|Looks Like Hell on Earth|
According to the best evidence we have, the Earth is 4.5 years old and it appears that primitive or proto-life appeared on Earth about a billion years ago, give or take a few million years. This is in itself quite remarkable, especially for the young Earth brigade who are categorically disposed toward a 6,000-year-old Earth. Moving on.....For most of the 3.5 billion years, after formation, the Earth was a seething cauldron of lava and therefore quite inimical to life. The evidence suggests that life burst on the scene soon after the earth cooled. Very early fossils found in sedimentary layers in Australia confirms this early start. But what would the first living ‘creature’ look like? Certainly, it would look nothing like the most primitive single-celled life existent today.
Before we tackle how life appeared and progressed by Darwinian selection, we must first address a very fundamental issue.
So what is life and how can we distinguish it from non-life? What are the special attributes, properties, or qualities that distinguish a living organism from a rock? This is a deceptively hard question. Perhaps we could define life at its most basic as: As a struggle against entropy. This is certainly the case. All organisms have to harness and utilise energy to prevent the almost irrepressible march of chaos. Although true, this definition is not particularly helpful. The definition that I find most useful, and scientifically satisfying, is: Life began the moment that a molecule containing information started to reproduce and evolve by natural selection. This may not be a satisfactory explanation to all, but it does underline the most basic condition that must occur when we contemplate what it means to be alive, and that is reproduction.
Entropy must be held at bay by all organisms even if for only a short time and this will require an energy source. Today’s living cells, no matter how primitive, must obey this cardinal principle, and energy production is facilitated, by most organisms, either by the ingestion of organic matter or by photosynthesis. The first living cells must have used a non-organic, non-living energy source. Today, deep in the seabed, where the sun doth shineth not, lurk volcanic vents. Belchings from the depths pour out immense amounts of heat and minerals. Around these vents live bacterial colonies directly harnessing the chemical outpourings, mostly hydrogen sulphide, to make the energy to control and direct their cellular processes. The earliest living entities must have harnessed such an external energy source and perhaps the first cell appeared under these circumstances.
It is likely that the first proto life was not cellular at all. Envisage a naked chemical able to assimilate other chemical elements to form a copy of itself and then release that copy into the environment for the process to continue anew. And the chemical candidate able to undertake this process was almost certainly ribonucleic acid (RNA).
RNA belongs to a chemical group, termed nucleic acids. There exists a big brother to RNA, deoxyribonucleic acid, or DNA. DNA is the molecule of choice for information transfer for the majority of living organisms present today. We are certain that RNA was the molecule responsible for early life for two reasons. First, RNA, unlike DNA exists in a single chain formation. However, RNA, just like DNA, is able to replicate itself given the right conditions. Secondly, unlike double-stranded DNA, RNA, in its single strand configuration, can fold to produce a range of conformations. This is crucial, as it is in this folded form that RNA can act as primitive enzymes that enable the attachment of amino acids to initiate protein construction.
I have blithely introduced complex organic compounds (RNA & amino acids) without nary an explanation. So the question remains: Would it have been possible?
Would the necessary organic precursor chemicals, essential for life formation, be present on the primitive earth? Experiments conducted in the 1950s (Miller & Urey) with simple equipment produced some very interesting results. They started with a ‘broth’ of simple inorganic chemicals together with an electric spark to mimic lightning. The sealed system was left to ruminate for several weeks and the resultant chemicals in the container analysed. What they found was astonishing. Under these conditions, it was found that a large number of amino acids had been produced. Amino acids are the units that when bonded together form a variety of proteins necessary for life. Other necessary organic compounds were also found. Subsequent experiments have modified the conditions of the experiment. These modifications were based on new knowledge concerning the nature and conditions of our primeval planet. Again amino acids were found together with a host of other organic chemicals including RNA. Thus it appears that the essential organic chemicals required to support life were likely present on the primitive earth.
Although proto-life based on raw replicating chemicals is a possibility, however, for life to progress there would have to be some form of encapsulation and this encapsulation would have had to form spontaneously according to the laws of chemistry, not biology. It is conceivable that oily compounds in the primitive environment could form microcels, bubble-like formations capable of enclosing the raw chemicals of ‘life’, even if it be for a short time. These constructs would have to contain hydrophilic (water 'loving') and hydrophobic (water ‘hating’) domains or moieties. This would be a necessary precursor for the formation of encompassing spheres. One theory suggests that very early life circumvented the ‘bubble stage’, for a period, at least, (or an eon), by clinging to dear life on clay. In this scenario, the necessary chemicals would adhere, albeit loosely to the clay and the close proximity of adjacent chemicals would facilitate chemical reactions. This could only be a temporary solution as further life progression would be dependant on encapsulation, of some form.
The final, and without a doubt, the most important property of first life would be the initiation of some form of primitive reproduction subject to Darwinian selection and hence evolution at its most basic level. Once selection occurs I think life’s development would take on a rapid course and in just a few million years perhaps, organisms, and when I talk of organisms, I mean simple forms of bacteria, as we would know it, would exist.
I found this topic very hard to write and whilst researching I was overwhelmed by the amount of research data and information out there in the scientific literature. This is a highly fluid and diverse area of research. Mayhap, in the future, I will find the time to consider other theories and ideas on this exciting and important area of science. And as for Panspermia............