Probing the Chemistry of Creation
Come, take a seat in the kitchen of creation and try to replicate the lost recipe for the origin of life.
Be warned. This is a hypothetical dish that must be prepared through trial and error from the raw chemistry of Earth and space--without benefit of conventional biology or supernatural intervention.
So, experiment. Stoke the primordial planet’s volcanic ovens. Stir its ocean caldron with wind. Boil it. Ice it. Season it with cyanide. Pepper the mix with comet dust and leaven it with time.
That is the task facing researchers trying to reconstruct the chemistry of creation.
To investigate the origin of life, some researchers are taking hints from the components of today’s biochemistry and trying to work backward to discover simpler organic molecules that can perform life’s essential functions.
In recent months, a NASA consortium of scientists has been shedding new light on the primeval biochemistry that existed before the first mating dance of proteins and DNA, which underlies all modern biology.
The group draws together biologists, chemists and geophysicists based at the research centers of “Biotech Beach†along the San Diego-La Jolla coastal corridor.
Its members have helped reset the clock of evolution and uncovered the earliest known evidence of life on Earth.
And in a crucial step with several other laboratories, other members have created a functional imitation of a primordial living molecule--a self-replicating substance that can be made to evolve and adapt without the help of DNA.
They have even evolved forms of DNA that nature did not. Last month two consortium scientists for the first time created a primitive molecule that can reproduce itself and evolve generation after generation in a continuous test-tube reaction. Some leading researchers now are confident that it may be only a decade or so before they can create life from scratch.
In the laboratory, however, life still requires a guiding hand.
Scientists have tried--and repeatedly failed--to create the conditions under which life can arise spontaneously. Nor have they been able to create a molecule that--unaided--can reproduce itself in a self-sustaining reaction. No one knows when anyone may achieve that ultimate goal.
This daunting laboratory enterprise, however, has become even more provocative as researchers eye primordial Mars and the frozen moons of Jupiter for havens of primitive life. Astronomers also yearn for signs of organic chemistry on any of the planets discovered recently around other sun-like stars.
Indeed, in some theoretical scenarios, the icy wastes of Jupiter’s Europa, recently scanned by NASA’s Galileo probe, bear a striking resemblance to the alien Earth of more than 4 billion years ago. Geologists suggest that life may exist in volcanic vents below Europa’s icy crust, just as bacteria live in Earth’s underwater volcanoes today and in lakes deep beneath the icecap of Antarctica.
The idea is seductive. Basic organic chemicals float in clouds of interstellar dust. They have been detected in the tails of comets like Hale-Bopp. And they have been discovered in meteorites as old as the solar system.
So it may seem only natural to expect that such interstellar chemicals seeded life on other planets. After all, life on Earth has taken up residence in so many unexpected places. Bacteria thrive in the absence of oxygen or sunlight, in boiling water, perpetual ice and subterranean depths, feeding readily on toxic wastes or on antibiotics designed to eradicate them.
But without a technical inkling of just how life arose in the one place it is known to exist, the search for life elsewhere in the universe is based largely on wishful thinking, researchers say.
More important for them, the effort to re-create the original chemistry of life offers the possibility of a scientific answer to one of humanity’s most persistent spiritual and philosophical questions: How did life begin?
Despite the daunting uncertainties, a recent series of advances has many researchers hopeful that it may be only a matter of time before science succeeds in creating life in the lab.
“We may never know how it happened on Earth, but I am confident that somebody is going to do it in the laboratory within the next 10 or 20 years,†said Leslie E. Orgel at the Salk Institute for Biological Studies.
Orgel, an authority on the origin of life, has joined with NASA and four other prominent specialists--Stanley Miller at UC San Diego, Gustav Arrhenius and Jeffrey L. Bada at the Scripps Institution of Oceanography, and Gerald Joyce at the Scripps Research Institute--to probe this enduring mystery.
Although they still may be far short of their goal, their growing understanding of life’s early chemistry offers the promise of powerful new drugs based on molecules custom-tailored by directed test-tube evolution.
More important, perhaps, the work of the NASA group reinforces the raw power of evolution as a natural force to spur change even in primitive molecules, forcing these collections of atoms to spawn creations more complex than themselves.
What seems most striking about this endeavor, researchers acknowledge, is that it is almost wholly an act of scientific intuition, constrained only by the rules of organic chemistry.
Should they succeed, they will have no way to know if they have actually learned how life began or if they simply invented a new prescription for its creation.
“If you had the correct answer to the origins of life, you would not be able to prove it is correct,†said chemist James P. Ferris at the Rensselaer Polytechnic Institute in Troy, N.Y., who works with Orgel.
“Our hope is to build systems that seem reasonable from what we know. We may be way off the mark.â€
Molecules Jockeying for Dominance
In the beginning, life may have existed as little more than a set of unusual molecules with the remarkable ability to store information, copy themselves and change in response to conditions around them. Long before there was even a single cell to divide and multiply, these individual molecules jockeyed for dominance.
