First off, let me be utterly clear. If someone believes that the deity of their choice created the Universe either recently or far in the past, created life in it, or any such thing I don’t have a problem with it. Believe that God created the Universe in six days about 6000 years ago? Knock yourself out. Believe that Izanagi shagged Izanami and the “drippings” made the islands of Japan. Okay, whatever.
I really am a near absolutist on religious freedom, including the freedom to believe things that I find…let’s just say “questionable” and leave it at that.
However some people go beyond having it as their religious belief and try to claim it as science. This without meeting even the most fundamental of scientific standards. (Quick: what evidence, if seen would lead to the conclusion that ones flavor of creationism or “intelligent design” was wrong? To be scientific it must be falsifiable, there must be some possible observation that would lead to the conclusion that the idea is false. And, yes, there are other things that people try to call “science” even “settled science” which fail to meet that standard. I call them on it too.)
I’m not going to go into the whole long series of arguments which justify the scientific basis on current theories of evolution and the origin of life (two separate things–evolution deals with what happens once you have living things while where life came from in the first place is a different set of theories).
First, we have the origin of life itself. Some people point to Pasteur’s work rebutting earlier theories about the “spontaneous generation” of life and say that “scientific principle” “proves” that life cannot appear from lifelessness. What they neglect is that Pasteur’s work, while important, was limited to the experimental apparatus of the time. A more accurate statement of his results would be “withing the space and time constraints of these experimental apparatus under these conditions life does not spontaneously arise.” It’s a little vaguer than the first and is sufficient to refute the then existent theories of spontaneous generation.
However take a much bigger “experimental apparatus”–either the entire primordial Earth, or even larger–all the planets similar to Earth in the entire Universe–and a much longer time span of millions to billions of years? What then?
Well, given enough time and a large enough sample size, some remarkably unlikely things can happen at least once. Rolling twenty 1’s in a row on a fair standard six sided die is a 1 in 3.66*10^15 probability–just under four million billion. At that likelihood Most people would say “impossible.” Roll a hundred million billion times though (big Universe) and you would expect it to happen about 27 times.
What likelihood are we looking at here? Well, consider what the simplest “life form” might be. At the most basic a “life form” is something that can replicate itself using stuff available around it. So, at the most basic, life would be something like a ribosome–a strand of genetic material (RNA or DNA for instance) carrying the “pattern” and a cluster of proteins that duplicate the genetic material and the proteins themselves. We know, for instance, that things like lightning and UV radiation can create amino acids and similar organic molecules from methane, ammonia, water, and hydrogen–all readily available in the universe. And, indeed, this has been experimentally verified. While there is not current single accepted theory on the origin of life, this does show that the “building blocks” of life as we know it can be produced by simple, natural processes from non-living source material. Once you have those “building blocks” and enough stimulation–through solar and cosmic radiation, lightning and other natural processes, they start interacting and it’s only a matter of time working on enough material before something self-replicating forms from pure random chance. How much time and how much material (maybe it takes a billion worlds of appropriate type to have enough material for life to form on one, or maybe every world with appropriate conditions develops life sooner or later) depends on how likely the particular combination that’s self-replicating can come up. The combination that makes “life” would be exceedingly, mind-numbingly unlikely.
But it’s a big Universe.
Once you have life, then, evolution comes into play. The replication is not “perfect” (pretty close, actually, amazingly so, but not quite). Differences from the original pattern will creep into it. Most of the times those differences will be lethal. But on rare occasions they’ll be beneficial. Accumulate enough differences and you have something that’s no longer even recognizable compared to the original pattern.
How much time? One person, in an effort to dismiss the idea of evolution being able to create the vast spectrum of life on Earth asked “how long does it take for a new species to arise?” He completely misunderstood the situation. It’s not “x years” and one new species, then another X years an a second new species, and so on. It’s rather, how long (on average) does it take for one species to split into two. You have one species, after X years you have 2. Then x years more each of the two splits and you have four. Then x years more and each of the four splits and you have eight. If X is a a million years, then in ten million you’d have over a thousand species. In twenty million years you’d have over a million. In a thirty million years you’d have a billion species. In about three hundred million years (which barely takes us out of the paleoarchaean era*) you’d have more species than there are particles in the Universe. Obviously, we never reach that point. Most of those species would never happen. However it serves to demonstrate that even a very slow rate of speciation–taking a million years for a new species to arise from an old one–is adequate to explain the vast diversity of life on Earth.
Now that steady rate of speciation is a horrible oversimplification. In reality a species could be adequately suited to its environment and change little over a long time indeed, then something changes in the environment makes it less fit and changes happen more rapidly until a new mostly steady state is achieved. A large population with enough cross fertilization so genes spread well through the whole population would change slowly, but a portion of that population, reproductively cut off from the main group would change faster so where once there was one species, eventually there becomes two.
None of this says that this must be the way it happened. And if you believe that it didn’t, that some god or gods (or aliens or whatever other “intelligence” your “intelligent design” calls for) managed things, then more power to you. But none of that is necessary to explain the observed data.
And the arguments about “impossibility” or “unlikelihood” fail simply from considering the scale in which that “unlikelihood” has in which to happen.
*Using 3.5 BYA as the earliest generally undisputed time for life on Earth.