• Choose a category
• All
• Classic-Fiction

By Root 711 0

the molecules’ stability and replication rates. These differences in the ability of molecules to stay intact (stability) and to allow for copies of themselves to form (replicability) will change the proportions of molecules in any region of space. If chemical and physical conditions in that region remain unchanged for long enough, the ratios of the different types of replicating molecules will eventually settle down to a fixed proportion. At that point, all the remaining replicating molecules in the region will be equally fit to survive—whether owing to their stability or replicability or varying combinations of both. In other words, a purely physical process has produced molecular adaptation: the appearance, persistence, and enhancement of molecules with chemical and physical properties that enable them to persist or replicate or both. Then, at some point, the chemical environment changes, slightly or greatly: temperatures rise or fall, new and different molecules diffuse through the region, the magnetic field strengthens or weakens. The process of adaptive evolution starts again, thermodynamically filtering for new stable, replicating molecules adapted to the new conditions.

As this process goes on, two other phenomena become inevitable: the size and complexity of the replicating molecules will increase. Eventually, molecules will emerge that enhance each other’s stability or replication through their chemical relations to each other. There are no limits to the repetition of this process, making bigger and more complicated and more diverse molecules. If conditions are favorable, the result will be really big assemblies of stable and replicating molecules—for instance, RNA and eventually DNA sequences and strings of amino acids (that is, genes and proteins).

The rest is history—that is, natural history. The process we have described begins with zero adaptations and produces the first adaptation by dumb luck, sheer random chance that the second law makes possible. Molecular biologists don’t yet know all the details of this natural history, or even many of them. Some have been known for a long time. It was in the early 1950s that two scientists—Stanley Miller and Harold Urey—showed how easy it is to get proteins, sugar, lipids, and the building blocks of DNA from simple ingredients available early in Earth’s history. All they did was run an electric current through some water, methane, ammonia, and carbon. Chemists have been designing similar experiments ever since, getting more and more of the building blocks of terrestrial life. Biologists have discovered that the simplest and oldest of organisms on the planet—the archaebacteria—probably first emerged at least 2.8 billion years ago and still survive in volcanoes at the bottom of the sea. It is there, in volcanoes at the bottom of the ocean, under the highest temperatures and greatest pressure, that we find chemical reactions spewing out boiling lava and producing the largest quantities of entropy on the planet. This is just what the second law requires to drive thermodynamic noise, and through it to find stable and replicating molecules in a world of random mixing.

How much like the evolution of recognizably biological things—genes, cells, replicating organisms—is the molecular process we have described? Well, recognizably biological evolution has three distinctive features.

First, natural selection usually finds quick and dirty solutions to immediate and pressing environmental challenges. More often than not, these solutions get locked in. Then, when new problems arise, the solutions to old problems constrain and channel the random search for adaptations that deal with the newer problems. The results are jury-rigged solutions permanently entrenched everywhere in nature.

A second feature of biological evolution is the emergence of complexity and diversity in the structure and behavior of organisms. Different environments select among variations in different adaptations, producing diversity even among related lineages of organisms. The longer evolution proceeds, the