Origin of the first cell
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If life is viewed from the point of view of
replicators, that is
DNA
molecules in the organism, cells satisfy two fundamental conditions:
protection from the outside environment and confinement of biochemical
activity. The former condition is needed to maintain the fragile
DNA
chains stable in a varying and sometimes aggressive environment, and
may have been the main reason for which cells evolved. The latter is
fundamental for the evolution of
biological complexity. If freely-floating DNA molecules that code for
enzymes
are not enclosed into cells, the enzymes that benefit a given DNA
molecule (for example, by producing nucleotides) will automatically
benefit the neighbouring DNA molecules. This might be viewed as "
parasitism by default." Therefore the
selection pressure
on DNA molecules will be much lower, since there is not a definitive
advantage for the "lucky" DNA molecule that produces the better enzyme
over the others: All molecules in a given neighbourhood are almost
equally advantaged.
If all the DNA molecule is enclosed in a cell, then the enzymes
coded from the molecule will be kept close to the DNA molecule itself.
The DNA molecule will directly enjoy the benefits of the enzymes it
codes, and not of others. This means other DNA molecules won't benefit
from a positive mutation in a neighbouring molecule: this in turn means
that positive mutations give immediate and selective advantage to the
replicator bearing it, and not on others. This is thought to have been
the one of the main driving force of evolution of life as we know it.
(Note. This is more a metaphor given for simplicity than complete
accuracy since the earliest molecules of life, probably up to the stage
of cellular life, were most likely
RNA molecules that acted as both replicators and enzymes: see
RNA world hypothesis. However, the core of the reasoning is the same.)
Biochemically, cell-like spheroids formed by
proteinoids are observed by heating
amino acids with
phosphoric acid as a catalyst. They bear much of the basic features provided by
cell membranes.
Proteinoid-based protocells enclosing RNA molecules could (but not
necessarily should) have been the first cellular life forms on Earth.
Another theory holds that the turbulent shores of the ancient
coastal waters may have served as a mammoth laboratory, aiding in the
countless experiments necessary to bring about the first cell. Waves
breaking on the shore create a delicate foam composed of bubbles. Winds
sweeping across the ocean have a tendency to drive things to shore,
much like driftwood collecting on the beach. It is possible that
organic molecules were concentrated on the shorelines in much the same
way. Shallow coastal waters also tend to be warmer, further
concentrating the molecules through
evaporation.
While bubbles comprised of mostly water tend to burst quickly, oily
bubbles happen to be much more stable, lending more time to the
particular bubble to perform these crucial experiments. The
Phospholipid
is a good example of a common oily compound prevalent in the prebiotic
seas. Phospholipids can be constructed in one's mind as a
hydrophilic head on one end, and a
hydrophobic
tail on the other. Phospholipids also possess an important
characteristic, that is having the function to link together to form a
bilayer
membrane. A lipid monolayer bubble can only contain oil, and is
therefore not conducive to harbouring water-soluble organic molecules.
On the other hand, a lipid bilayer bubble
[1]
can contain water, and was a likely precursor to the modern cell
membrane. If a protein came along that increased the integrity of its
parent bubble, then that bubble had an advantage, and was placed at the
top of the
natural selection
waiting list. Primitive reproduction can be envisioned when the bubbles
burst, releasing the results of the experiment into the surrounding
medium. Once enough of the 'right stuff' was released into the medium,
the development of the first
prokaryotes,
eukaryotes, and multi-cellular organisms could be achieved. This theory is expanded upon in the book,
The Cell: Evolution of the First Organism by
Joseph Panno Ph. D.
Same thing with the Big Bang. We cant go back in time to actually see these things happening so if people called them facts, then religous people would freak out cause there is no proof.
