Excerpt from Darrel Falk, “The BioLogical Evidence: Does Genetics point to Common Ancestry?” from Old-Earth or Evolutionary Creation?: Discussion Origins with Reasons to Believe and BioLogos, ed. Kenneth Keathley, J.B. Stump, and Joe Aguirre (Downer’s Grove, IL: InterVarsity Press), 182-183.
Another type of change, although much rarer, has been well studied by geneticists over the past sixty years. With this type of change, small blocks of DNA units are deleted or, in other cases, inserted. Insertions and deletions are created by cuts in the DNA followed by reattachment at the cut sites.
To illustrate the principle, imagine for a moment that you have a scar at a particular location on your body. Perhaps it is the result of a bad cut that occurred to your right little finger when you were seven years old. It’s at the tip and not the base of the finger. It’s on the inside of the finger near the middle, not on the left or the right side. It is a vertical cut, not horizontal, and it is one half inch long—not shorter and not longer. It is there because of a very specific event—an accident with a knife that happened when you were seven. The scar is not serving a purpose in your body. Its presence is solely the result of a particular event in history.
In a manner that is completely analogous to this the genome gets damaged, and insertions and deletions are like the scars on a damaged finger. They can be positioned exactly. Indeed they can be resolved to .00000034 millimeters (a single DNA unit) out of the five thousand millimeters of DNA in a typical human chromosome. Sometimes when the cut is healed ten DNA units are deleted, other times one hundred, and other times just one. Sometimes the healing is associated with a small insertion of defined length. But unlike the scar on your finger, DNA scars are passed on to subsequent generations and so can be tracked through an ancestral lineage.
Some of the scars in our DNA occurred fairly recently (evolutionarily speaking) in the human lineage. Some of us may share a particular scar because it happened in a shared ancestor deep in the past centuries. Others whose lineage does not include that same ancient ancestor don’t have the scar. There are other cases where all human beings share exactly the same scar. We can tell it’s been damaged and resealed because of certain trademark features that we observe in the laboratory when cuts are generated and resealed. We all share the exact same scar because it occurred in a single ancestor long ago.
Although many of the scars are unique to human beings, if our lineage can indeed be traced back to the same ancestral population of hominids to which the chimpanzee lineage can be traced, then there ought to be a set of scars that we share with chimpanzees. They would be a reflection of healing events that occurred in ancient populations of hominids that both humans and chimpanzees share. So do they exist? Yes, thousands of them. At a resolution of .000000034 millimeters the two species share many of the exact same scars. Furthermore, even if one imagines that sometimes a scar takes on a particular function in the body (which it can on occasion), usually the exact position of the scar would make no difference to its functionality. Most of the functions are not position sensitive—not at this resolution—and they work just as well if they are moved a little to the left or right. So the position of each scar is a function of a unique historical event that has been propagated through the ages and not a function of some essential design feature. The fact that there are thousands of these shared scars is the reason that virtually all geneticists are certain that we share them with chimpanzees because of single events that left behind scars still present in all descendants.
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