46 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”least 40-nucleotides long, for the cell to be able to replicate and to start a
primitive translation mechanism.^42 Because of the difficulty in transition,
it is hard to imagine an RNA World as a precursor of a modern Protein
World. Recently, scientists are starting to back down from a pure RNA
World in favor of a Ribonucleo-Protein (RNP) World, in which RNA and
peptides functioned in concert early on.^43
3.12 The Pros and Cons of a Protein-first Scenario
The temptation in favor of a protein-first probiotic world is obvious.
Proteins are versatile biopolymers that can perform a great number of
tasks. Enzymes, in particular, are the universal workhorse of a cell. Once
enzymes are in place, just about anything could happen. Furthermore,
some amino acids occur naturally in meteorites that are as old as the
birth of the Earth. However, although small peptides can occur spon-
taneously, big proteins can only be made in the ribosome through the
action of peptidyl transferase, a ribozyme. Theoretically, the difficulty of
making a functioning protein with the correct amino acid sequence far
exceeds that of nucleic acids, as there are twenty different amino acids
to choose from compared to only four nucleotides. Therefore, getting
an active enzyme by random assemblage of amino acids is impossible.^44
Another major drawback with protein is that the molecule itself cannot
serve as a template for replication, nor can it be used for any other kind
of information transfer.
3.13 In Search of a Confined Environment
As noted above, metabolism takes place optimally in a privileged envi-
ronment within an enclosure (the plasma membrane), which in turn
needs metabolism and energy supply for its synthesis and maintenance.
Here we hit another “chicken-or-the-egg” problem in addition to the
one between proteins and nucleic acids. All laboratory simulations for
the origin of life are conducted in test tubes, and it is all too easy to forget