Look out the window in any direction and what do you see? Trees, grass, lake, sky? Concrete, glass, asphalt, brick? Unless you’re looking at the raw face of a mountain cliff, the slope of a volcano, or some other feature carved solely by geology and tectonics, you are looking at the direct manifestation of DNA or its effects.
Trees and grass are easy to explain: organisms shaped by DNA. Lake and ocean are teeming with bacteria, plankton, fish, and the rest of the food chain founded on DNA, so that the color of the water reflects the life in it. The sky of this planet is colored by an atmosphere whose composition has been altered over half a billion years by the cycle of life. The cityscape and the interior of the room you’re sitting in have been shaped by hands and brains, which in turn were shaped by DNA.
DNA is found everywhere on this planet. And the wonder is, it’s all the same chemisty.
Yeasts, bacteria, amoebas, mushrooms, cornstalks, corn weevils, cats, and people all use the same system: DNA transcribes to RNA, which translates to proteins. Their genomes all use the same four bases (adenine, cytosine, guanine, thymine) in a three-codon reading frame to call out the same twenty amino acids into the peptide chains that fold into proteins.
The only difference is that single-celled organisms like bacteria keep their DNA in strands scattered throughout the cell body, while multi-celled organisms like corn and cats keep their DNA in coiled chromosomes inside special structures in the cell called the nucleus. Also, the structure of the ribosome—the RNA-structured molecule that reads the messenger RNA bases and puts the amino acids in order—differs between single- and multi-celled organisms. Many antibacterial agents work on inhibiting the ribosome, or some other unique structure of a bacterial cell, which is why they poison the bacteria but not the animal or human host.
The virtual identity of the DNA system used in all life on Earth is why we can say that human and chimpanzee genomes are 98% identical, while human and crayfish are 44% identical. They share that percentage of genes which are used for making identical proteins.
The amazing thing is that nowhere on the planet—not in the freshwater lakes sealed away under the Antarctic glaciers nor in geothermal vents on the ocean floor isolated by hundreds of miles from their nearest neighbors—can we find a competing genetic system. There isn’t any competition from a genetics based on the silicon atom, which is covalently almost as versatile as carbon. There’s no competing system using three bases or five, or arranged in a four-codon reading frame, or using only 18 of the amino acids, or building peptide-like chains from some other chemistry.
That is, there is no evidence on this planet of any evolution in the DNA system. No little side branches of chemically different competitors. Of course, the development of a chemical system to order the structure of molecules and store information about them must have preceded the first single-celled organisms. After all, you can’t make a self-constructing cell membrane without some organizing principle to tell the chemicals how to do it. In the first billion or so years after the Earth cooled from its conglomeration phase and acquired liquid water, there may have been competing systems—silicon based chemistries, different polymerizing chemistries—that auditioned for the job. It’s possible that the DNA-RNA complex was simply the best solution, beat out the competition, and became ubiquitous long before the first bacterium grew a protein coat. And given the fragility of these molecules, all the competitors were broken down and lost more than two billion years ago.
And there is slight evidence of evolution in the DNA-RNA complex itself. The ribose sugar ring that comprises the backbone of both DNA and RNA molecules has an oxygen-hydrogen (OH) group attached to the second carbon atom. That OH group is present in RNA (ribose nucleic acid), while the second carbon atom has only a single hydrogen (H) in DNA—and thus deoxy-ribose nucleic acid. It’s possible that RNA, with the OH in place, came first and that the oxygen atom in that position helped the string of bases react with other molecules. Because of this reactivity, RNA can do a number of jobs in the cell that actually preceded the development of proteins (see the above example of the ribosome). Loss of that oxygen atom made the resulting DNA less reactive and more stable, and therefor a better long-term storage medium. So the guess is that RNA preceded DNA in cellular evolution.
But this is a refinement of technique inside a single system. Nowhere can we find competing systems based on alternate chemistries and arrangements. Perhaps one day, in a corner of the world where no one has looked yet, we will find a five-base DNA, or a two-codon reading frame. But until then, the ubiquity of the DNA-RNA complex raises another tantalizing possibility.
Maybe the organizing principle of all life on Earth didn’t actually develop here. After all, we can find amino acids in the dust clouds emanating from the stars and complex organic molecules elsewhere in the solar system. Maybe RNA and DNA blew in on an ice crystal from some other solar system. Or maybe it was left behind as organic residue on the glove of an early star voyager who touched down on the recently cooled Earth.
Until we can find evidence of a terrestrial evolution of DNA from other competing chemistries, the possibility remains open that this most useful molecule is truly ubiquitous, found everywhere in the galaxy and perhaps throughout the universe.