R M Cullen
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How did life begin on Planet Earth?
The fossil evidence is that this planet is about 4.6 billion years old, bacteria (prokaryotes) appeared about 3.5 billion years ago, and more complex single celled life forms (eukaryotes) about 2 billion years ago.
Darwin’s theory of evolution by natural selection can not answer this question. Explaining the origin of variation has always been a major weakness. The theory says nothing about what happens until there is variation.
Darwin's theory begins "Once life appeared..." This is unsatisfactory but not fatal. The Big Bang Theory has a similar problem. It says nothing about the very first tiny fraction of a second after the universe began to exist. It begins "At time 0.0000...1 seconds the universe was.."
There are a host of related questions. Some of them are –
Did life begin spontaneously on this planet? Or,
Was there a designer or designers who introduced prokaryotes, and did they evolve into eukaryotes and then multi-cellular life?
Is it sufficient to introduce something less than a single celled organism? If so, what is the least that needs to be introduced?
If there was a designer or designers, did the designer need two attempts, with the introduction of prokaryotes being a blind alley, only leading to the evolution of different kinds of prokaryote?
The questions may even be important to a future team of scientists looking to introduce life to a barren world somewhere out in space.
However, these are not questions a Darwinian is willing to consider. Darwinians are committed to believing that life arose spontaneously on this planet as any other answer allows the possibility of evolution by intelligent design.
There are a number of theories as to how life might have arisen from inorganic beginnings on planet Earth.
The most famous of these is captured in the Miller-Urey experiment of the early 1950s where electric sparks (to simulate lightning) were passed through a mixture of water, hydrogen, methane, and ammonia. This mixture approximated the atmosphere of early Earth, as it was thought to be at the time. The experiment resulted in the production of organic chemicals, including many, but not all, amino acids.
It is a big step from production to reproduction, and the Miller-Urey atmosphere is no longer believed to resemble the atmosphere of early Earth. However, there is no requirement that the whole atmosphere be as envisioned by Miller and Urey, merely that it be like that somewhere.
The problem with all chemical soup theories, of which Miller-Urey is the best known example, is that a stage is reached where researchers currently have no real idea as to what happens next beyond ‘then, a miracle occurs’.
There are a number of other theories such as the idea that complex organic molecules can be formed on silica substrates (the ‘clay hypothesis’ of Cairns-Smith)
Or, that life arose near deep sea vents in the ocean. In this case, one would expect to see primordial life continuing to arise near deep sea vents. Unfortunately, man does not routinely take the required samples near deep sea vents
Darwinians face the problem that we observe only one type of life on this planet (and that is DNA based) so, it would seem that exactly one theory of the origin of life, from the many candidates, must be true.
One way of approaching the questions is to attempt to reach back in time by engineering bacteria with fewer and fewer genes in order to see what the minimal gene set is for ‘reproduction’.