Hey it’s Professor Dave, let’s talk about the origin of life.
We know that all living organisms on earth are made of cells, and that everything on
earth is related by some common ancestry.
But where did the first cell form, and how did it happen?
How do we even know that this is possible in the first place?
As our understanding of chemistry and biochemistry became more and more sophisticated in the
20th century, we began to learn about the components of the cell, especially DNA, which
we call the molecule of life, since it carries the genetic code for an organism.
We began to understand the processes it undergoes, like replication, so that every new cell can
have a copy of all the DNA, as well as transcription and translation, which dictate exactly how
DNA serves as a code for all the components that make a species unique.
We began to wonder, how could these molecules have assembled by themselves billions of years
ago?
A crucial experiment was performed by Stanley Miller and Harold Urey in the 1950’s to
shed some light on this mystery.
They set up a system that was meant to mimic conditions early in earth’s history, just
after it cooled down enough for liquid water to exist.
In this apparatus, they placed water, ammonia, methane, and hydrogen, all of which was exposed
to heat as well as an electric current, meant to simulate lightning.
After about a week, they found that the reaction mixture contained a number of different amino
acids, which as we remember from biochemistry, are the building blocks of proteins.
Since proteins include enzymes, which catalyze the formation of nucleic acids like RNA and
DNA, this offered good evidence that it is indeed possible that the basic materials needed
for life could have been generated spontaneously.
Furthermore, this experiment was only a week long, so half a billion years of molecules
sloshing around in the primordial soup could conceivably produce much more impressive results.
These findings prompted further speculation as to the specifics of how the first primitive
cell could have formed.
If small building blocks like amino acids and nucleotides came about spontaneously,
how did they then polymerize to form larger molecules like proteins and nucleic acids?
How did the plasma membrane of the cell come about, to encapsulate certain molecules and
define the dimensions of the cell?
Some believe that biomolecules first arose near hydrothermal vents at the bottom of the
ocean, where heat emanated from the hot core of the earth, catalyzing chemical reactions
and producing a variety of organic compounds.
Others say things are more likely to have begun in mineral-rich tidal pools where all
manner of catalysis could have occurred on various solid surfaces.
Some even propose that the first organic compounds were transported to the earth from space,
arriving on meteors.
This is called the panspermia hypothesis.
Whatever the case may be, we don’t know exactly what happened, but with a firm understanding
of biochemistry, it’s not much of a stretch to assume that the basic organic components
delivered from the Miller-Urey experiment were able to polymerize, and were then serendipitously
encapsulated in a lipid bilayer, which we know can form spontaneously because of hydrophilic
interactions between the polar heads and water molecules, and hydrophobic interactions where
the nonpolar tails hide away.
And there you have the first proto-cell.
This first cell must have been incredibly primitive compared to even the simplest ones
today, and subsequent cells grew in complexity over time.
This was due to the ability of nucleic acids to self-replicate, as well as mutate, which
is what led to the evolution of all the diverse kinds of life on earth over the eons.
We will learn more about evolution later, first we need to learn a lot more about the
components that can be found inside of different kinds of cells.
So let’s learn about all these components now.