The first living organism either
needed to find adenine (and the other nucleic acids) in its surroundings or synthesize
them from more common chemicals.
In the laboratory, chemists can create adenine in an
enclosed vessel from a concentrated solution of hydrogen cyanide and ammonia. Other
nucleic acids can then be created from adenine. The first form of life would be destroyed
by these conditions, and because there is simply no way to concentrate volatile gases in
an open environment (like the primitive earth), the conditions required to create large
yields of adenine in the lab do not model the conditions on the primitive earth.
Miller has suggested that adenine was synthesized from
hydrogen cyanide at very cold temperatures as water freezes, melts and re-freezes. While
this process may have produced some adenine, if the entire body of water ever melts, then
the adenine will again be too dilute. Furthermore, the freezing cycles described above are
limited to small ponds because the salts present in the ocean interfere with adenine
synthesis. Despite these concerns, adenine has been found in very small concentrations in
several meteorites. So nature can make it in small quantities; as a result, some adenine
was probably dissolved in the primitive ocean.
The problem is one of dilution. If the first
living molecule does not have the ability to synthesize adenine, then it will have to
search the ocean for decades, collecting adenine (and the other nucleic acids) before it
can replicate. Since the first living molecule must be simple, it would be more reasonable
to speculate that the nucleic acids float by the living molecule, and it simply collects
them. Such a molecule may have to wait several thousand years to accumulate the nucleic
acids that it needs to replicate just once. This is not a reasonable model for a living
system. The lifetime of the chemicals that make up the living molecule (assuming it is an
RNA molecule) is very short (see chapter 9). So such a molecule will not exist long enough
to replicate once.
Suppose instead that the primordial soup exists
locally in a small pond or puddle. In theory, with the appropriate concentration
mechanisms in place, such a puddle may have a concentration of nucleic acids many orders
of magnitude higher than that of the primitive ocean. If life evolves in the puddle, it
will quickly deplete the supply of free nucleic acids as it replicates. It will then run
into the dilution problem outlined in the previous paragraph.
This argument and several others like it
suggest that life was never as simple as many scientists have theorized. The enzymes that
synthesize adenine and the other nucleic acids almost certainly had to exist either before
or coincidentally with the origin life. Furthermore, there is quite a bit of evidence that
suggests that these enzymes did exist. Every single living thing (with the exception of a
few parasites who have lost the genes) shares the same genes that encode the enzymes
responsible for making nucleic acids. This means that the genes responsible for the
synthesis of adenine existed in the common ancestor to all living things, 2.5 to 4 billion
years ago.
Figure 13.1: The Tree of Life
For the reasons outlined above, life on the primitive earth was not
possible without the genes responsible for making nucleic acids, and these genes may have
actually proceeded life (figure 13.1). Even if life originated first, the problem remains
unsolvable. The metabolic pathway that will be discussed next is perhaps the best example
of an irreducibly complex system that can be found in life. The synthesis of adenine
requires 11 enzymes. If a single enzyme is missing, the yield of adenine is zero.
Therefore, all 11 enzymes must evolve together and become at least marginally functional
before natural selection can preserve and optimize the system. The implication is that the
very first step required to create the molecular knowledge is insurmountable. Chance and
natural selection cannot explain the evolution of this metabolic pathway.
next: Adenine
Synthesis
home: Intelligent Design and the origin of life
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