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The evolution of cells.
Evolution means change over time. This change involves divergence (going off in different
directions). The evolution of cells (and life) is not to be confused with the origin of cells.
Whereas we can observe evolution of cells (think of a bacterium that evolves to become resistant
to antibiotics), we cannot observe the origin of cells…..because it happened almost 4 billion years
ago.
The first cells made their mark on this planet about 3.8 billion years ago in the form of chemical
signatures in carbon-containing minerals that are seen only in the biochemistry of cells. If you
must have real, bona fide fossil evidence, then you can say that cells first left their mark on the
planet 3.5 billion years ago. In either case, it was a long time ago.
A brief summary of hypotheses regarding the origin of cells on earth: Scientists have long
speculated how the first cells originated. If we assume this occurred via natural phenomena (the
only testable hypothesis), we have some unimaginably significant obstacles. Scientists working
in the area tend to approach the origin of life from different perspectives. Abiogenesis is the
term applied to the origin of life from non-living parts. This would contradict the second part of
the Cell Theory (all cells from cells). This field of research is funded in large part by NASA. If you
pay attention to astronomical discoveries, you often hear about whether or not a celestial body
“might have liquid water” or “could support life.”
What we know: The first cells were relatively simple (bacteria were first). Yet all of the modern
cells, including those in your body, have some significant common features with these simple
cells, including (but not limited to) the plasma membrane, DNA, cytoplasm, ribosomes,
energetics (glycolysis, chemiosmosis), and information transfer (DNA>RNA>Protein>Trait).
Perhaps the most significant common feature of all life (including viruses) is the UNIVERSAL
GENETIC CODE. A “gene” from a human can be spliced into a bacterium, and the bacterium will
make the human protein because the CODE is universal. Human insulin (a protein) is made in
labs by engineered E. coli.
DNA carries instructions for making RNAs and Proteins. DNA doesn’t DO anything, really, it
carries coded instructions for making other molecules (RNAs and Proteins) that DO the work of
cells. The paradox is that the replication of DNA requires the work of RNAs and Proteins, and as I
said already, the instructions for making these working molecules is carried by DNA. Thus, we
can’t have DNA without RNA and Proteins, and we can’t make RNA and Proteins without DNA. It
has been hypothesized that an intermediate form of life existed prior to the first cells that had
neither DNA nor Protein, but it must have had RNA, which can function in both information
storage (like DNA) AND as a catalyst (like most protein enzymes). No such RNA-based life form
has ever been found, very strongly implying that if such an RNA-based, pre-cellular entity
existed, it has since gone extinct. For more info, google RNA World Hypothesis.
Prokaryote means “before the nucleus.” Cells with a nucleus did not appear (based on the fossil
record) prior to about 1.8 billion years ago. That means that for the first 2 billion years of life, all
cells were prokaryotic (bacteria and archaea). The evolution of eukaryotes is – like the origin of
life – shrouded in mystery. We know that mitochondria and chloroplasts are descended from
alpha proteobacteria and cyanobacteria, respectively, but we don’t know where the Eukaryotic
nucleus came from. Scientists argue about this a lot, but their arguments are not based on solid
evidence.
Obsession: Scientists working in the field of deep evolution (billions of years ago) seem to me to
be totally obsessed with their work. Many of them spend their entire careers working on one of
the many problems. These are brilliant people, but brilliance (in my experience) often comes
with combativeness (against those who disagree), and closed-mindedness (to opposing
hypotheses). I have personally spent a significant amount of time and thought on the various
hypotheses, and have come up with a very fuzzy, impressionistic image of how life might have
evolved. But I’m probably wrong. It doesn’t matter, because it helps me to sleep at night.
Back to divergent evolution: If you start with a small community of very similar individuals,
they will invariably go off in different directions over time. That is exactly what we see in
evolution. Adapt or die…..that’s the basis of Darwinian evolution. But the question that remains
(and about which Darwin never had a clue) is HOW organisms adapt. While it is true that
Darwin pointed out that even a small population of individuals will always exhibit variation, he
did not know the source of this variation. We now know that the variation is coded in the genes.
Some people think the source of variation is random mutation, with favorable mutations
surviving and unfavorable mutations dying out. It turns out that there is far more going on than
random mutations, and it has also been demonstrated in labs that mutation is not necessarily
random.
Ed’s view: The early earth was hot and nasty. Lots of volcanic eruptions, molten rock, and really
hot water (liquid and steam). Of the many atmospheric gases, oxygen could not have existed in
more than miniscule quantities. Chemical activity was driven by heat, radiation, and electricity
(lightning). The result was a “primordial soup” of energy-rich molecules. See section 17.4 in
your text. Despite the very significant problem of how these molecules organized, I see the RNA
World preceding the emergence of the first lineage of cells (bacteria). The transition from RNA
World to DNA>Protein world is difficult, because it would have been during this transitional time
that the “language” that is the Genetic Code evolved. Languages evolve communally, and I accept
the Carl Woese hypothesis of a community of pre-cellular entities that worked out a genetic code
together. Woese called these pre-cellular entities PROGENOTES. A lot of scientists disagree, but
they do not offer alternative explanations for the origin of the universal genetic code. This is all
speculative (with some evidence and a lot of hurdles), and people (including scientists in the
field, as well as yourself) are free to accept what works best for them…just as I have done.
As for the evolution of eukaryotic cells, I accept endosymbiosis as a very compelling explanation
for mitochondria and chloroplasts, but the rest of the complexity (most notably the nucleus), is
too mind-boggling for my pedestrian brain to deal with. Some scientists have proposed that the
nucleus is also a result of endosymbiosis, but the evidence for this is far to weak (in my opinion)
to be acceptable by me. The reason is that the DNA molecules we see in nuclei are linear (there is
a beginning and an end), whereas the Bacteria and Archaea (prokaryotes) have DNA molecules
that are circular. And while the Archaea DNA is more similar to nuclear DNA, there are still
significant differences. Some scientists propose that the eukaryotic nucleus was preceded by a
“eukaryotic line of descent” in which the eukaryotic lineage branched off from the bacteria
around the same time as the Archaea. This is the most acceptable hypothesis to me, but again,
there is no living organism without a nucleus, but with DNA structured in the way that
eukaryotic DNA is structured. Bottom line: The origin of the nucleus remains a mystery.