Origin of life
There are 3 possibilities for the origin of life, but only one falls into the realm of
science, because only one can be investigated with controlled experiments, and even
that is difficult. (The other 2 are the Panspermia hypothesis (life arrived via
meteors from somewhere else in the universe), and supernatural/divine creation.
We can only really consider the scientific hypothesis for the origin of life. Your text
calls it “Chemical Evolution”, but many scientists these days refer to the
spontaneous origin of life from inorganic molecules as “Abiogenesis.”
Many scientists are reluctant to admit how little we know about the origin of life, yet
it obviously happened, and the evidence very strongly supports that it happened
between 3.5 and 4.0 billion years ago. The question is how.
Oparin/Haldane and Urey/Miller: In the 1920s, Oparin (a Russian) and Haldane
(a Brit) hypothesized that in the primitive earth the conditions would have been
conducive to the spontaneous formation of complex molecules – the building blocks
of life. The most critical of their speculative notions was that there could be NO
OXYGEN in the earth’s primitive atmosphere (all atmospheric oxygen comes from
photosynthesis…no life, no photosynthesis). So what gases would have been
present? How about checking out the gases emitted from volcanoes around the
world and analyzing gases trapped in bubbles within volcanic rock. (Proportions
vary, but ammonia, methane, carbon dioxide, water, sulfate, and hydrogen are
consistently present in the gases emitted from magma.) In the 1950’s, Urey and
Miller (this is in your text) constructed an apparatus to simulate the conditions
proposed by Oparin and Haldane (figure 17.9, page 447). What they got (as did
many others who copied and modified their work), were amino acids, nucleotide
bases, urea, and aldehyde sugars. What they did not get is polymers and selfreplication. This remains a HUGE gap to fill.
The problem: ALL life on the planet is based on translating a universal code (think
language) from nucleic acid to protein. As you know from our unit on DNA
replication, there are numerous protein enzymes involved. Thus, DNA cannot be
replicated without proteins. Proteins are made on ribosomes (a tight cluster of
rRNA molecules and protein molecules) based on instructions from nucleic acids
(DNA>RNA). Thus, we can’t have DNA without proteins and we can’t have proteins
without DNA.
Enter RNA: In 1967, Carl Woese, first suggested that different types of RNAs could
have served both the information function (like DNA) and the metabolic function
(like enzymes). He suggested a long-extinct “protocell” without DNA or protein that
used RNAs for both information and metabolism. This is now referred to as the
“RNA World Hypothesis”. Woese was vindicated in this hypothesis with the
discovery of over 20 different RNAs with functions that include information storage
and catalyst, among many others.
Organization: The more one studies the complexity of even the simplest of cells,
the more unimaginable it becomes that such organization could have occurred
spontaneously. After all, polymers are dependent on catalysts, and catalysts are
polymers. Among the most extraordinarily complex and organized systems
required for cells as we know them is the organization of the universal genetic code.
RNA World to DNA/Protein World: Most scientists working on abiogenesis seem
to adhere to the notion of the RNA World hypothesis, but have gotten stuck on how
the transition might have occurred. Carl Woese spent his entire career working on
this. In a landmark paper from 1998 (The Universal Ancester), Woese proposed a
transitional cell-like entity he referred to as the “progenote.” The universal
anscester, according to this scenario, would necessarily have been communal, with
rampant gene sharing. During this hypothetical “progenote era” the genetic code
would necessarily have evolved. In 2006, Woese, with colleagues Nigel Goldenfeld
and Kalin Vetsigian ran numerous computer models to determine the probability of
the genetic code having occurred as a “frozen moment” in a single cell as many has
proposed, vs the probability that the genetic code could have self-organized
communally. Similar computer models have been generated by other researchers
with similar results: The probability that our genetic code evolved as a “frozen
moment” in a single cell is basically zero, however, the probability that the genetic
code evolved communally, while admittedly very low, was in fact greater than zero.
Despite his lifetime of achievements, Woese admitted that the origin of life is a
problem so enormous that he cautioned anyone from taking it on, lest they go mad.
(Yet the research continues.)
The bottom line is that your text’s presentation of how life could have originated
spontaneously is shot full of holes, and falls within the realm of speculation, whereas
Darwinian evolution (species have changed over time) is an empirical fact. There
are tidbits of evidence that provide some evidence that it could have happened
spontaneously, but proof of abiogenesis is non-existent.