Molecular properties - a functional cell requires many polymers, large long-chain molecules, built from a number of simple molecules (monomers). These molecules can hardly form spontaneously for several reasons. The formation of polymers requires bifunctional monomers (i.e. those that can combine with two others), and can be stopped by a small fraction of unifunctional monomers (those that can combine with only one other, thus blocking one end of the growing chain). Prebiotic simulation experiments produce five times more unifunctional molecules than bifunctional ones. Furthermore, many polymers (such as proteins, DNA or RNA) come in two forms, ‘left-handed' and ‘right-handed'. The building blocks of polymers essential for life need to have the same ‘handedness' - proteins consist of amino acids that are all ‘left handed', while DNA and RNA contain sugars that are only ‘right-handed'. Under the right conditions, an undirected environment that operates solely on the principles of physical chemistry can produce amino acids, but they are wrong for life. So created molecules are a blend of left and right hand forms, not the pure ones needed in living things. They could not form the specific shapes required for proteins, and DNA could not be stabilised in a helix and support life if just a small proportion of the wrong-handed kind was present. To produce the correct types of amino acids and sugars, life requires a certain type of proteins called enzymes. However, these complex molecules do not appear spontaneously, they can only be manufactured in a living cell. An equivalent of such a molecular machinery though, did not exist in the pre-life environment.
The cell components - even if all the right ingredients are present, there is still the problem of forming a functional cell components by random processes. Proteins and other structures necessary for life consist of many building blocks which must be instantly put together in a certain order. Out of a total number of possible protein structures (within an appropriate size range) only a tiny set have the correct properties from which a simple bacterium can be successfully built. The odds that they will be formed purely by chance is infinitely small. Let us consider the above mentioned enzymes. Just one is typically comprised of 300 amino acids. Even if it is assumed that a much smaller number of amino acids is needed to form a ‘primitive' enzyme, the probability of the required order in a single functional protein molecule arising randomly is estimated at 1043 (this is a modest estimate, some go as far as 10195). The simplest living cell must have at least several hundred enzymes and other proteins, which makes a chance arrangement of these molecules extremely unlikely. Silver writes that ‘the probability of a crowd of small molecules forming the needed large molecules to start the long, complex path to a single cell seems to be almost zero' (1998, p.349).
The cell membrane - the other necessary component of a living cell is the cell membrane. A universal ingredient of all cell membranes is the phospholipid molecule. This molecule can spontaneously form vesicles in water (‘bubbles' that resemble in shape the cell membrane), but no one has managed to reproduce this in the experiments that attempt to simulate conditions on the Earth when life began. Moreover, the cell membrane does not only maintain the physical unity of the cell, but also performs other vital functions (e.g. allowing energy exchange). This all requires a relatively complex structure even for the simplest imaginable functional cells that is highly unlikely to be a result of random chemical reactions.
Functioning of a cell - the above evaluates only the necessary parts, not a functional arrangement, i.e. one that works. Even if it is accepted that the components of a living cell were somehow formed accidentally, that would not be enough.
The simplest one-cell organism represents a level of complexity not found anywhere in the inanimate world including that created by humans (viruses, that are, roughly speaking, DNA coated in a protein, do not count, because they need other more complex cells to reproduce, so they could only appear or degenerate to this simplicity later in evolution).
All the components of a living cell need to be synchronised, to act in a union in order to maintain a cell. So, a cell cannot be built piecemeal, all the major constituents must have been created and assembled instantaneously for the cell to function. Without elaborate mechanisms that enable energy intake, chemical distribution, processing of proteins, and storing of genetic information to be passed on to the next generation, life could not exist.
So, the components on which these processes depend could not have evolved separately. Proteins cannot form without DNA, but neither can DNA form without proteins. Moreover, they could not exist independently for very long:
The large and complex molecules essential to life - proteins with dozens of hundreds of amino acids, RNA and DNA formed with long chains of nucleotides - do not appear spontaneously, even in carefully devised environments with high concentrations of monomers. Indeed, these macromolecules appear to be quite unstable. Even when two monomers link up or polymerise, they often will just as quickly disintegrate or depolymerise under water-rich conditions. (Hazen, 1997, p.165)
Most nucleotides (essential cell components) degrade fast at the temperatures that apparently existed on the early Earth. Polymers also quickly break down in water (water absorbing chemicals or evaporating water by high temperature would require either unrealistic conditions or would lead to the destruction of the polymers necessary to form a cell). This means that not only did they all need to be produced close to each other, but also within a very short period of time. DNA (and/or RNA), some proteins and cell membrane need to be formed at the right time and place and under the right conditions. However, as Silver point out:
It stretches even the credulity of a materialistic abiogenesis fanatic to believe that proteins and nucleotides persistently emerged simultaneously, and at the same point in space, from the primeval soup. We are in trouble enough without adding events of an astronomical improbability (1998, p.347).
Even if this was the case, many of the important biochemicals would, in fact, destroy each other (i.e. sugars and amino acids mutually react). Living organisms are well-structured to avoid this, but the ‘primordial soup' would not be. In other words, the cell is born out of co-ordinated complexity, not out of chaotic complexity.
The above indicates that an accidental beginning of life is implausible. Even a hard-line materialist, such as Crick, admits that ‘the origin of life appears to be almost a miracle, so many are the conditions which would have had to be satisfied to get it going' (in Silver, 1998, p.349).
- . Some of its properties seem to even anticipate complex life forms: ‘...the cell membrane is uniquely and ideally fit for its role of bounding the cell's contents and conferring on the cells of higher organisms the ability to move and adhere selectively to one another. These critical properties are also dependent on the size of the average cell being approximately what it is and on the viscosity of cytoplasm being close to what it is. The membrane is also fit, in that its selective impermeability to changed particles confers additional electrical properties, which form the basis of nerve conduction' (Denton, 1998, p.209).
- . To quote Silver again, ‘the basic problem facing anyone who is looking for the origin of life is to account for the formation of a complex, very highly organized, self-sustaining and self-replicating system out of a mixture of chemicals that, certainly in the early days of the soup, displayed none of these characteristics' (1998, p.340).