YELLOWSTONE NATIONAL PARK, Wyo.—Scientists who are trying to figure out the origin of life on Earth are getting nearer to their goal, but they concede that they may never solve this profound mystery. A crucial gap remains between the time when nothing was alive and the arrival of the first living creature.
Geologists recently have learned more about the conditions on our planet that could have made life possible as long as 4 billion years ago. And thanks to the DNA revolution, biologists now can trace the development of modern organisms—ranging from simpler forms such as mushrooms and flowers to more complicated ones such as humans—back to primitive microbes that lived in the ocean more than 2.7 billion years ago.
At some point in between, lifeless molecules—combinations of atoms—learned to eat, breathe, move and reproduce.
No one's certain when and how they did it, however.
"There are a lot of theories, but no solid explanation," said Tom McCollom, a geochemist at the University of Colorado in Boulder. "We may never have one."
"The rock record is too spare to allow us to determine the processes that were associated with the origin of life," Bruce Jakosky, a University of Colorado geologist, declares in a forthcoming book, "Astrobiology, Science and Society" (University of Arizona Press, 2006).
"The extremes are easy to identify and characterize—early on with no life, and later on with life—but the intermediate stages cannot be uniquely or readily categorized," Jakosky wrote.
Researchers are seeking to fill this gap with a purely scientific account of the origin and evolution of life, an evidence-based biological explanation that they say need not conflict with theological versions such as "intelligent design" or "creationism."
Advocates of intelligent design seize on missing pieces in the evolutionary record to support their argument that there must have been divine intervention. Scientists counter that a lack of some pieces of evidence, which may yet be found, doesn't mean that the theory of evolution is wrong.
"Everything we've done so far suggests that a natural origin of life is a very plausible event, even if we don't know yet what the sequence of events or chemical reactions was," Jakosky said in an e-mail interview.
"We have really only been working on this problem with a modern perspective for 50 years or so," he added. "The Earth had a vastly larger number of natural laboratories in which different chemical reactions could take place, and they probably operated for tens to hundreds of millions of years before something that we would recognize as life emerged."
At a workshop for science writers in Yellowstone National Park earlier this month, researchers described the latest thinking about the origin and development of life.
As early as 150 million years after the Earth formed, 4.5 billion years ago, the planet had cooled enough to have an atmosphere, an ocean and some dry land.
In this "pre-biotic stage," before living organisms appeared, volcanoes spewed lava and gases rich in hydrogen, sulfur, iron and other minerals. Chemical reactions between hot water and rocks produced more and more complex molecules.
"This environment clearly would have been dominated by chemistry and geochemistry with a complete absence of life," Jakosky wrote.
Later, probably sometime between 4 billion and 3.8 billion years ago, these inorganic molecules grew and clotted together in "protocells." These were little bags of chemicals that took in nutrients and discharged wastes. They formed organic compounds necessary for life.
At some point in this process came the crucial, still-unexplained step: the advance from clumps of molecules to living cells containing an early version of DNA known as RNA. DNA contains the instructions to make proteins, the building blocks of every living thing.
"We know least about this transition. This is where the `miracle' occurs," McCollom said, speaking figuratively, not literally, of a miracle.
Researchers speculate that there might not have been a single event that marked the origin of life, like an on/off switch.
"There is not necessarily a sharp boundary between living and nonliving at the time of the origin of life," Jakosky said. "There could have been a smooth, gradational boundary in the creation of life, with no place along the transition being a clearly definable moment at which life first existed."
Life on Earth may have begun only once, or it may have happened several times and been destroyed by cosmic rays or a bombardment by giant meteors. Perhaps the first creatures retreated to a temporary refuge under the ocean until conditions on the surface improved.
"The date of the origin of life is pretty much a question," McCollom said. "Did life begin before the bombardment, was wiped out and re-created or survived in the subsurface, or did it began after the bombardment?"
"I believe life began very early, before 4 billion years ago," said Norman Pace, a molecular biologist at the University of Colorado. However, he said the evidence for such a quick start was limited to "squiggles in the rocks" in Greenland and Australia that might represent ancient fossils. The oldest positive evidence for living microbes is dated at 2.7 billion years ago.
Once the first cells were established, they began the process of evolution, as Charles Darwin described it. Random changes in a microbe's DNA gave it an advantage over rival organisms that was passed on to the lucky one's descendants. Single-celled organisms clustered in colonies that became tissues and organs, and eventually formed multi-celled creatures such as ourselves.
"Microbes set the stage for us," said David Des Marais, a biochemist at NASA's Ames Research Center in Mountain View, Calif. "They prepared the Earth for us to be able to develop."
Steven D'Hondt, a marine biologist at the University of Rhode Island in Kingston, summed up the situation: "There is a tremendous amount that we don't know, but we know a whole lot more than we did a few years ago."
For more information online, go to www.resa.net/nasa/origins(underscore)life.htm
(c) 2005, Knight Ridder/Tribune Information Services.
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