All life known to science is found to encode the fundamental information for its construction and reproduction as a linear string of nucleic acid, either RNA or DNA. This encompasses all cellular life as well as viruses, which are not unanimously believed to be alive. It does not include prions, which have few advocates as living items. This chemical commonality suggests the possibility of a single common ancestor for all organisms alive today. This in turn points to the concept of a single origin of life.
First the evidence that all living things share one common ancestor. Knowledge of the chemical complexity of the fundamental organic molecules necessary for life has shown how remarkably difficult it would be for life to be generated spontaneously. Yet this is the orthodox theory for the origin of life on Earth. The Urey-Miller experiment [ref 1] demonstrated the formation of amino acids from water, methane, ammonia and hydrogen, a hypothetical simulation of the early atmosphere. This experiment has its advocates and detractors. Criticisms generally focus on the supposed content of the early atmosphere. However, as far as the true generation of living organisms is concerned the far more pressing difficulty is how a 'soup' of amino acids, however created, could be induced to spontaneously form complex proteins, or how a solution of dNTP molecules could similarly be induced to form DNA strands that would code the proteins necessary for their own replication. It is not my purpose here to propose a solution to this dichotomy, but to suggest that the problems inherent in theories of abiogenesis indicate with overwhelming likelihood that life originated on Earth only once.
Figure 1. Stanley Miller carrying out the Urey-Miller experiment in 1953
It is a fundamental empirical observation that life is incredibly efficient as its own replication and the consumption of the resources available to it. This observation is the strongest evidence for a single origin of life. Given our current knowledge of the chemistry of early of Earth we must assume that the probability of life originating at any one place and time would be extremely low. It therefore seems sound to suggest that once life did form, as it evidently did, it would have ample time to replicate, spread and consume before a second genesis could occur. Once the possible abiogenesis sites were already occupied by the first wave of living organisms the life span of any randomly generated organic molecule would be vanishingly small before it was consumed by some hungry creature. In addition, experiments such as the Urey-Miller experiment have shown that an atmosphere with even a small proportion of oxygen is unsuitable for the production of organic molecules. The presence of oxides in rocks dated to the time at which we expect life to have originated suggests that the presence of life immediately began to create an oxygenated environment. This would have drastically reduced the possibility of a production of significant quantities of organic molecules. Indeed some creationists have cited this as evidence that life could not have arisen naturally because oxygen would have broken down the necessary compounds before they could polymerize. This however neglects that the probable source for an oxygen rich atmosphere would be life that had originated shortly beforehand. All of these factors limit the abundance and life span of organic molecules once life had originated once. Given that a second creation would require quantities of complex molecules to be available for a sufficient length of time to organize themselves into the incredibly complex structures necessary it is not hard to see that a single origin is more likely.
Obviously the argument so far is a probabilistic one, and almost by definition must be one. It also relies on a large number of assumptions about the availability or lack of particular compounds on the early Earth. To support such a hypothesis it would be useful to find some form of further chemical evidence of common origin. One such piece of evidence is the observation that life is asymmetrical in its choice of organic molecules. Many organic molecules occur in two mirror image forms, labeled by the sense in which they polarize incident radiation. These are D (dextral) and L (levorotary). D molecules polarize light in a clockwise sense, L molecules anti-clockwise. Synthetic generation of these molecules always produces the two types in equal abundances. However, it is observed that nearly all organic molecules occurring in, and used by, living organisms are L type. The orthodox theory for this observation is that L molecules were present in the original genesis of living matter, presumably by chance. Once this had happened the life that had been created would naturally only use and produce L type molecules and thus the symmetry of the system would be broken. It is easy to see that the probability of subsequent creations producing the same L type life as before according to this theory is ½. Therefore a hypothetical system with n creation events has an immediate probability negation factor of (½)n. This can hardly be deemed to rule out such possibilities in itself but provides support to the earlier argument.
Besides the chemical evidence there are a host of events at various stages of the evolutionary tree that seem to indicate a single origin. Perhaps the most striking of these are the double genetic structure of eukaryotic cells and the triple genetic nature of plant cells. Taking the example of eukaryotes, all such organisms share the common feature of mitochondria, with their own separate genetic data. Similarly for chloroplasts in plant cells. This separate DNA points to the origin of these organelles as independent organisms that first inhabited cells parasitically, and later symbiotically. It would be an enormous coincidence if this had happened more than once in history with the same original organisms adapting to the same roles independent of each other. Thus these features provide almost irrefutable evidence that all eukaryotic cells at least have a common origin.
