The Extreme Complexity of the Simplest Living Cell

Revision 2010/10/01 By Don Stoner

I was recently asked whether I believed that prokaryotes (the simplest known living cells -- pictured right) could have come into being naturally, or if an intelligence of some sort was required. This turns out to be a very interesting question. The genetic code, with its attendant machinery, makes a good place to start. The code itself appears to be very "stable" (very little variation, since most mutations to these critical components are lethal) across all forms of life, right down to, and including, the prokaryotes. Here is a small part of what makes up that code:

1) The DNA molecule: This is a base-4 information storage molecule, comprised of phosphates sugars and the four chemical code constructs abbreviated A, T, G, and C -- paired A-T and G-C like ladder rungs, connecting two sugar-phosphate helix. Every three of these pairs (43 or 64 possible combinations) specifies either one of 20 amino acid molecules or a marker to "stop" making a protein. The code has some redundancy (some different combinations will code for the same amino acid) and also "start" markers (which are comprised of combinations of the other codes). This code is able to specify all of the details of any living creature; but the code won't do anything by itself.

2) The RNA Polymerase molecule: This one puts the DNA molecule to shame in the same way that a CNC (Computer Numerical Control) router dwarfs the simple little thumb drive that holds its program, when complexity is compared. RNA Polymerase, with the help of many other special molecules, hunts along the DNA until it finds a legal start sequence, attaches itself, and "crawls" along the chain matching bits of RNA (similar to DNA but different in a few critical points) to the pattern on the DNA, connecting them together, and thereby constructing very specific RNA chains (specified by the exact pattern of the DNA molecule. These chains fold into complex and specific shapes (DNA doesn't fold like this), some of which are themselves little machines. Three types of these produced RNA molecules are m-RNA, t-RNA, and r-RNA.

3) The Messenger RNA molecule: m-RNA is, like DNA, a data carrier -- except that it doesn't replicate (except indirectly in the case of viruses) or do many other things the DNA molecule does. Unlike DNA's information, RNA's information is easily accessible (but also more subject to damage). It is short-lived but can be read by another piece of cellular machinery; specifically the r-RNA molecule.

4) The Transfer RNA molecules: t-RNA hunts around the cell for specific amino acids. There are about 60 different kinds of t-RNA and each kind hunts down a particular one of the 20 amino acids used by the genetic code. (Multiple different t-RNAs can hunt for the same amino acid.) Each t-RNA is also coded with three RNA markers (kind of like a bar code) which identify it so that a ribosome can figure out what kind of amino acid it is supposed to be carrying.

5) The Ribosomes r-RNA: Ribosomes (Detailed image at left), whose structure includes more than fifty different proteins in addition to their RNA, are tiny nano-tech numerically-controlled molecular assembly machines. They take individual amino acids (sorted and delivered to them by t-RNA molecules), select the correct ones (using the pattern supplied by m-RNA molecules and the "bar codes" on the t-RNA), align them (using little alignment jigs), then connect them together into a chain (driving away water molecules, that would inhibit chemical bonding, by hitting them with tiny little hammers that fly back and forth at about 10 or 20 times a second), and finally check for errors (discarding incorrectly assembled amino-acid protein chains). These tiny machines are powered by ATP molecules which prokaryotes produce directly in their cell wall and cytosol. In other types of cells (called eukaryotes), this ATP is produced in a different piece of chemical machinery called mitochondria (which have there own slightly different DNA and genetic code). Prokaryotes do not have mitochondria. This is how Ribosomes produce the many different kinds of proteins, including RNA Polymerase (see #2 above) and the 50 or so different proteins which they themselves require.

That's just a tiny bit of the automated CNC chemical factory that is part of every "living" prokaryotic cell. Remove or damage any part in this critical loop and the cell won't work. Give it DNA that doesn't contain the correct templates to produce every one of these machines and the cell won't work. Is it possible to come up with a simpler set of machines that can perform a bootstrap function? How difficult might that be?

I've been trying for years to get my workshop properly equipped so that I can reproduce any machine in the shop. So are many other similarly bent hobbyists. We've made some amazing progress, but we aren't there yet. (Even though we are all intelligent designers.) Pictured at right is the RepRap, an experimental machine which is an attempt at a machine that can construct a copy of itself. The "parent machine" (left) produced all of the low-temperature plastic parts which were used to make the "child" machine (right). The metal parts, drive belts, motors, electronics, and the heating element were made by other machines; and assembly was done by humans. This is an impressive toy; but it is still a mere shadow of the final goal.

A CNC machine (left) can automatically make many mechanical parts; but it cannot make the parts for a working copy of its own electronics. Other machines make microelectronic components (right), but cannot make the necessary metal parts. Still others can make flexible drive belts, copper wires, heaters, and all of the other parts that machines require. Obviously the entire planet's commercial output is already up to the task of reproducing all of its machines. This really is possible, if we include the intelligent human assemblers, operators, and designers; but how often might we expect this level of technical capability to simply fall together completely by chance?

It is certainly true that the the Urey-Miller apparatus generates an interesting mass of gloop; It is also true that genetic engineers (intelligent) can rearrange the cellular machinery to produce some fascinating critters; but did all of the bare minimal necessary parts just fall together (well enough to bootstrap life) without any help?


Who Made God?

But this isn't exactly the sort of situation where we should be confidently asserting there was no "extraterrestrial scientist," or other intelligence involved. I'm betting with the odds. I'm also surprised when others won't even allow for the obvious. Something is seriously wrong here. I think the opinion pendulum "somehow" got "stuck" to one side.