DDD: Darwin, Dust, and DNA  updated October 3, 2005  D. Carrigan carrigan@fnal.gov (subject line must be sensible)

	DNA and all of biology constitute one of the five great pillars of our hundred years of science. While the discovery of evolution arose in the eighteenth century, our period has shed important light on the underpinnings of evolution. These underpinnings include the discovery of DNA, the idea of endosymbiosis, and the recognition that extreme geological environments expand the the opportunities for the origin of life. DNA gives both the molecular basis for genetics and the means for generating the chemical stuff of life, the "Big Bang of Biology". Endosymbiosis and environments can explain how early life evolved so rapidly. The evolution of the geology and climate of the earth drives biological evolution. Geology is the interplay of the dust, dirt, water, and gas that formed the earth originally and then became our home. There is a feed-back between the biota and the evolving geology so that biology has been an integral factor in the changing earth. An extremely interesting question is whether life exists elsewhere in the universe. If so, can the signatures of life be detected and studied? The study of this possibility is called astrobiology.
	In the mid 1800s Darwin (and Alfred Wallace) formulated the modern theory of evolution. Darwin's theory is based on "survival of the fittest." Typically different environments and competing populations cause animals and plants to evolve to meet a challenge. Laymen are sometimes confused by the the use of the word theory for evolution. One sometimes hears the statement "its' only a theory." This misses the point. Scientifically, evolution is on the same high plane as the theory of electricity and magnetism developed about the same time by Maxwell. That theory exactly predicts everything about how TV, household electrical power, and most other electrical and magnetic phenomena work. At the beginning of our 100 years biology was a science entirely separate from physics although a few people like Schrödinger were beginning to link the two subjects. Darwin’s "Origin of the Species" includes no mention of DNA. For that matter Mendel's work on genetics had not yet appeared. Around 1953 the picture changed. Watson and Crick unfolded the fantastic structure of DNA , deoxyribonucleic acid.
	•Watson and Crick (using important information from Wilkins and Franklin) found that DNA proteins contain sequences of 20 amino acids interlaced in two strands, the famous double helix. These amino acids are the building blocks of life. The structure Watson and Crick found was like a computer tape made up of combinations of four bases. These four DNA bases, adenine [A], guanine[G], cytosine[C], thymine[T] give two combinations - AT (2 hydrogen bonds) and GC (three hydrogen bonds). The number of bits/base pair is ln_2(20)/3 =  4.32/3 where 3 is the number of base pairs in a codon (a triplet of base pairs) so that the information content per base pair is 1.44 bits. A typical base pair with the associated backbone and bonds weighs 600 Daltons. This is a fantastically efficient energy storage medium compared to some sort of silicon device. The human genome contains about 3 billion base pairs but the actual information content is more like 0.05 Gbytes because of junk DNA. This is on the order of the size of a Microsoft program. The code-like character of DNA and its structure forged by quantum mechanics links DNA to the other great pillars of science.