The idea that we are not alone in the universe took strong root in the imaginations of philosophers and scientists in 1543. In that year Copernicus demoted the status of the Earth from its position at the centre of the universe to that of a mere satellite of an ordinary star.
The thinking was, and remains, that since there are probably very many Earth-like planets in our galaxy, why should Earth be unique in harbouring intelligent life?
Now that we have developed the capacity to actively search for life elsewhere in the universe, we are seeking evidence of simple life on Mars and there is also a programme called the Search for Extraterrestrial Intelligence (SETI) that scans the cosmos for radio transmissions from alien civilisations. No radio messages have been received to date.
The more experience we get under our belts the more critically we can think about this matter. Our lack of success to date in detecting the presence of alien civilisations is causing some scientists to re-visit the question first posed by the Italian physicist Enrico Fermi (1901-1954). If alien civilisations are common should this not be obvious to us?
SETI is based on the idea that civilisations elsewhere in the universe, at or beyond our level of development, will try to communicate with the rest of the cosmos. It is believed that the preferred mode of communication would be radio broadcast, so SETI scans the sky hoping to pick up a message.
So far a search of the cosmos extending 4,000 light years outwards from Earth has failed to detect any signals that a civilisation at our level of development might be capable of transmitting. Also, a search extending 40,000 light years outwards has failed to detect stronger signals that might come from more advanced civilisations.
Despite the fact that SETI is at an early stage (our galaxy is 100,000 light years in diameter), the silence to date is beginning to trouble scientists who had predicted that advanced life is common in the cosmos.
A typical SETI estimation is that our galaxy contains 1,000 radio-transmitting civilisations. If so, making conservative estimates of the rate at which civilisations rise and fall, and knowing the age of the galaxy (12 billion years) it can be calculated that billions of technological civilisations must have existed over the history of the galaxy.
We have listened in vain for sounds from current alien civilisations since 1960 and we have no evidence that any advanced civilisations existed in the past. Why, as Fermi asked, is this?
It seems very likely that some very advanced civilisations, if they existed, would attempt to colonise the galaxy. Calculations show that a civilisation capable of interstellar travel at one tenth the speed of light - which should be readily attainable - could colonise the entire galaxy in five million years. This period represents only 0.05 per cent of the entire age of the galaxy, but there is no scientific evidence that Earth was ever visited by extra-terrestrials.
There is, of course, the fascinating speculation that life was seeded on Earth four billion years ago by visiting aliens. If this is the case, we have no evidence that they bothered with us in the meantime.
The alternative scenario to the more usual optimistic one of a galaxy studded with advanced civilisations is that, whereas life may be common in the galaxy we are the only example of intelligent life. This position can be supported by considering the evolution of life on Earth.
The Earth was formed about 4.6 billion years ago and there is evidence that a simple unicellular form of life existed as long as four billion years ago. In other words, life began as soon as conditions on the Earth moderated enough to be compatible with life.
Many scientists have concluded that given the right conditions nature finds it easy to start up life, and of course this will happen in the universe everywhere the conditions are right.
But how much complex development might we expect from such simple life. Again consider the evolution of life on Earth. For over three billion years life did not evolve beyond the unicellular stage. Multicellular animal life did not appear until 700 million years ago.
It is therefore reasonable to conclude that the development of multicellular life is very difficult and therefore improbable. Although life could arise on a planet it might never evolve beyond a unicellular form.
We know that chance played a prominent part in determining how life evolved on Earth. The reign of the dinosaurs on Earth was ended by some catastrophic change - the favourite current theory is that this was a catastrophic collision of an asteroid with Earth.
The dinosaurs ruled the Earth for 140 million years, but never produced a civilisation. Neither was there anything in their general biological logic - which leaned on brawn rather than brain - to suggest that they would ever evolve significant cleverness.
Their chance demise allowed the mammals to develop which eventually allowed human civilisation to arise. So life could begin on a planet, might evolve to a multicellular animal stage but might never go on to evolve self-conscious intelligence.
I still think it is improbable, but we humans may well be the only intelligent life in the galaxy. This possibility is awesome to contemplate. If we are alone in the galaxy, we have inherited the most precious gift imaginable and have an unavoidable responsibility to nurture and to spread our light. The possibility that we are alone should make it unthinkable for us to foul our nest on Earth or to behave badly towards each other.
The topic of alien civilisation in our galaxy is discussed in detail in a fascinating article by Ian Crawford in the July 2000 issue of Scientific American.
(William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.)