500 MILLION YEARS AGO

Earth experienced relatively warm "greenhouse" conditions, following earlier ice ages in the Precambrian. Sea levels were high, and large portions of the continents were flooded by shallow inland seas, but the planet was not completely covered by water. Significant landmasses existed, although life was still almost entirely confined to the oceans. Indeed, life had not yet significantly colonised the land. However, all this was about to change as an increasing number of animals saw the benefit of living on land.

Speaking of moving onto land, some readers might be wondering, why make the move? One possible explanation is that some animals were tempted by the smogasbord of "all-you-can eat" insects that had already followed the plants onto land. We know small shrub-like plants (but no trees) had adapted to life above the water quite happily thanks to the Sun in providing light to help convert carbon dioxide into food using chlorophyll. The insects simply followed the plants for safety and food, which in turn gave other animals the impetus to follow the insects into the new environment.

Or, the other likely explanation is because predators were forcing a number of animals to seek shelter and protection near the surface of the water and along the coastal regions where the predators had not yet adapted to the new environment.

480 MILLION YEARS AGO

Much of Australia is covered by water.

455 MILLION YEARS AGO

Plants have well and truly established themselves along the edges of mostly fresh water supplies and following the rivers back to their sources at lakes and near mountain ranges, and exit points long the coasts, so long as temperatures are warm enough and adequate minerals in the soil and carbon dioxide in the atmosphere with sunlight were in plentiful supply. These plants were small, non-vascular, and lacked the complex structures seen in later vegetation such as trees. Bu that too would also change as new and larger forms of plants would appear at around this time as competition for the sunlight encouraged plants to evolve into new species.

450 to 440 MILLION YEARS AGO

This was the time for the first known major extinction of animals and flora. Scientists estimate that around 85 per cent of all species and up to 60 per cent of genera had perished for whatever reason. Even the highly successful trilobites, the great scavengers of the seas and oceans, were not entirely flourishing at the end of this period.

The magnitude of this extinction is second only to another more devastating event that would occur just prior to the rise of the great dinosaurs, but certainly more devastating than the demise of the dinosaurs around 65 million years ago. As Bruce Lieberman, a paleontologist from the University of Kansas, can surmise from this extinction:

"The Ordovician mass extinction is the second most severe extinction that has ever afflicted life on this planet. Far more severe than the extinction that eliminated the dinosaurs." (Quote obtained from the documentary titled Mega Disasters - Gamma Ray Bursts. Produced by Creative Differences for History Television Network Productions, © 2007 A&E Television Networks.)

With no definitive evidence to prove what caused this extinction, some scientists have widened the scope of possibilities beyond the usual extensive volcanic activity through the break up of more tectonic plates or even an asteroid impact to suggest something in the universe may have influenced life on Earth.

Today, scientists from the Astronomy Department at the University of Kansas have considered the possibility that somewhere between 6,000 and 10,000 light years away from the Earth there may have been a merging of two highly dense neutron stars or the sudden collapse of a very heavy star exceeding 20 times our Sun and the resulting star formed had directed one of its beams of energetic gamma ray bursts (GRB) emerging from the poles of the rapidly spinning star in the direction of the Earth.

Some scientists have proposed more unusual hypotheses, such as the possibility of a nearby gamma ray burst (GRB), which could damage the ozone layer and expose life to harmful radiation. However, this idea remains speculative and is not widely accepted as the primary cause of the Late Ordovician extinction. Enough of these GRBs do take place roughly 300,000 times per year somewhere in the visible universe. These highly directional and confined nature of the gamma ray beams make them extremely rare to directly hit a planet like the Earth. But should one of these beams hit the Earth, it would have devastating consequences for life at the surface of the planet.

If such a cosmic event did take place at this time, animals swimming in shallow seas would have died out almost immediately from the high radiation doses coupled by a sudden drop in world temperatures as nitrogen and oxygen atoms in the atmosphere broke apart by the radiation and later combined to form a thick fog of nitrous oxide compounds, thereby reducing sunlight from reaching the surface for several month (and so making it harder for plants to survive and insects to flourish). While those living in deeper waters would have had a better chance of surviving.

To give an idea of the kind of radiation doses a GRB at 6,000 light years away could unleash on the Earth, astrophysicist Stan Woosley of the University of California in Sanata Cruz, said:

"It's like having 3,000 megatons of [nuclear] bombs go off in the Earth's atmosphere simultaneously." (Quote obtained from the documentary titled Mega Disasters - Gamma Ray Bursts. Produced by Creative Differences for History Television Network Productions, © 2007 A&E Television Networks.)

Should the GRB occur at a distance of, say, 1,000 light years, Woosley can confidently say:

"The mega-tonnage would be like 100,000 megatons of nuclear bombs. It's like standing a couple of miles from a Hiroshima [atom] bomb everywhere on the surface of the Earth."

At this distance, it should be enough to destroy nearly all life on the surface except for anything living deep underground and in the deep oceans. If the source of the GRB is only 100 light years away, the radiation would literally strip away the atmosphere, push the oceans apart to form massive tidal waves, and land masses would melt. Only bacteria in the very deepest oceans of the world may continue to survive.

One thing in our favour is that our Milky Way is not a very old galaxy. Any of the bigger and/or older stars are more likely to be found closer to the centre of the galaxy compared to us lying more toward the edge of the galaxy. However, even at distances of 50,000 light years away, gamma ray bursts directed on Earth will have significant impact on how advanced life can survive.

We have to be grateful for the rarity of such an event. Scientists put it down to a probability of roughly once every 350 to 500 million years based on current research into this field by a number of American astrophysicists in 2010.

Very rare indeed, but it can happen.

How would we fair if such an event were to take place today? Not very good. Some scientists believe it would be enough to send humans back to the Dark Ages with much of our technology becoming worthless by the sudden surges in electrical current as the radiation hits the Earth. If people don't die from the radiation doses unless they live in highly protected areas, those who survive may have to burn books to keep warm, possibly fight for the remaining food in the supermarkets and, if populations are sufficiently reduced, to scavenge for the remaining food, clothing and other resources generated by the previous profit-motivated society until the sunlight returns and people learn to go back to subsistence living and growing food on the land, and then maybe technology might return again.

No doubt a frightening scenario for those not prepared for such a potentially cataclysmic event.

440 MILLION YEARS AGO

Whatever caused the extinction seem to have been short-lived. By 440 million years ago, plants grew once again to create a rich and diverse almost forested environment.

The extra plant growth may have also aided in the formation of extra oxygen. During this time, the level of oxygen in the atmosphere rose sufficiently for the Sun's powerful ultraviolet rays to convert a fraction of the oxygen into ozone in the upper atmosphere. Ozone is an important substance for it helps to shield living organisms from the harmful effects of ultraviolet light (a form of radiation considered by some scientists as one of the most important factors in shortening the lifespan of all living organisms as well as modifying the genes in DNA). Without ozone, life, especially for the animals, would probably not have reached out and made the first tentative steps on dry land.