I thought some more about the evolution argument I discussed yesterday, and decided that, the argument is sufficiently general to be addressed by general logical principles, and does not require detailed knowledge of the science of DNA. The argument is that Unqualified appeals to similarity do not demonstrate common descent any more than they demonstrate common design. Which is absolutely correct. The argument “X’s are similar, therefore X’s have a common descent” lacks a middle, and is therefore invalid. To make it valid, we need a premiss that connects the term ‘similar’ to the term ‘have a common descent’. The premiss ‘X’s that are similar have a common descent’ would clearly do, but is clearly untrue. For example, pebbles that have been washed over by the tide for many years are all similar in that they are smooth, but they have no common descent. Snowflakes all display a common form, but have no common descent, etc.
The real principle we need to appeal to is that things which are similar are (or are highly likely to be) similar by a common nature or cause. The scholastics had a wonderful word for this: ratio, which has a common meaning that cannot be translated by any single word, but which in the present context would be translated by ‘reason’ or even ‘explanation’. When we see things with an identical or similar structure, there should be a explanation or reason for this. Pebbles washed by the tide are smooth for the very reason that they are washed by the tide, being knocked against other pebbles, smoothing the pointy bits. Snowflakes are similar because they are all water, which has a consistent molecular structure (at least I assume so, otherwise I’ll leave this to experts).
Primates and humans share similar DNA. There should be a reason for this. One reason is that God made the DNA similar. But is there a simpler explanation? Surely there is. Science tells us that DNA is a highly complex molecule. Common experience tells us that DNA is very ‘weak’ – it degrades quickly outside its natural environment, which is a living organism. So DNA cannot randomly occur. We also know, from common experience, that it can occur by reproduction. Humans reproduce other humans, primates reproduce primates, plants reproduce plants. (I have a spider plant in the attic that I have replicated for more than 20 years by the usual well-understood techniques). So, one simple explanation for the similarity of DNA is that it replicates itself. It reproduces its form in matter (Carbon, Hydrogen, Oxygen etc. molecules).
Thus, the simplest explanation of the similarity of form of DNA is by replication or reproduction. This does not require invoking a designer who caused the similarity.