Shadows of Forgotten Ancestors

Biology is much more like language and history than it is like physics and chemistry. Why we have five fingers on each hand, why the cross-section of the tail of a human sperm cell looks so much like that of a one-celled Euglena, why our brains are layered like an onion, involve strong components of historical accident. Now you might say that where the subject is simple, as in physics, we can figure out the underlying laws and apply them everywhere in the Universe; but where the subject is difficult, as in language, history, and biology, governing laws of Nature may well exist, but our intelligence may be too feeble to recognize their presence—especially if what is being studied is complex and chaotic, exquisitely sensitive to remote and inaccessible initial conditions. And so we invent formulations about “contingent reality” to disguise our ignorance. There may well be some truth to this point of view, but it is nothing like the whole truth, because history and biology remember in a way that physics does not. Humans share a culture, recall and act on what they’ve been taught. Life reproduces the adaptations of previous generations, and retains functioning DNA sequences that reach billions of years back into the past. We understand enough about biology and history to recognize a powerful stochastic component, the accidents preserved by high-fidelity reproduction.

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DNA polymerase is an enzyme. Its job is to assist a DNA strand in copying itself. It itself is a protein, configured out of amino acids and manufactured on the instructions of the DNA. So here’s DNA controlling its own replication. DNA polymerase is now on sale at your local biochemical supply house. There’s a laboratory technique, polymerase chain reaction, which unzips a DNA molecule by changing its temperature; the polymerase then helps each strand to reproduce. Each of the copies is in turn unzipped and replicates itself.¹⁶ At every step in this repetitive process, the number of DNA molecules doubles. In forty steps there are a trillion copies of the original molecule. Of course, any mutation happening along the way is also reproduced. So polymerase chain reactions can be used to simulate evolution in the test tube.* Something similar can be done for other nucleic acids:

In the test tube before you is another kind of nucleic acid—this one single-stranded. It’s called RNA (ribonucleic acid). It’s not a double helix and does not have to be unzipped to make a copy of itself. The strand of nucleotides may loop around to join itself, tail in mouth, a molecular circle. Or it may have hairpin or other shapes. In this experiment it’s sitting mixed with its fellow RNA molecules in water. There are other molecules added to help it along, including nucleotide building blocks for making more RNA. The RNA is coddled, jollied, handled with kid gloves. It’s extremely finicky and will do its magic only under very specific conditions. But magic it does. In the test tube not only does it make identical copies of itself, but it also moonlights as a marriage broker for other molecules. Indeed, it performs even more intimate services, providing a kind of platform or marital bed for oddly shaped molecules to join together, to fit into one another. It’s a jig for molecular engineering. The process is called catalysis.

This RNA molecule is a self-replicating catalyst. To control the chemistry of the cell, DNA has to oversee the construction of factotums—a different class of molecules, proteins, which are the catalytic machine tools we’ve been discussing above. DNA makes proteins because it can’t catalyze on its own. Certain kinds of RNA, though, can themselves serve as catalytic machine tools.¹⁷ Making a catalyst or being a catalyst gives you the biggest return for the smallest investment: Catalysts can control the production of millions of other molecules. If you make a catalyst, or if you are a catalyst—the right kind of catalyst—you have a long lever arm on your destiny.