Recent data from the revolution in DNA sequencing has shown that so called 'higher animals' and multi-celled organisms in general share many genes with bacteria, particularly those related to very basic functions such as metabolism (humans sharing around 200 genes with bacteria). It is hard to imagine that if prokaryotes and eukaryotes had been created independently that they would share so many fundamental cell processes. Given the complexity of many of the cells metabolic systems one would expect that independent origin would be revealed by somewhat differing processes. The commonality between all living things at the genetic level is demonstrated by the methods used to amplify DNA samples or produce artificial proteins. Genes from higher organisms, such as humans or pigs, can be transplanted into the DNA of a bacterium with relative ease, whereupon the mechanisms employed by the bacteria for transcribing its own genome to produce proteins will happily transcribe the foreign DNA and produce the appropriate proteins as well. It is remarkable that organisms with such divergent histories that fill such different ecological niches should use such a coherent data read-write system. Again it is very hard to believe that this would be the case had these organisms not arisen from a common ancestor which employed the same system.
The argument so far centres on demonstrating that all organisms alive today must share a common ancestor, and that the independent creation of organic life is extremely unlikely given what we know about organic chemistry and the environment of the early Earth. However, on a more theoretical level we should examine the possibility of the origin of life forms based on a different chemical system that may or may not have contemporary descendants. As mentioned in the introduction to this essay, the arguments considered so far exclude the concept of prions as a form of life. One can debate what exactly constitutes a living organism. Indeed it could be argued that the case for a single origin of life is tautological, given that we define only those things that share a common chemical system for the storage of data to be 'alive'. It is hardly surprising therefore that we find that all living things share a chemical heritage!
Figure 2. Prions: What is our definition of life?
Most biologists would argue that for something to be considered alive it must metabolise. In other words in should use resources from its environment to produce its own energy and means of reproduction. An entity such as a virus falls very much on the dividing line of this thinking, whereas a prion would be deemed not to be alive. It is arguable however that this reflects the bias of biologists' background. Personally, looking at the subject from a more physical viewpoint I would argue that a definition of life based on the transmission and preservation of information would be more rigorous, especially if we were to confront more novel phenomena that showed the traits of life. According to this type of definition it is possible to class prions as a life form. They transmit the information inherent in their particular shape to other proteins with high fidelity. They have been shown to diverge into separate forms, allowing the possibility of evolution via selection of one form over another. Once they have taken their shape they hold it and thus show evidence of the preservation of information. The information is not of such a convenient linear and numerical form as found in DNA but it is our bias that prefers digital data that is easy for us and our computers to deal with, not a necessity of nature. Part of the bias against prions may be due to their simplicity as single molecules that essentially crystallize similar proteins in their environment into an identical form. However, is this not what we would expect at the creation of life? One hardly believes that life was created with complex multi-cellular organisms bounding over the uninhabited Earth!
Ultimately the question of whether life on Earth originated in one single event becomes a matter of semantics. Whether we believe that it did or not depends on what we believe life to be. What we believe life to be simply reflects what we feel to be important and novel enough to be classed as such a high status phenomenon. It seems probabilistically implausible that carbon based organic life originated in multiple independent events on Earth. Thus by our usual definitions a single common origin of life is overwhelmingly likely. On the flip side of the argument our definition of life seems to be designed to produce that very outcome. Perhaps because carbon based life is so prevalent on Earth we are blind to other possibilities. The standard definition of life may be challenged by the search for extra-terrestrial life, especially as self-styled astro-biologists may find themselves more willing to bend their definitions given the opportunity to announce the discovery of 'aliens'! We may then be forced to reconsider the status of entities such as prions that have been treated as purely lifeless phenomena. The recasting of life in terms of information might also allow the consideration of new entities such as computer viruses and even human ideas, as in the meme theory proposed by Richard Dawkins [ref 2]. One might also wonder whether a question that ultimately comes down to definitions, and is therefore simply words about words, can be classed as science at all!!
Figure 3. Would the discovery of information transmission via non-carbon based systems on other planets such as Titan make us reconsider our definition of life on Earth?
There is of course one particularly popular theory of the origin of life that postulates no origin of life on Earth, namely that life originated elsewhere in the universe and then arrived at Earth via a meteorite or comet. This theory comes in various guises ranging from the deposition of organic molecules via a meteorite to the deliberate seeding of the planet by intelligent life elsewhere. The more elaborate versions have little or no evidence to back them up but the concept of organic molecules arriving from space is probably at least as plausible as their spontaneous generation on Earth. Allowing for generation elsewhere in the universe gives scope for several extra billions of years in which generation could occur, as well as shielding the growing compounds from chemicals and radiation that could have destroyed them in the early stages of their growth.