Map of life expectancy at birth from Global Education Project.

Thursday, August 10, 2006

Kick start?

We don't know how life on earth got started. Creationists seem to consider this a major embarassment, but is it really so surprising that we can't be certain of what happened more than 3 1/2 billion years ago? I think I might have committed a typo a few days back and said 2 1/2 billion, but in fact, the oldest fossils -- of mats of cyanobacteria called stromatolites -- are 3 1/2 billion years old. As I said, that is not long, in cosmic terms, after the rain of big rocks from the formation of the solar system slowed down enough for the crust to cool and the oceans to condense -- maybe a couple of hundred million years, at the most, probably less.

Well, cyanobacteria are fully formed, complicated prokaryotic cells. They didn't just appear, they must themselves be the product of some process leading from nonbiological chemistry to a self-replicating system capable of evolution. On the one hand, that doesn't seem at all hard to believe: once you get a molecule that can catalyze the creation of copies of itself, but with the possibility of imperfect copying, you have the possibility of evolution leading to greater complexity. It only needs to happen once. It could be a very low probability event, but with trillions of chemical reactions going on all over the earth every day, improbable events happen a lot. Once you have a self-replicating evolving system, obviously, if it can reproduce faster than it is destroyed, the rest is history, or rather paleontology.

On the other hand, it has turned out to be hard to come up with an example of such a self-replicating system that can be shown to arise spontaneously, under conditions likely to have existed on the early earth, and not just any self-replicating system will do. We need a pathway to the actual life we have, Life on Earth, LoE. Now, there are fun parts to thinking about this, and there are boring parts. You're just going to have to accept a little bit of both.

The part that I'm afraid may be relatively boring is understanding the essential components for LoE, so I'll do that in this post and get it over with. (Those of you who already know all this, go read Effect Measure.)

First, you have an enclosed compartment formed by the cell membrane. This is essential to keep the complex biochemical sytems inside together, in a small volume where the parts can interact effectively, protected from chemical hazards or other life forms that would eat them, with control over what gets in and what goes out. All LoE has cell membranes formed from a double layer of compounds called phospholipids, but the kinds of phosopholipids in the membranes of archaea are different from those in bacteria. (Eukaryotic membranes are more like archaeal membranes, which is why it seems probable that the Eukaryotic cell arose from an archaean enclosing symbiotic bacteria.) Because the two early domains of life have different membrane lipids, it may be that the membranes arose after the rest of the system was in place. If this is so, it demands that we explain how the biochemical system was enclosed and protected before the development of the cell membrane.

Second, you need peptides. These are strings of subunits called amino acids. Very long ones are called proteins. (Chemicals consisting of such strings of sub-units are called polymers.) Depending on the sequence of amino acids, the proteins fold in complicated ways in solution to make all sorts of shapes. Proteins constitute basic structural elements of cells; catalyze (i.e., cause to happen or greatly accelerate) various chemical reactions including those involving other proteins, DNA and RNA; proteins and shorter peptides carry signals within and between cells; and basically carry out the business of life. There are hundreds of kinds of amino acids in the universe, but only 20 occur in LoE and make up biological peptides.

Third, of course, you need genetic information stored in the form of DNA. DNA, as you probably know, consists of two very long strands made out of four kinds of units called nucleotides, named Adenine, Cytosine, Guanine, and Thymine -- A,C,G,T. A binds with C, and G binds with T, so the sequence of one strand determines the sequence of the other. One strand contains the information which specifies the structure of the proteins - in a code consisting of three "letter" units of A,C,G, and T, which specify amino acids -- and the complementary strand is essential to reproduction of the DNA.

Fourth, you need the mechanism of transcribing proteins from DNA, which is carried out by three forms of RNA, called messenger, transfer, and ribosomal RNA. RNA is like a single strand of DNA. (Thymine happens to be replaced by the molecule Uracil, probably due to some ancient accident, but it scarcely matters. U, like T, binds with G.) Messenger RNA is assembled by "reading" its sequence off part of an active strand of DNA -- and proteins do the business of activating the strand and separating it from its complement so it can be read, so as you can see there are all sorts of complicated feedback loops. In the ribosome, a structure made of RNA, proteins are assembled by transfer RNA, which consists of small pieces with just three nucleotides, corresponding to the code for an amino acid. Each piece of transfer RNA binds to the appropriate amino acid, so, as the transfer RNA lines up along the strand of messenger RNA, the amino acids are assembled in the correct sequence to make a protein.

Whew. This is all very complicated. Every living thing works this way, however, using the same DNA code, the same RNA, the same 20 amino acids -- although, as I have said, there are two basic kinds of cell membranes. So we conclude that every living thing has a common ancestor. The Last Common Ancestor, a single cell, or perhaps a biological system enclosed in some other sort of cavity, whose descendants sallied forth and conquered the earth. How do we know all this? We learned it by studying nature, by looking very hard at reality. You won't find it in ancient books because people in ancient times didn't know as much as we do now. Learning more over time is called growing up.

How do you get the LCA in a few tens of millions of years, from a warm ocean and an atmosphere full of methane and carbon dioxide? Stay tuned.

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