By Robert C.B. Miller
Is life rare or is it common? The search for life on Mars, elsewhere in the solar system and in the Milky Way continues apace. Planets on nearby and quite distant stars have been discovered and investigated to determine whether they are in the habitable zone where life is possible – not too far from their star so that they are not too cold and not too close so that they are not too hot. Hope is expressed that life may be found in the water beneath the ice shells of Europa, Enceladus and Titan the moons respectively of Jupiter and Saturn.
Searchers, some funded by internet billionaires, are checking the electro-magnetic spectrum for radio signals which are artefacts and the product of intelligent life. Leading public intellectuals, such as Stephen Hawking, have opined on the risk that alien intelligent life might discover us and do us harm. So far no convincing evidence of life has been found.
But there is a puzzle about life. Biologists have difficulty in explaining how abiogenesis, the emergence of living from non-living material, is possible. It is now often forgotten that one of the great triumphs of modern science was the discovery that life could only come from life. No longer could it be believed that life could just emerged from non-living material as Aristotle thought. But in turn this discovery created a problem. If life did not emerge from the non-living, how is it that the world is full of living things? One apparently obvious solution to this puzzle is that it was the result of evolution. But this cannot be the case, evolution presupposes the existence of life on which the Darwinian machinery of mutation and adaptation can work.
Much energy has been used by biologists to discover how life originated and it was expected that this would be a relatively simple matter. Life, it was thought, could be shown to be the result of a chemical process that converted bare chemistry into biochemistry. But after much effort and theorising, none of the putative accounts developed have been accepted and all remain controversial. The origin of life remains an unsolved problem for chemists and biologists. In any case any the all must remain what cautious biologists call ‘Just So Stories’. The reason is that there is very clear evidence that abiogenesis occurred in the extremely distant past and consequently that almost all evidence of the process must have long disappeared.
But two related facts are clear, however life originated. First all life uses the same DNA building blocks and second all life descended from a single common ancestor. This means that the ‘tree of life’ with species branching from a common origin in a hierarchy pruned by extinction and expanded by well-known evolutionary processes is a well-established conclusion of biology.
The Two Strange Two Facts about Life
This leads us to two remarkable facts about life on earth which deserve much more attention for the extraordinary facts that they are.
First, that the appearance of life on earth took place a very long time ago – give or take a few hundred million years – between 3.8 and 4.1 billion years ago. This great age is significant as it seems that life appeared on earth very shortly about 600 million years after the planet formed 4.54 billion years ago and an even shorter period, perhaps a few hundred million years, after the earth became habitable. This distant date should be put in the perspective of the age of the universe which is estimated at about 13.8 billion years. This means that life appeared at about 70% of the age of the universe on a planet which had only just become habitable for life.
Second, there is very strong evidence that life appeared on earth only once. This is guaranteed by the ‘tree of life’ and the common DNA building blocks of all life. There is evidence on earth of only one kind of life and no evidence of any others.
In one sense there is of course nothing extraordinary about these facts. If life was to appear there seems no reason for it to appear at any one time rather than another provided conditions were right. Thus if life appeared as the result of a highly unlikely series of events and conditions then it would appear reasonably for it to appear 70% through the existence of the universe rather that at the beginning. The more unlikely are life producing events and conditions then it is more likely to appear latter rather than sooner. Thus if I have six fair dice then I am more likely to have roiled six sixes by the 100,000 roll than I am at the first roll. But while this argument may appear reasonable in the perspective of the life of the universe, it seems extraordinary in the context of life of the earth. It looks as if someone rolled six sixes on the very first roll of the dice.
But now the argument gets strange. As we have seen there is strong evidence that life emerged on earth only once. Given that life emerged on earth almost as soon as it became possible, it seems that it was the ideal environment for the appearance of life. But if life emerged rapidly in an ideal environment it is deeply puzzling why it has not emerged again in the 3.8 billion years since it first appeared. If it has emerged once, then why not twice or indeed many times? If life appeared so soon after it became possible then it suggests that the odds were not as long as was thought and that it might only be necessary to roll a single six rather than six sixes for life to appear. But if that is the case and the die was kept rolling then one would expect a regular supply of sixes after the first die roll. But as we have seen this appears not to have been the case. Why indeed are there not multiple forms of life? Why is there not a strand of life on earth based on silicon rather than carbon, or even boron? Why are there not a number of different types of carbon based life? There are none and no evidence that that any ever existed.
The evidence leads to contradictory conclusions. Life appears to be the result of a natural process which operates rapidly (or even immediately) when conditions are right. But in that case it is puzzling that it has emerged only once on earth. But its single emergence seems to suggest that conditions and process that leads to the appearance of life are extraordinarily rare. In other words, life appeared once on earth almost as soon as it became habitable, suggesting that it a high probability event. But since it has not appeared since this implies it was a very low probability event.
What is (or was) going on?
Could the puzzle be resolved by the fact that life did not emerge on earth until about 70% of the age of the universe had elapsed. This might suggest that the odds of life appearing are very long, given both the size and the duration of the universe. It is some 92 billion light years in diameter and has perhaps 100 billion galaxies each with 100 billion stars.
But the early appearance of life in the history of earth, a few hundred years after it became possible suggests that life should emerge similarly rapidly implying that the galaxy and the universe should be teeming with life. Wherever it can be sustained life should appear rapidly. But here we run into a version of the Fermi paradox. There appears as yet to be no evidence of any kind to suggest that there is any other life outside the solar system. This may change with, for instance, the discovery of significant amounts of oxygen in an exoplanet. Let us assume, for the sake of argument, that signs of life are discovered on a significant number of exoplanets within the Milky Way. Would it dissolve the paradox that the appearance of life on earth seems both probable and improbable at the same time?
Plainly it would not solve the puzzle and it would actually exacerbate it. The appearance of life on exoplanets would shorten the odds of life appearing on earth – and indeed on all habitable planets in this and other galaxies. What it would not do is to explain why life appeared only once on earth at the very beginning of earth’s habitable period. If the odds of life appearing are so short why has it only appeared once in the (evidently) ideal conditions of earth? The paradox remains, indeed it is more puzzling than it was before. Life appears highly probable – its emergence being evidently a short odds event. But if the odds are short, why has it only appeared once in the 3.9 billion period of earth’s habitability – a period which amounts to 30% of the lifetime of the universe? As we argued above one would expect that there would be a series of new appearances of life. The evidence is clear, there have been none.
The Paradox of Life
Where does the paradox of life leave us? One remote possibility is that new versions of life might be discovered on earth – suggesting that the appearance of life was really a short odds event. But this solution does not look promising. As we have seen all life has the same DNA structure and the tree of life with descent from a common ancestor appears a very robust conclusion of biology.
What then? One possibility is that the appearance of life may not be subject to the same probabilistic causal regime as other biological events. It may just be odd, but that is indeed an odd conclusion.