So, by this theory, the first living thing may have been a spreading patch of discolored clay at the edge of a drying lagoon or a mineral-like formation building up on a submerged volcanic vent. There was no hint that, in time, this self-sustaining chemical reaction would dominate the planet.
“The story [of life] has its beginning at the point . . . when molecules first began to undergo Darwinian evolution,†said Gerald Joyce, an expert in molecular evolution and a senior partner in the NASA collaboration.
By attempting to understand the composition and character of these forerunner molecules, researchers are exploring a realm that today exists only in theory.
Certainly, researchers agree, life did not start out with the sophisticated biochemical machinery of DNA, which today allows the molecular information of life to be carried inside a universal genetic code and copied every time a living cell divides.
But in trying to determine how DNA evolved into being, researchers must answer a riddle. Which came first: The DNA that stores genetic information or the proteins that enable it to copy itself?
Some researchers have sidestepped that question by conceiving a molecule that encompasses both.
Orgel and Nobel laureate Francis Crick--the co-discoverer of DNA--proposed that life might have started with a crucial organic molecule called RNA.
Today, RNA is merely a cog in the master machinery of life. It serves as an intermediary in the transcription of genetic information encoded in complex molecules of DNA, aiding in the manufacture of vital enzymes, proteins and hormones. But once it may have functioned alone.
It is possible that RNA-based life forms evolved into the more stable structure of DNA.
Nobel laureate Walter Gilbert dubbed this hypothetical kingdom of RNA-based life forms the “RNA world,†and evidence suggests that this theory may be close to the truth.
“What makes it particularly tricky is that we don’t ever expect to have fossil evidence of the RNA world,†said Joyce. “We don’t literally expect to ever see direct physical evidence that the RNA world existed on this planet or any other, for that matter.â€
Nonetheless, several researchers have shown that, in the laboratory, RNA can be made to copy itself without the assistance of the genes or protein enzymes so necessary to the function of living cells today.
Joyce and his colleagues have induced these RNA molecules to undergo primitive evolution.
The ability to force the RNA molecules to adapt to changing conditions bolsters the theory of the RNA world, experts said.
“It has the look and feel of what the RNA world in the test tube would look like,†Joyce said of his team’s most recent experiments.
That does not say necessarily that life started with RNA. It may have only been one of many molecular “species†that contended for resources in the primordial pools.
Indeed, Orgel and his colleagues are investigating the possibility of primitive protein-based molecular life, instead of more familiar RNA or DNA.
But even such primitive forms require a certain level of complexity to function. And evolution cannot begin without the right kind of raw material.
“How do you start evolution without the help of evolution?†Joyce said. “How do the chemical reactions bootstrap themselves to the magic moment when . . . evolution begins?â€
Life Amid Rubble of Young Solar System
Whatever its original form, life on the embryonic Earth arose earlier and perhaps faster than anyone had imagined, researchers at the Scripps Institution of Oceanography recently determined.
New chemical evidence from the planet’s oldest known sedimentary rocks suggests that life was thriving 3.8 billion years ago, dangerously close to--or even during--the eons when the infant planet was bombarded by the rubble left over from the formation of the solar system.
It may have taken 10 million years or less to go from a primordial soup of inanimate organic chemicals to the first bacteria whose remains form those earliest known fossils, researchers say. This is far faster than the billions of years that long had been conjectured.
Indeed, life may have arisen more than once, some speculate, somehow making the transition from inanimate mineral to living organism under conditions that almost certainly would be lethal to the life dominating Earth today.
Researchers are exploring ways these pre-biotic compounds--as the nonliving precursors of the first molecular life are called--may have developed from more basic chemicals.
“What is the nature of the first genetic material and how do you make the building blocks?†asked Miller of UC San Diego. He has spent almost 45 years trying to duplicate the original chemical formula for creation.
As much as anyone, it was Miller who took the question of life’s origin beyond speculative metaphysics or theology and into experimental laboratory chemistry.
While a graduate student in 1953, he conducted a now-legendary experiment in which he filled a beaker with his best guess of the Earth’s early atmosphere, jolted it with electricity and produced a rain of amino acids critical to all living things.
Today, as one of the collaborators in the NASA project, he still is trying to answer the question that launched his career.
“You have the pre-biotic soup. There is still this question of what is in the soup,†he said.
Using gases that may have dominated the planet’s early atmosphere, Miller and his collaborators so far have managed to create 13 of the 20 amino acids utilized for organic life in his test-tube Earth.
But researchers must work blindly, for time has erased any clues to the nether world in which life first arose more than 4 billion years ago. Not everyone thinks he has the right idea about the early atmosphere’s composition.
“Since we have no record of Earth’s earliest history, we have to speculate,†Arrhenius said. “But it has to be based on what we know of geophysics and geochemistry. Researchers in this area--chemists in particular--commit the sin of ignoring the ground rules.â€
Researchers are certain the early Earth and its atmosphere were radically different from what exists today.
There probably was no oxygen in the atmosphere, so it would have been poisonous to life as it is known now. Scientists also agree that the sun was 25% dimmer, while its ultraviolet radiation may have been up to 32 times more intense. Those differences are important because even a slight decrease in the sun’s brightness could drop temperatures to 40 degrees below zero.
