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.