Shadows of Forgotten Ancestors

We can believe in animal or human souls without holding to evolution, and vice versa. But if we examined life more closely, might we be able to understand at least a little of how it works and how it came to be, purely in terms of its constituent atoms? Is something “immaterial” present? If so, is it in every beast and vegetable, or just in humans? Or is life no more than a subtle consequence of physics and chemistry?

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One educated look at how the molecule is shaped and you can figure out what it’s for. Even at the molecular level, function follows form. Before us is a detailed blueprint of breathtaking precision for building complex molecular machines. The molecule is very long and composed of two intertwined strands. Running the length of each strand is a sequence made of four smaller molecular building blocks, the nucleotides—which humans conventionally represent by the letters A, C, G, and T. (Each nucleotide molecule actually looks like a ring, or two connected rings, made of atoms.) On and on the sequence goes, for billions of letters. A short segment of it might read something like this:A​T​G​A​A​G​T​C​G​A​T​C​C​T​A​G​A​T​G​G​C​C​T​T​G​C​A​G​A​C​A​C​C​A​C​C​T​T​C​C​G​T​A​C​C​A​T​C​A​C​C​A​C​A​G​A​C​C​T​C​C​T …

Along the opposite strand there’s an identical sequence, except that wherever nucleotide A was in the first strand, it’s T in the second; and instead of G it’s always C. And vice versa. Like this:T​A​C​T​T​C​A​G​C​T​A​G​G​A​T​C​T​A​C​C​G​G​A​A​C​G​T​C​T​G​T​G​G​T​G​G​A​A​G​G​C​A​T​G​G​T​A​G​T​G​G​T​G​T​C​T​G​G​A​G​G​A …

This is a code, a long sequence of words written out in an alphabet of only four letters. As in ancient human writing, there are no spaces between the words. Inside this molecule there are, written in a special language of life, detailed instructions—or rather, two copies of the same detailed instructions, because the information in one strand can surely be reconstructed from the information in the other, once you understand the simple substitution cipher. The message is redundant, bespeaking care, conservatism; it conveys a sense that whatever it is saying must be preserved, treasured, passed intact to future generations.

Almost every issue of leading scientific journals such as Science or Nature contains the newly uncovered ACGT sequence of some part of the genetic instructions of some lifeform or other. We’re slowly beginning to read the genetic libraries. The library of our own hereditary information, the human genome, is also becoming increasingly revealed, but there’s a lot to read: Every cell of your body has a full set of instructions about how to manufacture you, encoded in a very compressed format—it takes only a picogram (a trillionth of a gram) of this molecule to specify everything you’ve inherited from your ancestors, back to the first beings of the primeval sea. Yet, there are almost as many nucleotide building blocks, or “letters,” in the microminiaturized genetic information in any of your cells as there are people on Earth.

All words in the genetic code are three letters long. So, if we insert the implicit spaces between the words, the beginning of the first message above looks like this:ATG AAG TCG ATC CTA GAT GGC CTT GCA GAC ACC ACC TTC CGT ACC …

Since there are only four kinds of nucleotides (A, C, G, and T), there are at most only 4 × 4 × 4 = 64 possible words in this language. But if the order in which the words are put together is central to the meaning of the message, you can say a great deal with only a few dozen different words. With messages that are a billion carefully selected words long, what might be possible? You must take care in reading the message, though: With no spaces between the words, if you start reading at the wrong place, the meaning will surely change and a lucid message might be reduced to gibberish. This is one reason the giant molecule has special code words meaning “START READING HERE” and “STOP READING HERE.”

As you watch the molecule closely you observe that the two strands occasionally unwind and unzip. Each copies the other, using available A, C, G, and T raw materials—like the metal type stored in an old-fashioned printer’s box Now, instead of one pair, there are two pairs of identical messages. As well as utilizing a language and embodying a complex, redundantly encoded text, this molecule is a printing press.