conditions for supporting life always coincided. From observations that living phenomena and the environment change together he inferred that ‘Life and its domicile change together’, adding that ‘life but waits in the wings of existence for its cue, to enter the scene the moment the stage is set’ (Lowell, 1909: 69). From the assumption that life will accompany the conditions which support life, Lowell was able to secure his hypothesis that Mars was inhabited by an appeal to empirical claims regarding a favourable atmosphere, signs of vegetation and so on. This inference is fallacious. We cannot infer actual life from evidence that a planet is habitable. I could live in Spain but I do not. Nevertheless, this view concerning the necessity, or predestination, of life has been echoed by exobiologists throughout the twentieth century. Thus it is argued that if conditions favour life it will emerge, and if it emerges then it will lead to intelligence. In reply it must be asserted that proof of the existence of extraterrestrial intelligence must be direct; it cannot be inferred from descriptions of conditions in which intelligent life would be comfortable.

One of the problems in identifying life is that early forms of life do not have the qualities associated with more advanced forms. No definition can provide a fine demarcation line between life and non-life at the primitive molecular level. Yet one of the preconditions of advanced forms of life is the emergence of multicellular organisms. For most of the Earth’s history life was unicellular. Multicellular organisms appeared about 700 million years ago.

One thing is certain; life has evolved on Earth and continues to do so. More-over, life on Earth can survive in extremely inhospitable conditions, such as the bottom of the oceans, in frozen wasteland and on mountain tops. In this respect Earth can be viewed as a laboratory which, if the laws of physics and chemistry are universal, can provide experimental models of how life can develop. But Earth is not the only laboratory. Suggestions have been made that Saturn’s satellite, Titan, could be investigated as a precursor site for emergent life. When the Voyager spacecraft, launched in 1977, flew past Titan, it revealed spectro-graphs showing the presence of organic molecules, including complex hydrocarbons, acetylene, and hydrocyanic acid, which are fundamental building blocks of life. Titan’s diameter is 30,000 miles, half that of the Earth; it has a dense atmosphere, essentially composed of nitrogen. This would be the case on Earth but for the presence of oxygen of biological origin, which is mixed with the nitrogen. But because Titan is far from the Sun its temperature is -170ºC. Titan could be a good model of a primitive Earth in a state of deep freeze (Heidmann, 1989: 59).

The origins of life

Our galaxy is at least 12 billion years old. The Earth is 4.6 billion years old. Life on Earth can be traced back to about 3.85 billion years ago. Rocks 3.5 billion years old contain fossil organic molecules suggesting a rapid emergence of biotic activity. There are stromatolites, found at the North Pole, about 3.8 billion years

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old, and perhaps even older ones have been discovered in Western Greenland. Life on Earth must have appeared fairly quickly, less than 700 million years after the planet’s formation. The early atmosphere contained an abundance of biogenic compounds but very little oxygen. There are fossil remains of the earliest oxygen-breathing eukaryotic cells which have been found in Siberian rocks about 725 million years old and about 800 million years old in the Grand Canyon (Gribbin, 1994: 92).

How life emerged is very much a matter of speculation. There are three possible options: [1] it began as a supernatural act; [2] it emerged from natural physico-chemical processes; or [3] it came to Earth from external sources. The first suggests an arbitrary change which, like accounts of miracles, is not refutable, but is not compatible with scientific inquiry. The second suggestion can be pursued, using available knowledge about the development of life on Earth. At a chemical level life involves the interaction between nucleic acids and proteins. Darwinian theory stresses that the root of all life is the ‘primordial germ’, from which more complex forms developed. In the 1930s John Haldane in England and Alexander Oparin in the USSR suggested that life was originally formed in warm volcanic oceans in the early Earth. They spoke of a ‘warm dilute soup’, but our knowledge of this phase is very hazy as there are no traces left of the period. There has been no success in showing that these alleged early conditions actually occurred. Biologically significant small molecules have been formed in experiments, in the 1950s  and  1960s, under conditions believed  similar to the primitive Earth.

Some SETI exponents often appeal to the following argument. Life began with a combination of certain chemicals with carbon and water, which are abundant throughout the universe. If life appeared within the first billion of Earth’s 4.6 billion-year history, it seems likely that it will occur on similar planets, given a favourable environment. Support is also drawn from  laboratory experiments in which the early conditions on Earth are replicated. For example, in 1952 Stanley Miller and Harold Urey successfully synthesized amino acids in large quantities and thus demonstrated that a primitive atmosphere of methane, ammonia, water vapour, and molecular hydrogen bathed in ultra-violet light and charged with electric shocks akin to lightning, can replicate conditions whereby the early building blocks, ‘precursor’ molecules, like glycine and alanine, are formed. Various formulations of the Miller–Urey experiment have succeeded in synthesizing 18 of the 20 amino acids needed for life. Yet despite some initial successes involving the synthesis of polymers and more sophisticated precursors, attempts to simulate the ‘accidental’ conditions in which life allegedly started have not gone far. Critics of the ‘warm ponds theory’ frequently point out that scientists have so far failed to duplicate the spontaneous origins of  life in laboratory conditions. However, the theory might be defended by using assumptions familiar to SETI. Life has had millions of years to evolve spontan-eously on Earth, or even in some other region. But attempts to duplicate it in the laboratory have only involved a few thousand ‘man-hours’ to date. So put a few

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