One central question that proponents of biological evolution must answer is: How did the information in DNA get there? We might disagree on how much information is present in living things or even on how to quantify or define it, but pretty much everyone agrees that the DNA of living things contains information not found in non-living things and that complex organisms have more of it than simpler organisms.
So how did that information get there? Can information be added, bit by bit, through unguided natural processes? That’s what evolution claims. But, so far, this step by step addition of information has not been observed. Until it is observed, the idea of common ancestry of all life is merely conjecture.
Of course, even if we could show that there are some instances in which information is added by unguided natural processes, that would be necessary to support common ancestry, but not sufficient. In other words, you have to have that if evolution is true, but it's not enough, by itself, to prove it. In order to even begin to make a solid case for biological evolution, one must show three things. If even one of these prerequisites is false, the modern theory of evolution (i.e. common ancestry via unguided natural processes) cannot be true. If any one of them cannot be shown to be true, evolution remains unverified and thus belongs to the realm of speculation, not science.
1: It is possible to add biological information.
Think of Dawkins’ proverbial "Mount Improbable" which looks like an impossibly steep cliff on one side, but can be climbed by tiny little steps up the back. This is the evolutionist's concept of adding biological information in tiny steps, so that over time they add up to all the complexity we see among organisms today. However, you cannot climb Mount Improbable unless it is, at some level, possible to go UPWARD. If you only ever take downward or level steps and cannot step upward, you will never get to the top, no matter how much time you have or how shallow the slope is. Thus, in order to prove evolution, it is necessary (but not sufficient) to show that it is possible to add genetic information through unguided processes.
2: There are more upward steps than downward steps (or at least a way to get more upward steps than downward steps at least some of the time).
We know that genetic information is corrupted and destroyed through mutation. We know that there are many, many harmful effects when an organism's DNA is altered. In order for evolution to overcome this downward current toward catastrophic information loss, there must be an even stronger current upward (at least some of the time) in order to explain how the information was able to increase from that present in a tiny and simple organism to the information in a larger and more complex one.
Again, using the Mount Improbable analogy, it is impossible to get to the top by taking three steps downward for every step upward. There must be an overall trend toward increasing information. So even if there are a few examples of increasing information, they are not sufficient to prove evolution. If evolution is true, these upward changes must be the rule, not a rare exception.
3: There does exist a gradual genetic pathway that can be climbed in tiny, incremental steps.
In order for evolution to be true, not only does information have to be added over time, but each successive change must occur in a living organism and it must be conserved by being passed on to offspring. Thus, the change cannot kill the organism or seriously disable it, or the change will not be passed on. This must be the case for EVERY step in the entire evolutionary sequence, no matter how small. At every step you must have a functional organism. Thus, the changes must be gradual enough that the tiny upward steps (if they exist) can achieve each new level without killing or disabling the organism.
To use a simplistic analogy, if one tries to change from one word to another by changing one letter at a time (cat to cot to dot to dog, for example), there must, at every step, be an actual word that can be reached by changing one letter. In the Mount Improbable analogy, this means that there can be no upward jumps in the trail. If the maximum possible upward step is 6 inches, then there can be no 6 foot cliffs along the trail, or even 7 inch steps. If ever there is a step which requires more information than unguided evolution can provide, then evolution is falsified in that instance. It cannot account for the change in information if that is the case.
The problem with these three points is that, not only have they not been proven, but they aren't even adequately addressed by modern science. There has been a bit of discussion on point 1, with a handful of examples that might show information increase. However, even these are controversial and far from settled. There has yet to be shown any clear-cut examples of the addition of information by unguided, natural means. There are plenty of examples of genetic change that benefits the organism, but none that clearly do so by increasing information. In other words, while these examples do show that organisms change, it is not clear that these cases show the KIND of change that is necessary to account for how the current genetic information in living things got there in the first place.
So point 1 above has not yet been conclusively proven. We don't even have adequate evidence that upward evolution (increasing information) is possible. But that's just the first step.
Points 2 and 3 aren't even talked about, much less proven. In fact, there is good evidence that point 2 is false. Simply the fact that there are only a few cases where an increase in information is even postulated, compared to the huge number of examples where genetic change is known to destroy information, suggests that the downward current is stronger and that genetic information is far more often destroyed than built.
Furthermore, point 3 is usually just assumed without any discussion. The assumption appears to be that the theoretical gradual pathway of increasing complexity is real, without bothering to determine whether the proposed intermediate steps produce a functional living organism or whether or not there are information cliffs, so to speak, that cannot be bridged by gradual means.
Unfortunately, the fact that someone can imagine a gradual pathway of increasing information that results in a living organism at every step doesn’t mean it actually exists. Science requires more than mere speculation to make a valid theory. Until these issues are addressed and evidence is provided that all three prerequisites are true, it is more than premature to speak of evolution as fact – it is downright dishonest.