Ask A Scientist Molecular Biology Archive

name         Jordan T.
status       other
age          20s

Question -   I am reading a book in which there is a discussion about
a chemical experiment and I need a bit of clarification.  In this
experiment, Stanley Miller at the University of Chicago took a mixture of
 ammonia, methane, and hydrogen, shot electricity through it and found
 that amino acids had been formed.  This proved that life could have
 evolved out of nonlife on primitive Earth.

The counterpoint offered to this is that ammonia, methane, and hydrogen
were not abundant on primitive Earth but, rather, water, carbon dioxide
and nitrogen dominated the atmosphere and the experiment wouldn't work
with that mixture.  The man who brings that up seems very confident that
it invalidates the experiment.

My confusion arises because it seems to me that H2O, CO2, and N2 could be
split by electricity and the resulting atoms could realign themselves to
yield NH4, methane (a carbon compound), and H2.  So, in my mind, the
experiment is not invalid, it just skips a step.

I would very much appreciate clarification of why this is not possible.
----------------------------------------
      Several points:
1.     For starters, no one knows what the composition of the atmosphere was
in early geological times. Harold Urey and Stanley Miller (Urey's graduate
student) did the "sparking" experiments that you refer to. Many other
groups, for example biologist Carl Sagan (of "billions and billions of stars
fame"), subsequently performed similar experiments using different types and
amounts of starting gases, different energy sources, different total
pressures, and on and on.
  NONE OF THESE EXPERIMENTS "PROVES" THAT LIFE ON EARTH STARTED BY ANY ONE 
 OF THESE MECHANISMS. Their results are conjectures (Conjectures in science are
fine, scientists do it all the time. But do not confuse conjecture with
"proof" or with "cause -----> effect" ). Conjectures are educated guesses.
What these primordial atmosphere conjectures demonstrate is: It is NOT
IMPOSSIBLE that life MIGHT have started by one or more of these mechanisms.
That is a far weaker claim than: Life DID start by one of these mechanisms.
2.   No one even knows whether there were bodies of liquid water on earth
during those eons. If there was not, where and how did liquid water come to
cover the majority of the surface of the earth?
3.    And then there is the problem of oxygen (O2) gas. Oxygen gas is very
inhospitable to small organic molecules. It tends to oxidize (read that
burn) them back into CO2, N2, and H2O. But if there were no free oxygen (O2)
gas, when and how did it become 20% of our atmosphere and how did these
primordial life-forms survive in its presence?
4.    And what about the problem of chirality? Any molecule that has a
point or plane of symmetry with respect to reflection has two configurations
that are un-superimposable (mirror images like a pair of gloves -- one for
the left hand, and one for the right hand). If you try to put the right hand
glove on the left hand it does not fit. The thumb and little finger are in
the wrong places. Both of these configurations are formed in these high
energy atmospheric model experiments. The two configurations are
distinguished by whether the molecule rotates a beam of linearly polarized
light to the right (clock-wise) or to the left (anti-clock-wise). However,
virtually all molecules involved in life processes (sugars, proteins,
enzymes, etc.) are LEFT HANDED. The body cannot even digest the right handed
forms, and in principle if you were fed a diet composed solely of the right
handed forms, you could starve to death! Why did Nature choose the left
handed form to create life? If both forms are produced in equal amounts
(which is the case in many chemical reactions that are not sensitive to the
"handed-ness" of the molecule), why aren't there two complementary life
forms on earth -- one left handed and one right handed? Is this in some
complicated way related to the fact that Nature chose to make the Universe
out of matter, rather than anti-matter. The best current cosmology theories
predict that matter and anti-matter were formed in equal amounts, but the
Universe is almost all matter and anti-matter must be "created" by nuclear
processes and other artificial high energy interactions.
     I am not sure I clarified your concern about the primordial atmosphere,
but I hope I did clarify that there are more unanswered questions than
answered questions.

Vince Calder
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