Charles Darwin published the “Origin of Species” in 1859 and established evolution as the central organizing principle of biology. The molecular basis of evolution became clear about a century later with the understanding of the structure of Deoxyribonucleic Acid – better know as DNA.
DNA is the stuff of inheritance, and changes in DNA are the stuff of mutations and ultimately evolution. The code of life is defined by a simple alphabet consisting of only 4 letters and a grammatical structure which demands words can only be three letters long. Although this means there are only 64 words (called codons) in the DNA dictionary, “sentences” in the genetic code can be crazy long, literally tens of thousands of words long. The functional unit of DNA are strings of codons called genes which specify instructions about life.
Most importantly, the code is shared by all life. The codons mean the same thing in an aardvark and a zebra, from simple bacteria to you and I. Closely related organisms have closely related sequences of codons. This has allowed confirmation or rearrangement of the cladistic relationships of life. Not only can whole organisms be compared but also individual organs.
The evolution of the eye can be seen by comparing DNA across many organisms. Devotees of the idea of intelligent design, i.e, the god did it crowd, have suggested that an organ as complex as an eye cannot have arisen by evolution. They proffer the idea of irreducible complexity. It’s the old “what good is half an eye” argument. Mammalian eyes, just as one example, have several parts including a retina, an iris, a lens, a pupil, etc. Arthropods have compound eyes with multiple lenses and retinas.
There is a range of types of eyes that serve different functions and therefore have different levels of complexity, but in the last analysis they all share one common feature and that is the detection of light. The basic requirement for light detection, shared across all life, is a group of closely related molecules called rhodopsins. If light shines on this molecule, it changes shape and that triggers a signal to the brain that says light! Multiple copies of the molecule allow greater sensitivity and features such as a lens add acuity to the detection. Slight variations in the structure of the rhodopsin allows for detection of different wavelengths (colors). That the rhodopsin occurs across all kinds of life is seen in the gene which codes for the production of the molecule.
Here is the really interesting part. The gene for rhodopsin synthesis occurs in forms of life that have no eyes, such as a primitive organism know as cyanobacteria. These ancient bacteria have been around for billions of years. Why would a bacteria that couldn’t care less about detecting light have rhodopsin? It turns out the bacteria use this molecule for an entirely different reason.
Minor mutations in the rhodopsin gene allowed for the “repurposing” of the molecule to serve as a light gathering structure, rather than the function it serves in bacteria. This repurposing of structures is not an uncommon feature in evolution and allows for small changes to make big differences. Life is not irreducibly complex.