The Tale of the Ancient Molecule
From Paul Davies The Fifth Miracle
Inside each and everyone of us lies a message. It is inscribed in an ancient code, its beginnings lost in the mists of time. Decrypted, the message contains instructions on how to make a human being. Nobody wrote the message; nobody invented the code. They came into existence spontaneously. Their designer was Mother Nature herself, working only within the scope of her immutable laws and capitalizing on the vagaries of chance. The message isn't written in ink or type, but in atoms, strung together in an elaborately arranged sequence to form DNA, short for deoxyribonucleic acid. It is the most extraordinary molecule on Earth.
Human DNA contains many billions of atoms, linked in the distinctive form of two coils entwined in mutual embrace. This famous double helix is in turn bundled up in a very convoluted shape. Stretch out the DNA in just one cell of your body and it would make a thread two meters long. These are big molecules indeed.
Although DNA is a material structure, it is pregnant with meaning. The arrangement of the atoms along the helical strands of your DNA determines how you look and even, to a certain extent, how you feel and behave. DNA is nothing less than a blueprint--or, more accurately, an algorithm or instruction manual-for building a living, breathing, thinking human being.
We share this magic molecule with almost all other life forms on Earth. From fungi to flies, from bacteria to bears, organisms are sculpted according to their respective DNA instructions. Each individual's DNA differs from others in the same species (with the exeption of identical twins), and differs even more from that of other species. But the essential structure-the chemical makeup, the double-helix architecture-is universal.
DNA is incredibly, unimaginably ancient. It almost certainly existed three and a half billion years ago. It makes nonsense of the phrase "as old as the hills": DNA was here long before any surviving hills on Earth. Nobody knows how or where the first DNA molecule formed. Some scientists even speculate that it is an alien invader, a molecule from Mars perhaps, or from a wandering comet. But however the first strand of DNA came to exist, our own DNA is very probably a direct descendant of it. For the crucial quality of DNA, the property that sets it apart from other big organic molecules, is its ability to replicate itself. Put simply, DNA is in the business of making more DNA, generation after generation, instruction manual after instruction manual, cascading down through the ages from microbes to man in an unbroken chain of copying.
Of course, copying as such produces only more of the same. Perfect replication of DNA would lead to a planet knee-deep in identical single-celled organisms. However, no copying process is totally reliable. A photocopier may create stray spots, a noisy telephone line will garble a fax transmission, and a computer glitch can spoil data transferred from hard disk to a floppy. When errors occur in DNA replication, they can manifest themselves as mutations in the organisms that inherit them. Mostly a mutation is damaging, just as a random word change in a Shakespeare sonnet would likely mar its beauty. But occasionally, quite by chance, an error might produce a positive benefit, conferring an advantage on the mutant. If the advantage is life-preserving, enabling the organism to reproduce itself more efficiently, then the miscopied DNA will out-replicate its competitors and come to predominate. Conversely, if the copying error results in a less well-adapted organism, the mutant strain will probably die out after a few generations, eliminating this particular DNA variant.
This simple process of replication, variation, and elimination is the basis of Darwinian evolution. Natural selection-the continual sifting of mutants according to their fitness-acts like a ratchet, locking in the advantageous errors and discarding the bad. Starting with the DNA of some primitive ancestor microbe, bit by bit, error by error, the increasingly lengthy instructions for building more complex organisms came to be constructed.
Some people find the idea of an instruction manual that writes itself simply by accumulating chance errors hard to swallow, so let me go over the argument once more, using a slightly different metaphor. Think of the information in human DNA as the score for a symphony. This is a grand symphony indeed, a mighty orchestral piece with hundreds of musicians playing thousands of notes. By comparison, the DNA of the ancient ancestor microbe is but a simple melody. How does a melody turn into a symphony?
Suppose a scribe is asked to copy the original tune as a musical score. Normally the copying process is faithful, but once in a while an eighth note becomes a quarter note, a C becomes a D. A slip of the pen introduces a slight change of tempo or pitch. Occasionally a more serious error leads to a major flaw in the piece, an entire bar omitted or repeated perhaps. Mostly these mistakes will spoil the balance or harmony, so that the score is of no further use: nobody would wish to listen to its musical rendition. But very occasionally the scribe's slip of the pen will add an imaginative new sound, a pleasing feature, a successful addition or alteration, quite by chance. The tune will actually improve, and be approved for the future. Now imagine this process of improvement and elaboration continuing through trillions of copying procedures. Slowly but surely, the tune will acquire new features, develop a richer structure, evolve into a sonata, even a symphony.
The crucial point about this metaphor, and it cannot be stressed too strongly, is that the symphony comes into being without the scribe's ever having the slightest knowledge of, or interest in, music. The scribe might have been deaf from birth and know nothing whatever of melodies. It doesn't matter, because the scribe's job is not to compose the music but to copy it. Where the metaphor fails is in the selection process. There is no cosmic musician scrutinizing the score of life and exercising quality control. There is only nature, red in tooth and claw, applying a simple and brutal rule: if it works, keep it; if it doesn't, kill it. And "works" here is defined by one criterion and one criterion only, which is “replication efficiency”. If the mistake results in more copies made, then, by definition, without any further considerations, it works. If A out-replicates B, even by the slightest margin, then, generations on, there will be many more A’s than B's. If A and B have to compete for space or resources, it's a fair bet that A will soon eliminate B entirely. A survives, B dies.
Darwinism is the central principle around which our understanding of biology is constructed. It offers a clear explanation of how a relatively simple genetic message elaborates itself over the eons to create molecules of DNA complex enough to produce a human being. Once the basic manual, the precursor DNA, existed in the first place, random errors and selection might gradually be able to evolve it. Good genes are kept, bad genes are discarded.