1) Demonstrating that one thing has happened naturally (in this case, life) says nothing about whether everything should be explained naturally.
2) Darwinian evolution is a theory with massive difficulties.
The second point may need some clarification!
The idea that natural selection can drive a degree of evolution within species is easily-provable scientifically – species tend to adapt and change across generations because fitter individuals within the species are more likely to survive, reproduce and, therefore, pass on their characteristics to the next generation (natural selection).
What is lacking is strong evidence for the theory that natural selection can account for the emergence of all living things from simpler life-forms (i.e.,Darwinian evolution).
Of course, people have cast doubts on Darwinian Evolution for decades – but there are now signs that these doubts are being acknowledged more seriously in the academic world (which has tended to resist criticism or even questioning of Darwinian evolution).
For example, in the secular, peer-reviewed science journal, Biological Theory, Depew and Weber wrote an article on “The Fate of Darwinism” in which they acknowledged that all attempts so far to adequately explain how natural selection could have driven large-scale evolution (including “Population Genetical Darwinism”) have simply failed to account for “how major forms of life evolved”.
As they put it, “genetic Darwinism...in its current versions...can no longer serve as a general framework for evolutionary theory. The main reason is empirical. Genetical Darwinism cannot accommodate the role of development in many evolutionary processes.”
The crucial thing to note is this: these authors still hope that the theory of large-scale evolution by natural selection can be saved. They have no desire whatsoever to abandon their belief in Darwinian evolution. Yet they recognise that, at present, biologists cannot even explain how Darwinian evolution can account for the development of complex life-forms, let alone prove it.
The difficulty with finding a purely natural explanation for life is that life is so mind-bogglingly complex – and as our understanding of biology increases, the level of complexity we’re observing is only
getting greater and greater.
The complexity of life is far greater than many people realise – and far more remarkable than anything designed by mankind. It is simply very difficult to explain naturally.
In conclusion, it should not be assumed that the theory of Darwinian evolution provides a solid basis for philosophical naturalism. Life is far more mysterious than naturalism cares to admit!
Another problem for naturalists is that, even if the emergence of complex life-forms like us could be explained by natural selection, that would do nothing to explain how life itself (remarkable as it is) could have originated naturally. But that’s for another time!