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Genetic homologies are similarities between the genetic codes of different living organisms which cannot be explained simply by functionality. There are an abundance of particular examples, but the biggest example is the common genetic mechanism itself: Why should all living creatures share the same basic genetic structure? It is not chemically determined. It has been shown scientifically that there are (probably many) other alternatives that would work for coding proteins (which is essentially what DNA/genes do). For instance, DNA uses only four bases out of the many naturally occurring bases (>80). It has been shown that other base codings are possible. Why does all of life use DNA with the same four bases?
Why should every single life form on the planet share the same basic genetic mechanisms? It actually goes beyond just the genetics since there are other biochemical aspects which are shared by all life: for example, the common use of ATP as an energy storing molecule. There are other alternatives to this molecule, some of which are even more efficient at storing energy.
If individual life forms arose independently, why would all life share these basic characteristics? The shared biochemical mechanisms are among the widest shared homologies in life. Most of the homologies described in previous sections are shared by some subset of living organisms, but when we get down to the molecular level, we find homologies that cut across all living organisms. Once again, if life forms arose independently, there is no reason to think they would all share these characteristics, especially given that alternatives are known to exist. These homologies argue strongly for common ancestry to all life.
As previously noted, the primary creationist argument against biochemical homologies is some sort of "divine" plan - and the same rebuttal applies. There is no coherent, scientific argument that would explain this evidence other than the theory of common descent. Non-scientific explanations are, of course, possible and creationist arguments generally fall into this category. However, they do nothing to detract from the evidential support for evolution since evolution operates within the realm of science.
(A bibliographic list of sources and recommendations for further reading will be coming soon...)
This FAQ information provided by Lance F..
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