Researchers Getting Closer to Finding the Roots of Life
Volcanoes spewed steam and other gases. Sunlight and lightning changed the gases into other molecules, which rained into oceans. And in shallow pools near barren seashores, increasingly complex chemical reactions produced Earth’s first living organisms.
“They were even simpler than the simplest bacteria,” says UCLA paleobiologist J. William Schopf.
He said they probably looked like round, microscopic bags made of membrane. They contained chemicals that let them make copies of themselves and obtain food energy from their bleak surroundings.
Scientists who paint this picture of the origin of life say they still do not know exactly how complicated mixtures of chemicals gradually assumed the properties of life sometime between 4.6 billion and 3.5 billion years ago.
But in the 34 years since Stanley Miller subjected gases to electrical sparks to produce amino acids--the chemical building blocks of life--researchers have got much closer to figuring out how life began.
“We’re trying to reconstruct a historical process,” said Miller, a chemist at UC San Diego. “So we’re never going to find the piece of rock where life started.”
Scientists eventually will demonstrate a plausible explanation of how life started by actually synthesizing a very primitive life form in the laboratory, said Miller, whose famous 1952 experiment as a University of Chicago graduate student earned him a place in biology textbooks.
“I’m optimistic that in the near future we will understand what happened 4 billion years ago on the primitive Earth,” said chemist Andre Brack of France’s Center for Molecular Biophysics.
Researchers have a pretty good idea how simple chemicals evolved into complex organic chemicals that contain carbon but are not living substances, said Boston University biologist Lynn Margulis, co-author “Microcosmos,” a book about evolution.
She said scientists also know about the evolution from bacterial life to people, but there is a gap between chemicals and very simple bacteria. “That gap is being narrowed,” she said.
Recent advances in understanding how chemicals became life were outlined in July when 285 researchers from 22 countries met in Berkeley for the eighth International Conference on the Origin of Life.
The most exciting findings concerned the way complex chemicals first were able to make copies of themselves, setting the stage for the reproduction of living cells; how organic chemicals might have evolved into the membranes essential for cells to exist, and how the earliest organisms could have collected, stored and used the energy they needed to survive.
The scientists also debated what conditions on early Earth made life possible.
The theory that life arose from chemical evolution--increasingly complex chemical reactions that produced sugars, nucleic acids, proteins and other vital molecules--was developed in the 1920s by Russian biochemist Alexander Oparin and British biologist J. B. S Haldane.
The still-predominant theory holds that ultraviolet sunlight, lightning and other energy acted on gases in Earth’s early atmosphere to produce more elaborate chemicals, which rained into oceans and created a “primordial soup” of organic chemicals that eventually spawned life.
UC Berkeley’s Melvin Calvin first tested the theory in 1950. But it was Miller, working for Nobel laureate chemist Harold Urey, who made the breakthrough.
He bombarded a mixture of hydrogen, methane, ammonia and water vapor with electricity and produced four amino acids, the organic acids commonly found in the proteins of all living things. The gases used in Miller’s experiment were those from which the solar system is believed to have condensed.
But in recent years, atmospheric chemists argued that hydrogen, methane and ammonia would have been destroyed fairly quickly in Earth’s early atmosphere. They say volcanic eruptions yielded an atmosphere made up mostly of nitrogen, carbon dioxide and water vapor.
The problem is that life’s organic building blocks would not easily form in such an atmosphere, although Miller and others still believe they did.
Those who disagree offer minority theories about the source of the organic chemicals needed for life.
Chemist A. Graham Cairns-Smith of Scotland’s Glasgow University believes life arose from clay minerals that could reproduce themselves, store and transfer energy and serve as lifelike factories that assembled organic chemicals.
Other scientists argue that life started near chemical-rich undersea hot springs.
Some contend that the chemicals needed for life came from meteorites that struck Earth. Most scientists believe molecules from meteorites simply augmented organic chemicals that formed on Earth.
“The general feeling is that the real business (of life) started on Earth in liquid water,” Brack said.
Conditions for Life
While debate continues on the conditions for life, research is starting to show how chemicals became organisms.
Biologists define an organism as something able to maintain itself by consuming and deriving energy from its environment so it can grow and reproduce.
The blueprints that govern this growth and reproduction are stored in so-called nucleic acids--DNA, the material genes are made of, and RNA, which is, among other things, essential in the manufacture of proteins.
Some proteins become part of the cell as it grows; others are used to regulate the workings of the nucleic acids.
For years, scientists were stymied by a chicken-or-the-egg question: Which came first, nucleic acids or proteins?
Without nucleic acids, there is no information to reproduce and no way to create proteins. Yet without proteins, what would run the nucleic acid chemistry needed for reproduction?
A possible answer was offered earlier this year by Thomas Cech and his University of Colorado colleagues. They found that certain types of RNA, not just proteins, could produce the chemical reactions needed for life.
Miller said the discovery was exciting because it meant the first living cells could have arisen from chemicals much more easily than previously believed.
For a living cell to exist, it also must maintain itself as something separate from its environment. Membranes perform this essential task, preventing living things from collapsing into watery blobs.
Findings From Meteorite
Biophysicist David Deamer of UC Davis studied a meteorite that fell in Australia in 1969 and found it contained organic chemicals that can assemble themselves into fatty, membrane-like droplets and layers.
Because meteorites are remnants of the solar system’s birth 4.6 billion years ago, such molecules probably were also on the young Earth and could have developed into the earliest membranes, Deamer said.
The ability of these molecules to form a film on the surface of water suggests that they might have concentrated the chemicals needed for life from the mixture in Earth’s early oceans.
How did membrane-like bags of self-reproducing chemicals handle energy they needed to survive and grow?
Modern organisms use a substance called ATP to store and use food energy. But ATP is a complicated chemical. Scientists believe the first living cells had a simpler way of handling energy.
Biochemist Herrick Baltscheffsky of Sweden’s University of Stockholm suggested an answer: a simple phosphate molecule called PPi. He says this chemical can drive many of the energy reactions of living cells.
Fossils also may offer clues to life’s origins.
Nobody knows exactly when life started after the solar system and Earth condensed from a swirling cloud of dust and gas. But recent findings persuade scientists that life began earlier than 3.5 billion years ago.
UCLA’s Schopf and graduate student Bonnie Packer recently confirmed earlier, disputed findings that Earth’s oldest known fossils were left by bacteria that lived 3.5 billion years ago.
The microscopic fossils appeared to be cyanobacteria, slimy green bacteria capable of photosynthesis, which is the conversion of carbon dioxide and sunlight into oxygen and food energy.
2.8 Billion Years Old
The oldest previously known cyanobacteria fossils date back 2.8 billion years. Schopf’s discovery that they apparently existed 700 million years earlier means that the oxygen needed for the development of more complicated organisms was being produced earlier than thought.
More important, it suggests that the evolution from organic chemicals to simple bacteria to complex bacteria occurred much faster than once believed.
As researchers close the gap between chemicals and living organisms, they realize that “the division of matter into living and non-living is perhaps an artificial one,” said University of Maryland chemist Cyril Ponnamperuma.
Much information about how life started on Earth came from the space program. Scientists believe that future space missions should study the chemical evolution of organic substances in space, look for possible planets around nearby stars and keep trying to determine if life exists beyond Earth.
“The process that led to life on Earth must have occurred elsewhere in the universe,” Ponnamperuma said.
In a universe that contains some 100 billion galaxies, each with at least 100 billion stars, there must be at least a billion billion places where conditions might allow life to exist, said George Wald, a Harvard biologist and 1967 Nobel laureate.
“We find ourselves in a life-breeding universe,” he said.
