The likelihood of life

Science is about regularities in nature: patterns that can be described, explained and predicted. It deals less comfortably (if at all) with unique, one-off occurrences. We have concrete evidence for life on only one planet - our own. And for reasons given in the previous section, it is arguable that the evolution of all surviving organisms began from a single, unique origin-of-life process. ("Process" means a specific sequence of physical and chemical events. No biologist could believe that life arose from non-life in a single step.) We have no direct evidence about this process and there is no consensus about the sequence of events involved. Everything we can say about the origin of life is a mixture of inference and guesswork. Even our most basic questions about it invite speculation; we are in no position to argue from incontrovertible fact. The best we can do is to temper our speculations with scientifically informed reasoning.

One of the most fundamental questions is this: was the origin of life likely or unlikely? Both alternatives can be, and have been, supported by reasoned (though circumstantial) argument. Broadly speaking, before the closing years of the 20th century, a consensus of scientists believed that the process was astronomically unlikely. Since then, the consensus has perhaps swung the other way. But in this field, fundamental shifts of opinion are matters of fashion rather than advancing knowledge and insight. Both opinions remain legitimate.

First, there are good arguments that the origin of life on Earth was highly improbable. The order of events involved in the process is controversial, but there is broad agreement about what many of the events were. Organic molecules such as amino acids and nucleotide bases had to be present in the right environment. These had to polymerise so that proteins and nucleic acids formed. A dialogue had to be established so that the nucleic acids directed the synthesis of the proteins, and the proteins catalysed the expression and replication of the nucleic acids. These processes had to be enclosed in a membrane-bound system. Energy-providing and signalling apparatus had to be constructed. Organisation had to be imposed. And so

All or most of these events are inherently improbable. Few can be simulated in the laboratory and many can only be described in terms that are difficult to relate to the nuts and bolts of physics and chemistry. It is astronomically unlikely that a long sequence of individually improbable events could happen, by chance, in exactly the right order. Therefore, the origin of life was astronomically unlikely. (The probability that all our proteins could have been formed by chance, with the correct amino acid sequences, has been estimated at 10-40000, which is effectively zero.) This conclusion has the advantage of circumventing the "spontaneous generation" problem (see the previous section). Explaining why life stopped originating once it was established becomes a non-issue: it was simply too unlikely to have happened twice on the same planet.

There are equally strong arguments that the origin of life was probable. First, life began while the Earth was still very young, almost as soon (it seems) as the surface was able to bear liquid water. That is to say: as soon as life became possible, it happened. Second, the conditions on the early Earth probably resembled those in which the deep ocean vent archaea live today. However, the environment was so violent and catastrophe-prone -continual comet and large meteorite impacts, ultraviolet irradiation, incessant volcanic eruptions and earthquakes - that an individual newly-formed organism could have had little chance of surviving and leaving offspring. So it is highly likely that life was extinguished very soon after it began. Presumably, therefore, life must have begun on Earth many times, and only one of the original organisms managed to survive to become our ultimate common ancestor. If so, the origin of life was a common occurrence on the primitive Earth, so it could not have been unlikely. Third, it is now widely believed that life also began in other parts of the Solar

System - on Mars, for instance, and perhaps the Jovian moon Europa -though it might not have survived for long on these bodies. If life began on several planets or large moons in the same Solar System, it could not have been a particularly improbable event.

At present we have no way of deciding between these opinions. If future space probes reveal evidence of past life on, say, Mars or Europa, the "life is probable" position would be favoured. But the opposite view could still be maintained. Perhaps spores of life were, after all, transmitted from body to body in the solar system by meteorites. Failure to find evidence of past life in other parts of the Solar System would not affect the argument at all: absence of evidence is not evidence of absence. Of course, if it were firmly established that extraterrestrial life once existed and had a different molecular basis from ours (a different genetic code, different amino acids, or even alternatives to proteins and nucleic acids), then the "life is probable" option would become almost certain. But that is science fiction.

It is worth noting here that the sequence of DNA bases in the genome of any organism is "random". It is information-rich and unpredictable by any law - it is, in technical language, "algorithmically incompressible". But it is also highly specific; it carries semantic meaning, in the sense that it determines and directs the amino acid sequences of all the organism's proteins. No known law of nature allows, still less specifies, the production of "highly specific randomness". Nothing in science tells us how an object with semantic meaning can arise by physical and chemical processes. Evolution by mutation and natural selection produces highly specific randomness, but only by operating on organisms already in existence. This gives us no clues about how the first-ever organisms came to exist. More than one author has suggested that laws of nature crucial for explaining the origin of life remain unknown to us.

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