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commented on Jan. 7, 2012
Proteins are what you get when you string amino acids together, and we need a lot of them. No one really knows, but there may be as many as a million types of protein in the human body, and each one is a little miracle. By all the laws of probability proteins shouldn’t exist. To make a protein you need to assemble amino acids (which I am obliged by long tradition to refer to here as “the building blocks of life”) in a particular order, in much the same way that you assemble letters in a particular order to spell a word. The problem is that words in the amino acid alphabet are often exceedingly long. To spell collagen, the name of a common type of protein, you need to arrange eight letters in the right order. But to make collagen, you need to arrange 1,055 amino acids in precisely the right sequence. But—and here’s an obvious but crucial point—you don’t make it. It makes itself, spontaneously, without direction, and this is where the unlikelihoods come in.
The chances of a 1,055-sequence molecule like collagen spontaneously self-assembling are, frankly, nil. It just isn’t going to happen. To grasp what a long shot its existence is, visualize a standard Las Vegas slot machine but broadened greatly—to about ninety feet, to be precise—to accommodate 1,055 spinning wheels instead of the usual three or four, and with twenty symbols on each wheel (one for each common amino acid).1 How long would you have to pull the handle before all 1,055 symbols came up in the right order? Effectively forever. Even if you reduced the number of spinning wheels to two hundred, which is actually a more typical number of amino acids for a protein, the odds against all two hundred coming up in a prescribed sequence are 1 in 10260(that is a 1 followed by 260 zeroes). That in itself is a larger number than all the atoms in the universe.
Proteins, in short, are complex entities. Hemoglobin is only 146 amino acids long, a runt by protein standards, yet even it offers 10190possible amino acid combinations, which is why it took the Cambridge University chemist Max Perutz twenty-three years—a career, more or less—to unravel it. For random events to produce even a single protein would seem a stunning improbability—like a whirlwind spinning through a junkyard and leaving behind a fully assembled jumbo jet, in the colorful simile of the astronomer Fred Hoyle.
Yet we are talking about several hundred thousand types of protein, perhaps a million, each unique and each, as far as we know, vital to the maintenance of a sound and happy you. And it goes on from there. A protein to be of use must not only assemble amino acids in the right sequence, but then must engage in a kind of chemical origami and fold itself into a very specific shape. Even having achieved this structural complexity, a protein is no good to you if it can’t reproduce itself, and proteins can’t. For this you need DNA. DNA is a whiz at replicating—it can make a copy of itself in seconds—but can do virtually nothing else. So we have a paradoxical situation. Proteins can’t exist without DNA, and DNA has no purpose without proteins. Are we to assume then that they arose simultaneously with the purpose of supporting each other? If so: wow.
And there is more still. DNA, proteins, and the other components of life couldn’t prosper without some sort of membrane to contain them. No atom or molecule has ever achieved life independently. Pluck any atom from your body, and it is no more alive than is a grain of sand. It is only when they come together within the nurturing refuge of a cell that these diverse materials can take part in the amazing dance that we call life. Without the cell, they are nothing more than interesting chemicals. But without the chemicals, the cell has no purpose. As the physicist Paul Davies puts it, “If everything needs everything else, how did the community of molecules ever arise in the first place?”
It is rather as if all the ingredients in your kitchen somehow got together and baked themselves into a cake—but a cake that could moreover divide when necessary to produce more cakes. It is little wonder that we call it the miracle of life. It is also little wonder that we have barely begun to understand it.