How Did Life on Earth Get Started?

On an arid outcropping of basalt in northwestern Australia, some of the oldest rocks on Earth lie exposed to the fierce sun. Formed at the bottom of an ancient ocean, this volcanic material shelters what one scientist calls the "oldest robust evidence" of life. At a scientific meeting at Rockefeller University in May, Roger Buick of the University of Washington said that the 3.5 billion-year-old rocks hold traces of carbon that once made up living organisms.

Even before Buick's discovery, ample evidence indicated that life on Earth began while our 4.5 billion-year-old planet was very young. Simple organisms certainly flourished between 2 billion and 3 billion years ago, and claims of older evidence of life have periodically surfaced. But none have been universally embraced, and Buick's claim is so new that other scientists haven't fully reviewed it.

Yet even if the geologist is right about his rocks, his discovery would leave unanswered one of life's biggest mysteries: how life actually arose. While creationists attribute that spark of life to the hand of God, scientists are convinced there's a natural explanation. Yet as close as they've come to pinning it down, some admit the particulars may never be fully resolved. Others are convinced that we're edging closer to an answer—and to settling one of the oldest and most contentious questions in science and religion.

To solve the riddle of genesis, biologists, astronomers, geologists, and chemists are attacking the problem from all angles—even trying to re-create life from scratch. In recent years, institutions, including Harvard University, the Georgia Institute of Technology, and McMaster University in Canada, have formed "origins" institutes to probe the deepest history of life on Earth—and to search for life in the heavens. "The field is going through a minirenaissance," says chemical biologist Gerald Joyce of the Scripps Research Institute in La Jolla, Calif.

According to scientists, life began when chemistry begat biology—that is, when simple molecules assembled into more complex molecules that then began to self-replicate. But rocks that might harbor traces of such genesis events simply don't exist, says Buick. During Earth's opening act, space debris and cataclysmic volcanic upheavals destroyed the evidence, like an arsonist torching his tracks. The oldest known rocks are about 4 billion years old, yet even they formed roughly half a million millenniums after our planet's surface cooled and water first pooled into shallow seas. Scientists widely suspect that life began during that long, undocumented interval.

Theories about where and how life began range from the sublime to the bizarre. One camp says that deep-sea vents known as black smokers nurtured the first life. In the late 1970s, a team of researchers from Oregon State University unexpectedly discovered whole ecosystems thriving around a hot vent on the Pacific seafloor. Such vents, where molten rock from inside the Earth's mantle heats seawater to as much as 660 degrees Fahrenheit, could have provided the energy and basic organic molecules needed to spark life.

Another camp believes that ice—not boiling water—served as the cradle of life. Even the coldest ice contains seams of liquid. These watery pockets could have acted as test tubes for the earliest organic reactions. Experiments show that units of RNA—the genetic material that was probably the forerunner to better-known DNA—spontaneously string themselves together in ice, supporting this theory.

Still other scientists point to the skies. They argue that meteorites carrying amino acids and other important molecules seeded Earth with the necessary ingredient for life. Supporting the idea: high concentrations of amino acids inside meteorites found on Earth and in gas clouds in space. A wilder offshoot of this theory, called panspermia, suggests that whole bacteria—life itself—first evolved on Mars and then hitched a ride to Earth via small pieces of the Red Planet blasted here by asteroid or comet impacts. But no life has been found on Mars, and the one claim of fossil bacteria in a Martian meteorite, made by NASA scientists in 1996, has been almost universally rejected.

Perhaps the leading theory focuses on a much more prosaic realm: the slimy interface where the sea laps against land. If early oceans carried organic molecules—and they most likely did—the porous surfaces of shoreline minerals could have helped organize such building blocks into primitive structures. Eventually, these units could have replicated, forming thin films on the seashore rocks, says Robert Hazen, a researcher at the Carnegie Institution in Washington.

Scientists favoring one or another theory have tried to boost their case by attempting to re-create the elements of life—and even life itself—from the bottom up. Such work sparked to life in 1953, when two researchers cooked up a "primordial soup" of amino acids. In their University of Chicago lab, they applied simulated lightning to a pair of flasks that contained, respectively, an oceanlike solution and an atmosphere rich in methane, water vapor, ammonia, and hydrogen. That experiment and subsequent ones showed that simple chemistry can transform nonorganic molecules into the building blocks of life.

"The classic experiments done over a period of 50 years give us confidence that the building blocks would have been present," says Andrew Knoll, a paleontologist at Harvard University. "The big question is how do you go from there to something that can replicate itself." A pile of lumber, after all, is not a house.

To understand how the transition might have occurred, a handful of scientists are striving to re-enact it in the lab. Jack Szostak of Harvard Medical School is at the forefront of this work. In his lab, researchers fill test tubes with the barest ingredients of life, then watch the elements self-assemble into what look like primitive cells, hoping that they will begin replicating. "Sooner or later, life will be made in the lab," says Joyce, who performs similar experiments.

While none of these primitive cells have yet "gone critical" and started to copy themselves, the research has already paid real-world divi-dends, including one blockbuster pharmaceutical and perhaps more to come. The drug Macugen treats macular degeneration, the leading cause of blindness in the elderly, with a tiny snippet of the genetic molecule RNA. Since scientists like Szostak think RNA preceded DNA, they've invented ways of pushing RNA to evolve in test tubes. Under the right conditions, that process can produce an RNA molecule that's evolutionarily "fit" for a task like treating the biological cause of macular degeneration.

Hazen says that origins-of-life experiments may also help create synthetic cells that can churn out biofuels, which would be a boon in these days of energy crunches and concerns about climate change. "Life takes things like carbon dioxide and water and makes useful compounds, including fuels," he says. "If we understand in a basic way how that's done," he adds, then scientists might be able to build a primitive, fuel-producing protocell.

Such a self-replicating system, Joyce and Szostak contend, would constitute life. And while such a feat may not exactly recapitulate how biology began, it would be example No. 2—the first being the entire panoply of plants, animals, and other organisms that now dominate our planet.