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.