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Name: Bonnie
Status: Educator
Age: 50s
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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.

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.


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

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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