-sea trenches, and once in a while the plates slip against each other - the sudden movement can cause earthquakes and tidal waves. Island chains such as Hawaii are formed over "Hot-Spots" deep in the earth. As the continental plate that Hawaii is on moves slowly across the hot-spot, new underwater volcanoes push through the crust and eventually form a chain of islands. These islands are prime real estate for the growth of new coral reefs.
	A key to understanding the past
	One method marine scientists use to investigate the way the world was millions of years ago is called coral coring. By drilling a vertical core through the reef, it is possible to see relative periods of growth and decline in the reef. It is possible to date these periods of growth and decline as well as the evolutionary state of the animals and plants that inhabited the reef while it grew, by looking at fossils in the rock and measuring the types of atoms (or isotopes of atoms) present in the rock. The type of atoms present in the rock can tell scientists lots about how old the rock is, and what the climate was like at the time. All sorts of information can be obtained from rocks and coral cores, such as which times the Earth experienced rise and fall of the sea levels, as well as atmospheric changes and interruptive events such as ice ages, huge volcanic eruptions and meteors falling to Earth causing mass extinctions!
	Teaching old dogs....
	Coral reefs have been around for millions of years and have undergone a huge number of changes. It is important to understand the changing nature of a reef as an organism and how vulnerable it is to exterior influences. By understanding the history of reefs and the evolutionary changes they have gone through it can be argued that a reef is not a fragile ecosystem but one that is immensely capable of dealing with change and adapting to new environmental conditions. The world is still changing in radical ways - the plates are still moving and therefore the oceans and the continents are changing and moving. Did you know that the Atlantic Ocean is growing and the Pacific Ocean is shrinking? The rate that these things happen is so slow that it's impossible for us to perceive, but on a geological timescale we are simply going through another chapter in history. The way a coral reef ecosystem reacts is often a good indicator of a larger, global change. This is especially important today in assessing the added pressure the reefs face due to the growing influence of man, and the potentially rapid changes our activities may bring about.
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	The last 2 million years.
	Reef Geology