Miller-Urey Experiment
Hardly a conversation can be had about the origin of life on
Earth without mention of the Miller-Urey experiment. Very little is known about the conditions on
Earth during the time that life would have been forming. Harold Urey and his then graduate student
Stanley Miller were amongst the first scientists to postulate about early
conditions. They conducted an experiment
that has been repeated in its original and altered form for over five
decades. Their work has become seminal
for those studying the chemistry of the origin of life on Earth.
Although scientists continue to collect new data that sheds
light on the subject, there is still quite a bit of debate over the composition
of early Earth’s atmosphere. What does
the atmosphere have to do with the origin of life, you might ask? Well, the chemical composition of the
atmosphere strongly influences the types of chemical reactions occurring at the
surface of the planet, and consequently impacts the conditions under which life
would have originated. Based on work
published in The Origin of Life by the Russian scientist Alexander Oparin in 1938, Miller suggested that life was forming
during a time when Earth’s atmosphere consisted of methane, ammonia, water, and
hydrogen. This chemical makeup is quite
different from our modern atmosphere of nitrogen, oxygen, and other gases. In short, Miller introduced these molecules
into a sealed flask, applied an electric discharge, and allowed the system to
cycle for a week. What he discovered has
impacted origins research for over 50 years.
The original apparatus used by Miller and Urey was quite
simple compared to today’s standards. It
essentially consisted of two glass flasks connected by glass tubing. One flask served as the boiling flask, where
gases and other molecules could accumulate in a water phase. The other flask (located above the boiling
flask) served as a place where gases could accumulate and mix together. An electrical discharge, meant to simulate
lightning to produce free radicals, was provided by using an induction
coil.
The experimental procedure was also straightforward. Water was first added to the boiling
flask. Then the apparatus was evacuated
completely of air. Once the air had been
removed, hydrogen gas (H2),
methane (CH4)
and ammonia (NH3)
were pumped into the apparatus. Finally,
the water in the flask was boiled and the electrical discharge was
started. The entire system was allowed
to run continuously for a week.
Like any good scientist, Miller took copious notes of what
happened inside the apparatus during the weeklong experiment. After the first day, the water in the flask
turned distinctly pink. As the week
progressed, the solution inside became more and more red and also a bit cloudy. Once the experiment was complete, Miller and
Urey determined that the cloudiness, or turbidity, of the solution was due to
silica from the glass. The reddish
color, however, resulted from organic compounds that “stuck” to the
silica. Although difficult to see at
first, Miller also noted yellow organic molecules.
At the end of the week, Miller collected the contents of the
apparatus and tested the contents for amino acids using chromatography. Initial tests confirmed the presence of glycine, α-alanine, and β-alanine and suggested that
aspartic acid and α-amino-n-butyric acid had
also been produced. This list of amino
acids falls miles short of the 20 amino acids commonly used by life on
Earth. However, Miller and Urey both
suspected that other amino acids were also present, but in such small amounts
that their detection was difficult to impossible.
The intent of Miller was not to try and produce amino
acids. Rather, his intent was to explore
the early conditions on Earth and what the naturally occurring results would
be. What he discovered was that,
although the conditions he proposed are not optimum, organic molecule synthesis
could have been a natural consequence in Earth’s history. More importantly, Miller and Urey went on to
explore amino acid synthesis by developing more efficient apparati
and altering the initial atmospheric conditions in the simulated
environment.
Scientists studying the atmosphere of early Earth now
believe that the primary atmospheric constituents were different from those
first proposed by Oparin and later tested by
Urey. James Kasting
at Pennsylvania State University has suggested that the atmosphere on Earth
just after the succession of heavy bombardment would have been dominated by
carbon dioxide and nitrogen and contained small amounts of carbon monoxide,
hydrogen gas, and reduced sulfur gases.
Now instead of a simple set of glass flasks connected by tubes and
sealed, scientists use complex computer models and mathematical equations to
simulate the conditions of early Earth.
Unless we develop a time machine, we will never know exactly what the
planet was like. But through good
observations and critical analysis by all scientists in the field, we will
definitely arrive at feasible theories about the beginnings of Earth.
This is a picture of the apparatus used by Miller. I took it directly from his original 1953
article, but it is simple enough that we could re-create it no problem. The reference for the article is:
Miller, S.L. (1953). A production of amino acids under
possible primitive earth conditions, Science, 117, 3046, 528-529. It is on page 528.
