Most of the coding sequences in DNA is not used in the construction of a human body. The sequences are a result of errors in duplicating the DNA when it splits. It's called "junk" DNA. The same sequence of errors in the DNA appears in humans and primates.
Among many examples of genetic homologies, the most interesting are in what is frequently termed junk DNA. Junk DNA are basically pieces of DNA that have no function (or in some cases, such as introns, they produce no protein but may be involved in regulation of the gene). When the DNA is transcribed, these pieces of DNA either do not get transcribed at all or are only partially transcribed, with no final result (i.e., a functional protein) being produced. You can cut out or modify most of this junk DNA without affecting the organism.
There are several varieties of junk DNA including pseudogenes, introns, transposons and retroposons. In many organisms (such as human beings) the vast majority of their DNA is of the junk variety. As an example, in humans there is one particular family of junk DNA called Alu sequences that are repeated some million times or so, and this one family alone accounts for about 5% of our DNA. There are numerous other examples.
What's more, with much of this junk DNA we can make pretty good guesses as to how it came to be. A lot of it (such as pseudogenes) appears to be copies of other pieces of DNA that have mutated such that they are no longer functional. There are a variety of mutations that can result in non-functional genetic code, so junk DNA essentially represents errors in our DNA.
Why is junk DNA so interesting? An analogy from the courts may prove useful here. Proving that someone has copied copyrighted material can sometimes be difficult, as in some cases you would expect the material to be similar since it covers the same topic or comes from the same sources. For example, phone number databases would be expected to be very similar since they contain the same basic information.
However, one excellent way to determine whether something has been copied is if the errors in the source have been copied as well. While you could argue that, even if highly unlikely, the material is similar because it has similar function, it is very hard to explain why some material would have exactly the same errors as some other material if it were not copied. Companies that sell products such as phone lists or maps routinely insert fake listings to protect themselves from copyright violations.
The same can be said of DNA. It is hard enough to explain (if you don't accept evolution) why some functional pieces of DNA show great similarities. <hl>It is pretty much impossible to rationally explain why nonfunctional DNA, erroneous DNA, would be very similar between different species. Why would genetic code that doesn't do anything and which clearly appears to be the result of mutations be similar, or in many cases identical, between different organisms? </hl>The only explanation that makes any sense is if this DNA was inherited from a common ancestor. Homologies between junk DNA are probably the most powerful of the homology evidence for common descent, as common descent is the only rational explanation for them.
Just Curious:TCS NOTE: Obviously NOT the product of an intelligent designer; what's intelligent about errors?
There are many examples of homologies between junk DNA, a number of which can be found in Zeus Thibault's Proof of Macroevolution series. We will address but a few of them here.
Pseudogene equivalents are genes which are identifiable as some functional gene in another organism but which have a mutation which has rendered them nonfunctional. There are three sets of genes found in many species that have pseudogene equivalents in primates, including humans. They are:
* several odorant receptor genes,
* the RT6 protein gene, and
* the galatosyl transferase gene.
The mutations which made these genes inoperable are shared among the primates. It is important to keep firmly in mind that there are numerous mutations that can render a gene nonfunctional. Yet not only do primates have pseudogene versions of these genes that are functional in other creatures, but these pseudogenes have been made nonfunctional by the same mutations - they have the exact same errors in the genes. This makes perfect sense if this genetic material was inherited from a common ancestor. Creationists have yet to come up with a rational alternative explanation.
Summary of Biochemical Homologies
The biochemical homologies offer some of the strongest homology evidence for common descent. This is due the universal existence of some homologies across all life forms, as well as the large number of possible biochemical alternatives that life forms could have used, but didn't. While all homologies support the idea of common descent, some biochemical homologies such as those in junk DNA provide especially strong evidence, since their very nature makes it exceedingly unlikely that they would exist for any functional reason. Common descent offers a meaningful explanation for these homologies.