Earth Reveals Its Early History in Tiny Bites
Little by little, scientists piece together clues about the origin of organic life on the planet billions of years ago
SCIENTISTS trying to figure out the conditions under which organic life arose on our planet have a nearly impossible task. Stanford University geologist Donald Lowe compares their quest to ``taking a book and burning three-quarters of it, tearing it up, and having it written in a foreign language ... and then expecting someone to put together the story.'' Much of the ancient rock record has been destroyed and what remains is severely distorted.
``And so,'' Dr. Lowe says, ``people have been working on understanding these basic problems ... about life for a long time, and they advance in little bites.''
Some recent ``little bites'' have given new insight into how meteorites and comets bombarding the early Earth may have helped set up conditions for the rise of organic life. Lowe has advanced knowledge a bit by narrowing the probable range for the temperature under which the earliest organisms lived some 3.5 billion years ago. Earth's surface temperature then probably was above 20 to 30 degrees C (68 to 86 degrees F) and below 50 degrees C (122 degrees F), in his estimation.
Scientists view the rise of earthly life in the perspective of an evolution that began some 5 billion years ago. That's when the sun and planets formed from a contracting mass of dust and gas. Earth was a distinct planet by 4.5 billions years ago.
The oldest fossil microbes are found in 3.5 billion-year-old rock formations in South Africa's Barberton Mountains and in Western Australia. The oldest known signs of biological activity are traces of chemicals in 3.77 billion-year-old Greenland sediments. These suggest the existence of photosynthesis at that remote time. Thus organic life arose early in Earth's evolution. But how early is hard to estimate.
Maud M. Walsh of Lousiana State University, who has studied the Barberton fossils, told an early-life symposium at the American Association for the Advancement of Science (AAAS) annual meeting in February that ``as we delve into the rock record, we find a growing body of evidence that life began [at least] 3.5 billion years ago.''
Those fossils are remains of blue-green, filament-like bacteria perhaps capable of photosynthesis. They probably were the main source of the buildup of oxygen in the air and provided the stratospheric ozone shield that screens out harmful solar ultraviolet radiation. They were a crucial stage in the evolution of organic life because, Dr. Walsh says, ``the organisms allowed life to evolve further.''
Stanford's Lowe also studies the Barberton and Australia formations - remains of volcanic piles that once were typical early land forms. The bacteria, which appear to have been the dominant organisms, grew in mats on volcanic rocks.
At the AAAS symposium, Lowe pointed out that gypsum crystalizes from sea water only at temperatures below 50 degrees C. Gypsum is abundant in the old rocks, indicating a surface temperature below that level 3.5 billion years ago. Also, the rock record shows that weathering was very active at that time. This suggests a warm climate - probably above 20 to 30 degrees C, he said.
Lowe notes that scientists are not certain that the earliest microbes were photosynthetic. But by 3.5 billion years ago, it seems fairly certain that organisms were complex enough to suggest considerable evolution before that. The big unanswered question asks how that proceeded and when it started.
For some hundreds of millions of years after Earth formed, the infall of debris - giant meteorites and comets - probably was too destructive to allow organic life to evolve. But, at some point, biological molecules would have formed and begun to evolve in complexity.
James F. Kasting of Pennsylvania State University says that, by 3.8 billion years ago, the bombardment would have tapered off and may have helped set up atmospheric conditions favorable to this evolution.
In 1957, the late Harold Urey and Stanley Miller, who is now at the University of California at San Diego, passed electric sparks (simulating lightning) through ``primitive atmospheres'' rich in methane, ammonia, and water. They produced amino acids, the building blocks of proteins. This became the classic experiment showing that biologically important chemicals could arise under presumed early Earth conditions.
Geochemists later concluded, however, that sunlight would have driven methane to react with water to produce carbon dioxide and hydrogen. Methane was a key ingredient in the Urey-Miller experiments. It provided the carbon that is the backbone of biological molecules in an easily accessible form. It's much harder for carbon dioxide to fill this role.
Dr. Kasting says he now believes that comets and asteroids hitting Earth could have provided the needed carbon in a variety of ways. Carbonaceous meteorites and comets contain several percent organic carbon by weight. Most of this should have been oxidized to carbon monoxide during impact. Other chemical reactions with iron in some meteorites and reactions driven by the shock heating of impacts could also have produced carbon monoxide.
This gas is a reasonably good source of carbon in experiments seeking to imitate how primitive biological chemicals formed. A press release issued by the National Science Foundation, which supports Kasting's work, quotes Dr. Miller as saying that such synthesis is ``generally not very successful using carbon dioxide. [But] intermediate success is obtained with carbon monoxide.''
Unlike Lowe and others who have at least some fossils to study, Kasting has no fossil of the primitive atmosphere. He has to work with plausible scenarios based on the best current knowledge of what kind of chemistry would have been possible on the primitive Earth.
Now that studies of the moon and other solar system bodies show that all planets underwent heavy bombardment after the solar system formed, ideas of what may have happened on Earth have been revised. Kasting says some incoming objects probably would have been big enough for the impact to heat up the planet and evaporate much, if not all, the ocean and destroy any beginning life forms. But the impacts also were a rich source of carbon.
Considering the chemical possibilities in this light, Kasting concludes that carbon monoxide was probably 10 million times more abundant in the primitive atmosphere than in the air today. And this could have created conditions favorable to the rise of simple pre-life chemicals.
This kind of speculation puts boundaries on what is feasible when scientists try to penetrate the mystery of how earthly life arose. In this sense, it is one of the ``little bites'' by which Lowe says this difficult research advances.
But scientists doing such research also know how tentative their conclusions are. Lowe notes, ``Commonly a significant amount of what they do will be modified by later workers who come along, but there's a progression toward what we think is an understanding.''