The Origin of Life, Part II: Polymerization

Okay, let’s go on with the origin of life. In my last post I talked about Miller’s experiment. The significance of this experiment was simply to show that non-biological processes could result in the formation of organic molecules, including amino-acids and nucleotides. These molecules that Miller got, however, were still relatively simple. They thus only represented a first small step.

Amino-acids and nucleotides by themselves don’t get us very far because we need to get these simple molecules linked together. They act as building blocks to make the more complicated stuff that we are really made up of. The technical term for this process is polymerization. In other words, complex organic molecules, like proteins, or DNA, are polymers. They are long chains of building blocks(monomers).

Miller was able to make the building blocks, but living things need those building blocks strung together in polymers.

Ordinarily, in living things today, there are a series of specialized proteins, called enzymes, that are responsible for building these polymers out of the monomeric building blocks. What happened in the early Earth in the absence of these specialized protein machinery that could possibly lead to polymerization?

Polymerization in Laboratory

The first evidence that this was possible came fairly early on in the late 1950’s, and it was worked by Sidney Walter Fox. Fox took Miller’s experiment one step further. He was able to take amino-acids that might have been created in an experiment like Miller’s and get them to start joining together but only under certain conditions. In just the right proportions, in just the right temperature, the right amount of time that you might heat them, he could get short polymers of amino-acids. We call a polymer of amino-acids a protein, but we also call it a polypeptide chain. That’s simply because the chemical bond that link these monomeric amino-acids to form that chain is called a peptide bond.

What Fox was able to do is to get fairly short polypeptides, showing that you can get spontaneous polymerization. The problem was that Fox could only do this under a very narrow range of conditions. In Miller’s work, you could just throw a bunch of stuff into those flasks, and you get some sort of organic molecule. Fox’s work, however, required much more controlled conditions, conditions that are unlikely to have been that of the early Earth.

Origin in Clay?

Fox and a number of other scientists, however, speculated that maybe you could get more spontaneous formation of polymers if you had some sort of non-biological catalyst. A catalyst is just a term that refers to something that makes a chemical reaction run faster. What Fox and others suggested was that maybe there was something that was non-biological that could catalyze these polymerization reactions. Specifically, what they suggested was that perhaps there were certain kinds of clays that acted as inorganic catalysts.

Why clay? It turns out that some kinds of clay, when they dry out, form very regular ladder-like structures. Furthermore, these clays would also have weak electrical charges on their surfaces. These weak electrical charges can adhere organic molecules. The idea here is that sometime in the early Earth, the shores of a primitive ocean had a bed of clay. As organic molecules that were being created in that primitive ocean got accumulated into the shore, they adhered to that clay. And the clay, because of its regular order and the spacing, would increase the probabilities that you get some sort of spontaneous polymerization.

Wow, that’s an interesting idea. Is there any evidence that this could work? We don’t know what the primitive Earth was like at that scale, but it turns out that recent work by James Ferris, who is at the Rensselaer Polytechnic Institute, has shown exactly that this process does work under abiotic conditions. In a laboratory, Ferris and his colleagues have been able to synthesize not only short polypeptides, but also short stretches of DNA from the component building blocks that were created from experiments like those done by Miller.

The proteins and DNA that Ferris and other have produced are not functional. These are strings of monomers that have been polymerized, but they don’t make any sense. It’s not like a string that would do anything like a real biological molecule might. Nevertheless, it is a start. We can postulate that biological polymers could arise spontaneously.

So, let’s imagine that we have complex polymers. Let’s imagine that we’ve got a primitive ocean brimming with a whole bunch of organic polymers, what has been called the primordial soup. Let’s imagine that even some of these polymers, by chance, have come together as strings that might even have some sort of useful biological function, like modern polymers. Where do we go from there?

The experiments of Miller, Fox, Ferris and others had shown that this is possible, but even with all of this we still don’t have anything approaching what we would want to call life. Why not? Because we know that the organic molecules that make us up are not just a jumble of things floating around in a primordial soup, they are highly ordered. They come in highly ordered packages. This is going to be the subject of my next post, cells. Stay tuned.

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