There is a popular mode of thinking which believes that, left to its own devices, the Earth regulates itself so as to remain pretty much constant, maintaining benign conditions for all biological life. This thinking automatically damns most changes made to the Earth by humans.
However, the truth is that the long history of the Earth has been a story of continuous change, sometimes of dramatic intensity. This tendency of the Earth to change continues apace and has nothing to do with human activity. Analysis of the effects of humankind's activities on the environment only makes full sense in this context.
The Earth was formed about five billion years ago when material in the rotating disc around our sun coalesced under the force of gravity to form the planets. The early Earth was a violent place, regularly bombarded by comets and meteorites and riven by volcanic eruptions. The atmosphere was bathed in strong ultraviolet light from the sun as the protective ozone layer had yet to form high in the atmosphere.
Life began on Earth about 3.5 billion years ago when the atmosphere was devoid of oxygen. Oxygen was a deadly poison for the first life forms called anaerobes. About two billion years ago, new organisms arose, capable of harnessing the sun's energy to make nutrients. These organisms emitted oxygen to the atmosphere as a by-product.
The rise of new oxygen-friendly organisms changed the composition of the Earth's atmosphere and sounded the death knell of most of the original anaerobic organisms.
Life continued to develop from then to the present along the lines described in the Theory of Evolution by Natural Selection. The Earth today is populated by over 30 million biological species, each descended from the original life form which arose 3.5 billion years ago.
The evolution of life is a story of continual, gradual change, marked at regular intervals by dramatic changes. Anyone who believes that Mother Nature, if left undisturbed, operates the ultimate social welfare system for living organisms should ponder the fact that 99 per cent of all living species which arose in the history of life on Earth became extinct.
These extinctions largely occurred because species were supplanted by new forms better adapted to survive in changing natural conditions.
The early Earth was actively bombarded with asteroids and comets. Much of our water was possibly brought here by comets. Although relatively infrequent, significant collisions with asteroids have occurred in the meantime.
For example, there is excellent evidence that the extinction of the dinosaurs, 65 million years ago, resulted from a catastrophic collision of an asteroid with the Earth. It is certainly possible that such an event will happen again.
The physical planet Earth is a dynamic structure which is constantly changing. The crust of the Earth, bearing the continents, moves on a molten under-layer called the mantle. The crust is cracked and these cracks divide the crust into large pieces called tectonic plates. Most of our planet's earthquakes and most volcanos are located at the borders between plates where they bump into or slide past each other.
Despite extensive research, there is still no reliable way to predict when an earthquake will occur. Serious earthquakes occur all too frequently, with devastating effect. Unfortunately many of the world's fastest-growing cities are located close to major fault lines between tectonic plates.
The following are some of the major earthquakes and associated fatalities within the last few decades: Kobe, Japan, 1995 (6,000 dead); Armenia, 1988 (30,000 dead); Mexico City, 1985 (100,000 dead).
What about the future for life on Earth? In the very long term there is no future. Life on Earth depends on the sun just as surely as life on your TV screen depends on plugging the set into a power outlet.
Our sun will live for 10 billion years and is now halfway through its life-span. The sun will die in five billion years and this will be the end of biological life on Earth. If humanity hasn't figured out by then some way to vacate the Earth and to live elsewhere, it will perish at that stage.
LOOKING at the larger picture, the only guarantee is continual change, continual uncertainty and continual risk. The potential of the titanic forces which govern the universe to bring about change and to wreak havoc for any particular living species completely dwarfs any effect which humanity might bring about by its own hand. Of course, this does not absolve us of the responsibility to be careful.
By our own hands we have already damaged our protective ozone layer, pumped gases into our atmosphere which cause the Earth to warm up, and other gases which cause corrosive acid rain to fall on our heads. The fact that these effects are puny compared to what Mother Nature might unleash on us does not give us a licence to do as we please.
However, let us also remember that we do not stand apart from nature: we are embedded in it. We can neither dominate nature nor hope to leave it unmarked. Nature is affected by the very fact of our existence. We are a very small part of a huge canvas.
At this moment, a huge asteroid with our name on it could be heading towards Earth, just as an asteroid bearing the dinosaurs' name flew towards Earth 65 million years ago.
Every trouble-free day which arrives is a gift. We should accept these gifts with gratitude and take the care required to ensure that we don't foul our own nest.
William Reville is a senior lecturer in biochemistry and di- rector of microscopy at UCC