With so little else certain about early Earth, there is almost no limit on the scientific imagination. So, there is no shortage of theories about the original chemistry of life. None can be proven--or disproven--and each contains at least one seemingly fatal flaw.
One leading theory notes the consequences of a weaker sun shining on the primordial planet.
Bada and other scientists speculate that on the early Earth all but the ocean depths must have been frozen solid. Under that scenario, life may have developed around hot volcanic vents on the ocean floor. Crucial organic compounds could have been delivered by comets and meteors smashing through the icy crust.
Some researchers, however, question whether fragile organic chemicals could survive the fiery shock of a comet impact. Bada has suggested that they may have been protected by traveling inside the armor of special carbon molecules.
Gases spewing from the comet as it sped toward Earth also could have lingered at high altitudes. In research made public last month, Christopher P. McKay and William J. Borucki at NASA’s Ames Research Center suggest that shock waves generated by comet impact could turn trace gases into complex organic compounds in the atmosphere, much as lightning will.
In any case, the idea of comet chemicals brewing under an icecap has captivated scientists who think Europa may be home to life.
Intrigued by new images released by the Jet Propulsion Laboratory last month, some researchers speculate that organic matter--and, perhaps, extraterrestrial life--may be developing in an ocean concealed by the tiny moon’s miles-thick sheath of ice, kept warm by the heat of its core.
Some biological evidence on Earth lends credence to this idea.
During undersea volcanic eruptions, researchers often detect microbes that developed under the extreme conditions prevailing inside the Earth’s rocky crust. Some of these microbes grow in water as hot as 235.4 degrees. Indeed, the earliest known bacteria seem to have evolved in conditions of extreme heat.
In April, two German researchers offered new experimental evidence for the idea that life began around the red-hot lava of a volcano. By re-creating the chemistry of deep sea vents in the laboratory, they synthesized some key chemical steps necessary for the creation of biological molecules.
“The conditions of our reaction may be taken as a model for understanding the habitats of primitive forms of life on Earth or Mars,†concluded Claudia Huber and Gunter Wachtershauser.
But critics say the water circulating through those volcanic vents on Earth, heated by magma to more than 600 degrees, is so hot it would have destroyed any simple organic compounds.
Others researchers scoff at the thought that life developed in a deep freeze. They contend that the early Earth may have been kept warm by an atmospheric greenhouse effect caused by high levels of carbon dioxide.
But that idea also has problems.
For the necessary greenhouse effect, the early atmosphere would have required a level of carbon dioxide 100 to 1,000 times higher than today--high enough to quash most organic chemical reactions, researchers say.
In yet a third theory, some researchers argue that the oceans could have been kept liquid--a prerequisite for life--by the internal heat of Earth’s newly formed core. Others suggest that in the period when life is believed to have begun, the oceans may have been periodically vaporized in plumes of steam by the impact of comets and huge meteors.
Even the presence of liquid water--so crucial to the maintenance of life today--could have hindered its development on primordial Earth. Experiments show that water can interfere with the growth of the kind of complex molecules needed to develop life.
So many uncertainties are enough to frustrate the most dedicated optimist.
Forty-four years ago, when Miller first performed his ground-breaking “genesis†experiment, the creation of life “looked so simple and so easy,†he recalled.
“It turned out not to be easy,†he said.
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Looking for Life
Scientists--inspired most recently by new information from Jupiter’s moon Europa and a Martian meteorite--have been trying to figure out how life originated on Earth, as a first step in answering whether life could exist elsewhere in the universe. An unusual NASA consortium of scientists in San Diego and La Jolla is leading the effort to reconstruct the chemistry of creation. Here are three scenarios they are researching.
Impacted World: New evidence suggests that life on Earth arose close to--or even during--a time when the planet was bombarded by the rubble left over from the formation of the solar system. In fact, some researchers believe that comets and meteors were the source of the organic chemicals needed for the beginning of life. But the bombardment, so fierce it left scars still visible on the moon today, easily could have obliterated any developing life forms.
Ice World: Some researchers believe that, under a weaker sun, the oceans of primordial Earth must have been frozen. Today, even a slight decrease in the sun’s brightness would be enough to drop temperatures to 40 degrees below zero. Under that scenario, life might have developed around hot volcanic vents on the ocean floor. But critics contend that the water in volcanic vents is so hot it would have destroyed any simple organic compounds.
Water World: Other researchers contend that the early Earth may have been kept warm by an atmospheric greenhouse effect caused by high levels of carbon dioxide in the air. But that idea also has problems. To create the necessary greenhouse effect, the planet’s early atmosphere would have required the level of carbon dioxide to be between 100 and 1,000 times higher than today--high enough to quash most organic chemical reactions, researchers say. Other researchers say the internal heat of the planet’s core could have kept the oceans liquid--a prerequisite for life.
Source: NASA Specialized Center of Research and Training in Exobiology
Compiled by ROBERT LEE HOTZ / Los Angeles Times
