Moving on to a larger scale, we can look at the genes themselves and see that there are quite a number of genes that are shared by a wide variety of living things. These genes perform the same or similar functions in the genomes in which they are found, from the human genome to the fly genome. There would be no reason for organisms as different as humans and flies to have the same genes to perform similar functions, if not for the fact that they were descended from a common ancestor that possessed those genes. We’ve already looked at the cytochrome c gene; evolutionary scientists also invoke the opsin family of genes, which are used in light-sensing structures, more commonly known to us as “eyes”.
Perhaps the most famous of cases of stunning gene similarity among living things are the Hox genes. Essentially, Hox genes are genes responsible for laying down the basic body plan of an organism very early in its development; Hox genes, first found
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in the fruit fly, establish what will be the front and back of an organism, and where structures such as limbs and eyes will develop; interestingly, the order of these genes on the
chromosome perfectly correlates with the parts of the body that are influenced by particular genes. Not only are these genes found in virtually all organisms tested to date (including jellyfish), and not only are their sequences virtually identical in all organisms, but they are also found in the same order on the chromosomes of organisms studied. These genes are so identical that scientists have been able to take
Hox genes from
one species and express them in another species; the genes function perfectly. For example, the gene governing limb development in chicks, called Sonic hedgehog, can be expressed in a shark and govern the perfect development of a shark fin.
One last piece of gene-scale evolution evidence can be found in what some scientists call vestigial genes. Just as vestigial structures are structures that at one time had a function for an organism but now essentially serve no purpose, vestigial genes are genes that were once expressed in an organism to make a protein product, but now are silent. Two classic examples are the gene responsible for Vitamin C synthesis (found but completely nonfunctional in the human genome and in higher primates) and odorant receptor genes, used in olfaction. The human genome contains around 100 odorant receptor genes, of which roughly 70% are completely nonfunctional; indeed, humans, and primates in general, are less dependent on their sense of smell than many other organisms. Dolphins contain numerous odorant receptor genes, with not even one being functional; dolphins, in other words, have given up their sense of smell. Again, there would be no reason to have genes without any function whatsoever unless they were passed along through common descent.
