The Atheist's Guide to Reality_ Enjoying Life Without Illusions - Alex Rosenberg [30]
MOLECULAR SELECTION: NATURE’S OWN NANOTECHNOLOGY
Natural selection requires three processes: reproduction, variation, and inheritance. It doesn’t really care how any of these three things get done, just so long as each one goes on long enough to get some adaptations. Reproduction doesn’t have to be sexual or even asexual or even easily recognized by us to be reproduction. Any kind of replication is enough. In chemistry, replication occurs whenever a molecule’s own chemical structure causes the chemical synthesis of another molecule with the same structure—when it makes copies of itself or helps something else make copies of it. This can and does happen several different ways, both in the test tube and in nature. The way most directly relevant for evolution on Earth is called template matching—the method DNA uses to make copies of itself.
First step: When atoms bounce around, some bind to one another strongly or weakly, depending on the kind of attraction there is between them—their chemical bond. When the bond is strong, the results are stable molecules. These molecules can only be broken up by forces that are stronger than the bonds. Such forces require more energy than is stored in the stable bond between the atoms in the molecule. Breaking down a stable molecule takes more energy than keeping it together.
Second step: Occasionally, these relatively stable molecules can be templates for copies of themselves. Their atoms attract other atoms to themselves and to each other so that the attracted atoms bond together to make another molecule with the same structure. The process is familiar in crystal growth. Start out with a cube of eight atoms in a solution of other atoms of the same kind. They attract another four on each side, and suddenly the molecule is a three-dimensional cross. As it attracts more and more atoms, the crystal grows from a small cube into a large one. The crystal grows in a solution through “thermodynamic noise”: the increasingly uneven and disorderly distribution of atoms just randomly bouncing around in the solution as mandated by the second law. The atoms already in the crystal latch onto ones in solution in the only orientation chemically possible, making the nice shape we can see when they get big enough.
A crystal molecule doesn’t just have to grow bigger and bigger. Instead the molecule can set up chemical forces that make two or more other unattached atoms that are just bouncing around bond with one another, making new copies of the original crystal. Instead of getting bigger, it makes more copies of itself.
The process could involve more steps than just simple one-step replication. It could involve intermediate steps between copies. Think of a cookie that is stale enough to be used as a mold to make a cookie cutter that takes the stale cookie’s shape. It’s a template to make a new cookie. Make the new cookie, then throw away the cookie cutter, let the new cookie go stale, and use it to make a new cookie cutter. Make lots of copies of the same cookie using one-use cookie cutters. You get the idea. (In a way, this is just how DNA is copied in the cell, using each cookie cutter once, except the cookie cutter is not thrown away. It’s used for something else.)
Physical chemists and organic chemists are discovering more and more about how such complicated structures arise among molecules. They are applying that knowledge in nanotechnology—the engineering of individual molecules. Pretty soon they’ll be able to induce molecules to build any geometrical shape they choose. Often the molecule of choice in nanotechnology experiments is DNA. Inevitably, there are people like the late Michael Crichton who warn us that we will soon be overrun