Naturalism is the idea that life on earth spontaneously arose from simple proteins and then evolved through genetic mutation and natural selection into the various life forms we see today.
The theory of naturalism stands in contrast to supernatural theories involving the actions of one or more gods or non-physical beings creating life on earth.
For some time naturalism has been quite popular but it has a number of sticking points that make it hard to believe for many people. Perhaps one of the hardest things to explain using naturalist principles is how the original life form(s) began on this planet. If we do away with extraterrestrial intervention or the supernatural actions of a God or gods or other extra-dimensional beings, to focus on naturalistic explanations, we are left with only 2 options:
(1) that life spontaneously arose from non-life on Earth, or,
(2) that the seeds of life arrived on Earth via a meteorite and took root.
When the original idea of spontaneous generation was posited some centuries ago their was little understanding of how complex even the simplest of reproducing life forms needed to be . As a result the idea that basic molecules that existed on earth in primeval times may have formed together to form the earliest simplest forms of life didn't seem outrageous. However, now that we have a greater understand of how complex even the simplest forms of life need to be (see the diagram of the simplest type of self-reproducing life form on earth, prokaryotic bacteria, above) it is becoming increasingly clear that such spontaneous generation is not only an impossibility, but the idea is an absurdity.
Now that spontaneous generation of life has been rules out it leaves us with only one other naturalistic theory for how live began on earth. That theory posits that microscopic life arrived on earth from space via meteorites and such. This theory is called panspermia. The problem with this idea is that, although it is plausible it doesn't explain how that form of life originated on its home world. This leaves us with the same problems we have explaining origins on Earth using a naturalistic approach, and means that we will need to leave that approach if we are to understand how life arose on this or any other physical world.
Stuart Wilde addresses this issue here and comes to the conclusion that life probably arrived from another dimension (or dimensions) of being.
Sharka Todd