|
09/11/2001
Origins of Life. Part 2
Last week, I mentioned Stanley L. Miller''s landmark experiment
in 1953 that demonstrated the possibility that organic compounds
could have formed on earth prior to the appearance of life. With
this experiment, Miller became the father of what is now known
as "prebiotic" chemistry. I was curious to see whether Miller is
still around 48 years later and found that he''s alive and still
working on experiments related to the origin of life on earth. In
honor of his 70th birthday last year, Jeffrey Bada and Antonio
Lazcano wrote an article (posted on the University of California
at San Diego''s Web site) about the background of Miller''s first
experiment. Sometimes it''s a twisted route to a landmark
experiment.
Miller was a 23-year-old graduate student at the University of
Chicago when he attended a seminar by Nobelist Harold Urey.
Urey suggested that someone should try to synthesize organic
compounds in a reducing atmosphere. Four billion years ago,
earth''s atmosphere had no oxygen, which today makes ours an
oxidizing atmosphere. Miller was fascinated by Urey''s seminar
but thought experimental work time-consuming and messy.
So, seeking a theoretical project, he hooked up with Edward
Teller, father of the hydrogen bomb. Big mistake. Teller
assigned Miller one big problem - figure out how the chemical
elements were formed in the early universe. Miller spent a year
theorizing with nothing to show for it. Besides, Fred Hoyle,
whose death we noted a couple weeks ago, was on a team that
was solving the elements problem quite nicely at the same time
Miller was floundering. Also, Teller left Chicago for California.
Chastened by the experience, Miller remembered Urey''s seminar
and proposed carrying out an experiment with a reducing
atmosphere. He must have been surprised when Urey was
skeptical. Urey thought a graduate student should work on a
project with a greater chance of success. But Miller persisted
and Urey agreed, giving him a year to show some results. What
to do? Miller knew that chemists had been experimenting with
electric sparks in gas mixtures for decades. Miller also knew that
amino acids were the stuff of proteins and decided to focus on
them.
He and Urey designed an apparatus consisting essentially of a
couple connected flasks, one with water, the other with a couple
of electrodes to form the spark. By heating the water, Miller
formed water vapor. He then introduced a mixture of methane,
ammonia and hydrogen and sparked away for two days. Sure
enough, he found glycine, the simplest amino acid. Urey was
away at the time and returned, delighted by the result. Miller
sparked for a week in the next experiment and other amino acids
were formed. He''d only been working on the project for three
months!
As a Nobelist, Urey used his clout to try to get quick publication
in the prestigious journal Science. The editor agreed that six
weeks was possible and Miller quickly wrote the paper. Very
generously, Urey said not to put his name on the paper! His
reasoning was that Miller wouldn''t get the appropriate credit
otherwise. I once heard Urey give a talk at Bell Labs and he
seemed just that kind of guy. Well, 6 weeks went by and no
word from Science. A reviewer of the paper didn''t believe it and
just put it aside without comment! The paper did get published
and prebiotic chemistry was born.
Back to the chase. Continuing last week''s saga, we''ve got organic
compounds made on earth and/or from space. Now the tough
part begins. To form life, these simple compounds have to first
link up and react with each other to form more complex
compounds, the proteins and other stuff of life. But there''s a
problem - we''re in a very hostile environment. For starters,
there''s no oxygen or ozone in the atmosphere to shield against
ultraviolet radiation. UV radiation would likely tear apart any
complex molecules. So how did our simple amino acids link up
to form more complex, self-reproducing compounds?
Miller and others speculated that life was formed when the
amino acids and other compounds gathered in tidal pools along
rocky coasts. As these pools waxed and waned, the compounds
became concentrated into a soupy mixture that provided the
opportunities for linking up and reacting to form the complex
compounds. This was the prevailing view until the discovery of
those hot, smoky hydrothermal vents in the ocean depths, where
there''s no light or oxygen. Yet, the areas surrounding these hot
vents are teaming with those tubeworms and all kinds of living
creatures.
Many now believe that the first life originated around these hot
vents. But Miller and others have been skeptical of this idea,
saying that amino acids would decompose at the high
temperatures around the vents. In an article in the April 2001
issue of Scientific American, Robert Hazen acknowledges these
criticisms. In fact, he and Jay Brandes, a colleague at Carnegie
Institution of Washington''s Geophysical Laboratory, performed a
confirming experiment. They heated pressurized water
containing the amino acid leucine up to 200 degrees Centigrade
(that''s 180 degrees Fahrenheit above the normal boiling point of
water). Sure enough, the leucine couldn''t take the heat and
quickly decomposed. Score one for the critics.
But wait, Hazen and Brandes had a trick up their sleeves. They
repeated the experiment, only this time they mixed in some iron
sulfide, a mineral found around those hot vents. The leucine
lasted for days at the same high temperature - no decomposing!
The mineral had stabilized the amino acid under the most trying
conditions.
Rocks and minerals have long been suspected as playing a major
role in life''s origins. In the early 1970s, an Israeli group showed
that amino acids could attach themselves to clay surfaces and
link up in chains resembling proteins. Others have shown that
clays can serve to build the compounds that are the building
blocks of RNA, the molecule that takes DNA''s instructions and
makes proteins.
Some point to zeolites, minerals that have very porous
structures. Joseph V. Smith, also of the University of Chicago,
fellow is a zeolite enthusiast. Most zeolites are like to pick up
water in their pores. (I''ve used zeolites to dry organic solvents
for use in lithium batteries.) But some synthetic zeolites prefer
to pick up organics. Now Smith has found a naturally occurring
zeolite in Antarctica that likes organics. His hypothesis is that
organic monomers (single molecules) could assemble in such
zeolites and that the zeolites could catalyze linking up to form
polymers. The porous structure would provide nooks and
crannies to shelter these polymers from UV radiation.
Once again, we''re given a choice of possibilities for the
formation of precursors to life. In any event, it seems reasonable
that minerals have played a role in the origin of life on earth.
With cloning, the genome, stem cells and all that''s going on
today, it seems likely that this century will see a prebiotic
experiment yielding a self-reproducing compound. Are we ready
for it?
Allen F. Bortrum
|
