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We are moving progressively smaller in terms of the size of the structures whose homologies we're considering. Advances in the understanding of the biochemical processes of life have provided a wealth of evidence in support of evolution. Biochemical homologies provide some of the strongest evidence for evolution - partly because of the level of detail they provide, and partly because the nature of some of the homologies makes any explanation other than evolution seem even more farfetched than with the larger-scale homologies. There are a variety of different avenues of biochemical evidence for evolution, but most of them are either examinations of genetics or of proteins. First we'll look at general issues then move on to specific examples.
Protein Homologies
There is no need to spend much time on protein homologies because proteins are coded by genes, so in a sense, they are reflective of genetic homologies. However, they are being considered independently because there has been a lot of work done on examining proteins. The understanding of genetics is a relatively newer science, and while more and more data is becoming available regularly, at this point in time proteins may have been studied more than genes.
A protein is a string of amino acids. Proteins range in size from around 50 amino acids to thousands. Proteins are among the most important chemicals in life: in addition to making up a good chunk of the structure of many organisms, proteins are involved in regulating or controling many of the functions of a living organism. The characteristics of a protein are determined by the sequence of amino acids of which it is constructed.
There is a homology between all livings things regarding amino acids because the same twenty amino acids are found in most kinds of living things. These twenty are a small subset of the amino acids that occur naturally (~250) and there is no known reason why these particular twenty amino acids need to be used over some other subset of amino acids. If different life forms originated independently, there is no reason to think that the same twenty amino acids would be found in most of life. However, it does make sense if all life evolved from a common ancestor that happened to use these twenty amino acids.
Homologies can be found not only among the constituents of proteins but also among proteins themselves. One important point to note about proteins is that, in many cases, smalls changes in some of the amino acids that make up a protein do not appear to have much if any effect on the functioning of the protein. Thus we can have a set of proteins that do essentially the same thing but are not identical.
For some proteins it is estimated that changes in a significant number of the amino acids from which it is constructed will not affect its function. Hemoglobins are such an example because there are actually several types, all of which serve the function of binding oxygen in the blood and yet differ in their amino acid sequences. Hemoglobins are found in a wide variety of life forms and they are all very similar in structure. Given that there are so many different sequences of amino acids that could make a functional hemoglobin molecule, we can ask why the various hemoglobins among vastly different creatures are so similar. Evolution provides a meaningful answer.
As was stated before, there are lots of examples of protein homologies between different organisms, but genetic homologies are more interesting because genes code proteins (somewhat of an oversimplification but accurate enough), so they represent a more basic form of homology. It is to these which we will turn next.
(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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