systems would not have the protection of giant sweepers, which may even become attractors, and life would be frequently extinguished before it has a chance to develop.
What can be concluded from research on catastrophes on Earth is that the development of complex life-forms has to take place within periods between life-destroying volcanic or impact events, and that for various complex forms of life to develop, a planet will have to be free of volcanic action or impacts for long periods.
It may be that the most typical form of life in the universe is some form of hyperthermophilic life, which can survive in extreme conditions, and can withstand the volcanic and impact disasters which periodically wipe out all other life forms. Jakosky (1998) maintains that hyperthermophilic life is the norm and everything else has developed from it. Given the wide range of energy sources that are known to have been utilized by early life-forms, it can be plausibly suggested that life may be widespread throughout the universe. But whether or not it has developed along the complex lines found on Earth is still a major unresolved question.
The tenacity of life
Discoveries of exotic forms of life, which survive in extremely inhospitable conditions on Earth have given rise to speculations regarding the possibility that life may be found in similar inhospitable sites throughout the solar system. These speculations not only intensify arguments in favour of more detailed searches of planets, such as Mars, but also generate hypotheses regarding the origin of life on Earth.
During the past few years many thousands of different strains of micro-organisms have been discovered surviving in intolerable conditions, as deep as 3.5 kilometres beneath the Earth’s crust at temperatures of 113ºC (Pain, 1998: 28–32). This could suggest that living organisms might also exist in other apparently inhospitable zones in the universe. Although satellite images of Antarctica have shown it to be barren like Mars, towards the end of the twentieth century it was acknowledged that Antarctica is teeming with life. Living microbial organisms have been discovered in frozen Antarctic lakes, which are probably the most inhospitable sites on Earth, thus suggesting that similar forms of life might exist on icebound planets or comets.
There is so much evidence of early life on Earth, when its surface temperatures were extreme, that Darwin’s notion of life originating in a ‘warm little pond’ has to be ruled out. It is more likely that the earliest precursors of life were extremophiles, or hypothermophiles, which were unicellular bacteria that can withstand extremely high temperatures and high doses of radiation. Moreover, there are ecosystems on Earth that do not depend on photosynthesis, surviving within rocks deep in the ocean floor, relying upon combinations of oxygen and hydrogen sulphide. The implications for any search for extraterrestrial life is that
the scope of the search can be extended, including deep below a planet’s surface and within its rocks.
Investigations of extremophiles have generated two hypotheses concerning former life on Mars. The first view is that various forms of extremophiles survived the heavy bombardment of asteroids and comets which either eliminated or prevented the emergence of surface life on Mars. It is even possible, argues Paul Davies (1998: 24–9), that ejected rocks containing micro- organisms found their way to Earth thus enabling life to get started here. So it is possible, hints Davies, that Earth life actually started on Mars, and that we are the last Martians.
The second view, advanced by Jakosky, is more sceptical with regard to early life on Mars, as there was so little energy available from volcanism and plate tectonics – the kind of energy that might have triggered off life on Earth. Mars has a history of low volcanic activity, and has never developed plate tectonics. Many researchers believe that early cells formed within a period of 100 million years on Earth, drawing energy from chemical reactions as a result of volcanic activity, with photosynthesis occurring some 500 and 800 million years later. According to this view, deep-seated extremophiles on Mars would have less chance to surface and develop.
It is a very big step from primitive life to intelligent life. It took life on Earth 3 billion years to go from a single-celled to a multiple-celled stage, and altogether it took nearly 5 billion years for intelligent life to evolve on Earth. If this is the case on other planets orbiting a stable sun in a thermally habitable zone, then we cannot expect to find intelligent ETs near stars that are less than 5 billion years old. This might, however, be contested with reference to setbacks encountered on Earth which might not have been encountered on other planets. Thus extraterrestrial life could have evolved faster on other planets if it did not have to re-evolve several times. Hence the average genesis of intelligent life throughout the universe may be less than 5 billion years.
If intelligent living systems are diffused by means of space travel, the probability of intelligent ETs would be considerably increased within the 5-billion-year period. Travelling at speeds approaching the speed of light it would take 300 million years to reach the centre of our galaxy. That, of course, would be the absolute minimum time in which life could be spread around the galaxy. But even if we multiply this figure by ten it would still be quicker to spread life by diffusion than wait for its random development. The time may be further reduced if we allow for the possibility that some advanced civilization is spreading out towards us. SETI bioastronomers maintain that once life appears, then intelligence is likely to follow and flourish as it bestows selective advantage on those endowed with it. However, this appeal to human intelligence is a very weak link in the chain of assumptions underpinning SETI, and its likelihood of
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