Shadows of Forgotten Ancestors

But what’s the use of a message if nobody reads it? Through copying links and relays, the sequences of As, Cs, Gs, and Ts are revealed to be the job orders and blueprints for the construction of particular molecular machine tools. Some sequences are orders to itself—arranging for the giant molecule to twist and kink so it can then issue a particular set of instructions. Other sequences ensure that the instructions will be followed to the letter. Many three-letter words specify a particular amino acid (or a punctuation mark, like the one that signifies “START”) out there in the surrounding cell, and the sequence of words encoded determines the sequence of amino acids that will make up the protein machine tools that control the life of the cell. Once such a protein is manufactured, it usually twists and folds itself into a three-dimensional shape spring-loaded for action. Sometimes another protein bends it into shape. These machine tools, at a pace determined both by the long double-stranded molecule and by the outside world, then proceed on their own to strip other molecules down, to build new ones up, to help communicate molecular or electrical messages to other cells.

This is a description of some of the humdrum, everyday action in each of the ten trillion or so cells of your body, and those of nearly every other plant, animal, and microbe on Earth. The tiny machine tools perform stupefying feats of molecular transformation. They are submicroscopic and made of organic molecules, rather than macroscopic and made of silicates or steel, but at the molecular level life was tool-using and tool-making from the start.

The long self-replicating double-stranded molecule with the complex message is a sequence of genes, a little like beads on a string. Chemically, it is a nucleic acid (here, the kind abbreviated DNA, which stands for deoxyribonucleic acid). The two strands, wrapped around each other, comprise the famous DNA double helix. The nucleotide bases in DNA are called adenine, cytosine, guanine, and thymine, which is where the abbreviations A, C, G, and T come from. Their names date back to long before their key role in heredity was understood. Guanine, for example, is named unpretentiously after guano, the bird droppings from which it was first isolated. It is a double ring molecule made of five carbon atoms, five hydrogens, five nitrogens, and one oxygen. There’s something like a billion guanines (and roughly equal numbers of As, Cs, and Ts) in the genes of any one of your cells.

Except for some oddball microbes, the genetic information of every organism on Earth is contained in DNA—a molecular engineer of formidable, even awesome talents. One (very long) sequence of As, Cs, Gs, and Ts contains all the information for making a person; another such sequence, nearly identical, for a chimpanzee; others, not so different, for a wolf or a mouse. In turn, the sequences for nightingales, sidewinders, toads, carp, scallops, forsythia, club mosses, seaweed, and bacteria are still more different—although even they collectively hold many sequences of As, Cs, Gs, and Ts in common. A typical gene, controlling or contributing to one specific hereditary trait, might be a few thousand nucleotides long. Some genes may comprise more than a million As, Cs, Gs, and Ts. Their sequences specify the chemical instructions for, say, manufacturing the organic pigments that make eyes brown or green; or extracting energy out of food; or finding the opposite sex.

How this complex information got into our cells, and how arrangements were made for its precise replication and the obedient implementation of its instructions, is tantamount to asking how life evolved. Nucleic acids were unknown when The Origin of Species was first published, and the messages they contain were not to be deciphered for another century. They constitute the demonstration and definitive record of evolution that Darwin sought. Scattered in the ACGT sequences of the diverse lifeforms of our planet is an incomplete history of the evolution of life—not the blood, bones, brains, and the other manufactured products of the genetic factories, but the actual production records, the master instructions themselves, slowly varying at different rates in different beings in different epochs.