THE ORIGIN OF LIFE ON EARTH: AN UNSOLVED PROBLEM

There is now a consensus of informed opinion on some important dates in the Earth’s history. Several lines of astronomical evidence point to the ‘Big Bang’, the high-temperature, high-density event when the universe began, as occurring roughly 15 billion years ago. Then about 4.5 billion years ago, the Sun and then the Earth formed, essentially by the accretion of dust and small particles, and without living forms of any kind for its first billion years. About 3.5 billion years ago, life appeared: the oldest known fossilized microorganisms, found in datable rock formations in Western Australia, are of that age. So began life on Earth, with its defining characteristic, the ability to replicate itself through the storage and passing on of genetic information from generation to generation. Rather slowly the first primitive, single-cell microorganism led, through evolutionary changes that are now understood in outline, to the whole range of multicell plants and animals known today. Modern man and woman ( Homo sapiens ) is about 50 000–100 000 years old.

work in the 1860s established that ‘spontaneous generation’ of life does not occur in laboratory conditions, although his experiments did not exclude all possibility. He ‘did not think it impossible’ in 1878 and in 1871 had speculated that protein, a characteristic chemical type found in all living things, might have originated in ‘some warm little pond’. In the early 1900s suggested that life on Earth might have begun through the arrival of organisms from elsewhere in the universe (the theory of ‘panspermia’) and in the 1970s this proposal was revived by , but has found little support.

A valuable proposal was made by the Soviet biochemist and independently by , who argued that the early atmosphere of the Earth contained little free oxygen (O2 ), and that this was generated much later as a result of photosynthesis by green plants, a view now generally accepted.

By the 1950s a good deal of information more or less relevant to the problem of the origin of life had been accumulated. For example, reasonably well-based estimates of the origin, age and composition of the universe and of the solar system and of the Earth were available, and a range of simple organic compounds (but none characteristic of living forms) had been identified in the dust and gas of space beyond Earth’s atmosphere and in meteorites arriving on the Earth’s surface from the outer parts of the system.

In 1952 argued that the important energy source for early geochemical and prebiotic Page 358  chemical reactions was very probably the stream of solar ultraviolet radiation, together possibly with lightning discharges and meteor impacts; and that the early (‘primordial’) Earth’s atmosphere was composed mainly of CH4 , NH3 , H2 and H2O. The next year his co-worker passed electric discharges through this mixture and showed that within a week an impressive range of organic molecules was formed, including no fewer than 25 amino acids. Amino acids react rather easily together to form protein, whose presence is so ubiquitous in living systems.

However, the discovery by of the double helix of the nucleic acid DNA as the key material forming the genes, a discovery which effectively created the new science of molecular biology, generated a new difficulty for theorists of the origin of life. By 1958 Crick laid down the ‘central dogma’ of the new science: that the direction of flow of chemical synthesis in living systems is one-way and irreversible, in the sense: ‘DNA makes RNA, and RNA makes protein’. (Exceptions to this have proved to be rare, but cases are known: the retroviruses contain RNA as the genetic molecule, and this makes the more complex DNA, a reversal of the general rule.)

So the idea that a thin ‘primordial soup’ of amino acids, leading perhaps to proteins and on to the simplest living cells, could form a satisfactory model for the origin of life, is clearly unsatisfactory. In any case, as A G Cairns-Smith pointed out, it is hard to see how a thin soup could organize its amino acids and proteins in a way describable as ‘life’ rather than merely undergoing unfruitful chemical interactions.

Despite much effort by many investigators (theorists, analysts and experimentalists), the question of how life originated remains very open indeed. There is fairly widespread belief that the simpler class of nucleic acid, RNA, probably formed at an early stage in the path to the first, and doubtless very simple, living cells. From RNA, it is not difficult to see in general terms how protein and then simple cells may have developed; and thereafter, evolution may have followed a broadly understood network of increasing complexity. But how did the RNA, a rather complex biopolymer, originate? A credible route to it from the fairly simple organic molecules that might have been available has yet to be devised.

The mystery remains. It may prove to be unsolvable, permanently. Likewise the question of whether life only exists on Earth and if not, whether it began here or came here, may be unanswerable.