Ask A Scientist© Molecular Biology Archive

name         Bonnie
status       educator
age          50s

Question -   School text regarding Harold Urey and Stanley Miller's
experiment on the origins of life.  Placed gases found in atmosphere 3.6
billions years ago (no oxygen, no unicellular organisms),water to
represent ocean, plus electrical current.  Week or so later dark liquid
containing building blocks of life carbohydrates, proteins, and nucleic acids.

Question:  How do I explain no oxygen or hydrogen in the Earth's
atmosphere 3.6 billion years ago?  Water did not evaporate?  Original life
forms did not need oxygen to survive and yet first life forms lived in the
ocean where there was oxygen?  Kids and I don't get it.
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When water evaporates it doesn't turn into hydrogen and oxygen! This is an
example of a physical change, not a chemical one. In a physical change,
matter only changes state or size, etc., ie. new molecules are not formed.
So, yes, the theory states that the earth was very hot and it follows that
water evaporated. But the oxygen and hydrogen in the water was not free.
And, not all organisms require oxygen for life. There are many bacteria
(probably the first life forms) that cannot tolerate molecular oxygen and are
killed by it. As far as geological evidence goes, rock that dates back to
that time by radiometric dating does not contain oxides, ie. elements
combined with oxygen. Oxides do date to the time that single celled
autotrophs (photosynthesizers) and algae first show up in the fossil record.
As they gave off oxygen as a waste product, it began to fill the air. So,
there was no oxygen in the atmosphere before plant like organisms evolved and
plants came after bacteria.

Vanhoeck
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Urey/Miller used methane (CH4), ammonia (NH3), hydrogen (H2), and water
(H2O) as a model primitive atmosphere and direct current sparks (lightning)
and heat as energy sources. The absence of free O2 was based upon the
premise that the abundance of hydrogen would react with any free oxygen in
such a model atmosphere, in much the same way that we do not find very much
free hydrogen in our contemporary atmosphere due to the abundance of oxygen.
It is very likely that free O2 could be formed transiently by the
electrical sparks decomposing water.

I don't think their intent was to show that IT HAPPENED exactly like their
model, but that it did not defy the laws of chemistry and physics that IT
COULD HAPPEN in such an environment. They did not use the Sun as a direct
source of photochemical energy, although it certainly was out there, because
the best guesses were/are that not much light actually made it to the
surface of the earth. Besides if you allow sunlight into the model, the
photochemical formation of complex organic compounds would not be a surprise
at all.

Actual life forms came much later in geological history, and I don't think
anyone has a very firm conviction what their biochemistry was.

Vince Calder
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When we say that there was no oxygen in the early atmosphere, we are
referring to elemental oxygen, O2. Yes, water (H2O) was present, but that's
chemically quite different from oxygen. All the elemental oxygen now in the
earth's atmosphere was put there by photosynthetic plants and bacteria. It
was generated from water using the energy from sunlight. In the early
earth, before life was present, this process had not yet taken place.

If this doesn't answer your question, please clarify and ask again.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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