Cosmology, in partnership with geology, has aged the Earth at approximately 4.5 billions years. During the early formation of the Earth, its molten hot core would have been too unstable to support life of any kind. Therefore, it might be surprising that scientific evidence, in the form of the fossil records, suggests that simple biological life appeared on Earth as early as 3.8 billion years ago. However, many discussions of the evolution of life start and end with Darwinian natural selection. However, this perspective ignores the equally important issues of how life initially separated itself from inanimate chemistry and how human life may ultimately come to control its own evolution.
In the context of biological life, 'abiogenesis' is the study of how life on Earth could have been triggered from what might seem little more than a cause and effect chemical process, which really has nothing to do with evolution by natural selection. In 1924, a Russian biochemist, Alexander Oparin proposed that living cells may have come into existence through a sequence of chemical reactions. The basis of Oparin's idea was that lightening may have triggered gases in the atmosphere of a young Earth to form simple organic compounds. These compounds then subsequently formed increasingly complex molecules, e.g. proteins, which in-turn organize into living cells. In 1953, Stanley Miller and Harold Urey tested Oparin’s hypothesis by conducting an experiment that attempted to simulate the atmospheric conditions on the young Earth. Without going into too much detail, this experiment essentially relied on combining water vapour with ammonia, methane, and hydrogen and the then subjecting this mixture to a 50,000 volt spark. The resulting residue from this experiment was a tar-like substance that contained a collection of amino acids, the building blocks of life.
So does the Miller-Urey experiment provide proof for abiogenesis?
Subsequent analysis, based on a more detailed understanding of Earth's primitive atmosphere has now thrown up a number of problems with the Miller-Urey experiment. For example, it is now known that oxygen must have been presence, but its presence would have prohibited the development of organic compounds. So while most life has come to require oxygen to survive, it has also required many evolutionary adaptations that would not have been present in the most primitive cell-like structures. Subsequent geological evidence now indicates the presence of substantial quantities of oxygen and if these quantities are used proportionally in the Miller-Urey experiment, no amino acids are produced.
But surely there might have been some isolated localised environments?
Another major difficulty with the Miller-Urey experiment is that it cannot produce the right kinds of amino acids. The amino acids that comprise living proteins are of a left-handed form, yet in experiments, like Miller-Urey, an equal mixture of left-handed and right-handed amino acids are always produced. In fact, all known natural mechanisms in which amino acids are produced, always produce amino acids in roughly the same proportion of right and left-handed forms.
Ok, so we don't understand all the details, but what about the basic principle?
Well, even if we ignore some of the details, we would also have to ignore the fact that a living cell requires hundreds of specialized proteins that need to be precisely coordinated. We would also have to account for a mechanism in which DNA, RNA, cell membranes and a host of other chemical compounds could have come into existence, while not overlooking the equally important issue of how they came to be in the right molecular locations to perform their respective functions.
So what is the conclusion, is abiogenesis wrong?
No, it is simply the case that, at this point in time, abiogenesis is an unproved hypothesis and there is nothing wrong with hypothesis within scientific methodology providing the degree of conjecture is openly detailed and discussed.
So what are the alternatives to abiogenesis?
In terms of an overview, we might narrow the options to just two: 'panspermia' and 'creationism'. The hypothesis known as panspermia has forwarded the idea that life might have emerged in one location, then spread between habitable planets. However, this idea does not necessarily suggest that all life originated just once and then subsequently spread throughout the entire Universe. While it is entirely possible that life on Earth did start due to a 'migration' of primitive bacteria-like cells, the question of how this life actually originated remains unanswered. If panspermia and abiogenesis are rejected, you are pretty much left with only one other alternative, i.e. creationism. In the context of the present discussion, we shall describe creationism as a belief that the universe, and all life, within it was created by some form of deity for lack of a better word. Note, this definition of creationism should not be confused with the religious idea of 'intelligent design' that opposes the subsequent process of evolution by natural selection.
So where to from here?
It would appear that, for the moment, creationism is a belief that sits outside the scope of science. Of course, science might be better placed to refute this belief if a theory could be developed that described, in detail, how life might have emerge from the cocktail of inanimate matter known to have existed on the young Earth some 4 billion years ago. Alternatively, future research might provide solid evidence supporting the idea of panspermia, although in this case, the question of how life was actually created is left unresolved. With this said, it seems that we can only proceed on the basis of what evidence currently exists today, i.e. that life on earth seems to have appeared some 3.8 billion years in the form of primitive cells. Although along the way we might still conjecture on the nature of the following questions:
- What is life?
- How did life start?
- How did life evolve?
- Does life have to die?
- Will life continue to evolve?
- If so, how?
As a point of principle, our starting point for this wider discussion is the current state-of-play of scientific knowledge. In terms of the basic building blocks of life, this means a review of molecular and evolutionary biology.