Stars forming in the Orion Nebula.

Tony Hallas

In the latest scientific version of Genesis, life begins, paradoxically, with an act of destruction. After 10 billion years of guzzling the hydrogen in its core, a sun-size star runs out of nuclear fuel and becomes unstable. It goes through a series of convulsions and expels a shell of searing-hot atoms—including hydrogen, carbon, and oxygen. The star fizzles into an inert cinder, and its atoms drift off, seemingly lost in the interstellar gloom.

But next the story takes a surprise turn, from destruction to construction. Some of those rogue atoms float into a nearby gas cloud and stick to fine grains of dust there. Even at a frigid –440 degrees Fahrenheit, the atoms bump and crash into each other, merging to form simple molecules. Over millions of years, one relatively dense region of the cloud begins to collapse in on itself. An infant star takes shape at the center. In the surrounding areas, temperatures rise, molecules evaporate from their icy dust grains, and a new round of more intricate chemical reactions begins.

Then comes the most wondrous part of the whole tale. Those reactions weave the simple atoms of hydrogen, carbon, and oxygen into complex organic molecules. Such carbon-bearing compounds are the raw material for life—and they seem to emerge spontaneously, inexorably, in the enormous stretches between the stars. “The abundance of organics and their role in getting life started may make a big, big difference between a giant universe with a lot of life, and one with very little,” says Scott Sandford of NASA’s Ames Research Center in Moffett Field, California, who studies organic molecules from space.


The notion that the underlying chemistry of life could have begun in the far reaches of space, long before our planet even existed, used to be controversial, even comical. No longer. Recent observations show that nebulas throughout our galaxy are bursting with prebiotic molecules. Laboratory simulations demonstrate how intricate molecular reactions can occur efficiently even under exceedingly cold, dry, near-vacuum conditions. Most persuasively, we know for sure that organic chemicals from space could have landed on Earth in the past—because they are doing so right now. Detailed analysis of a meteorite that landed in Australia reveals that it is chock-full of prebiotic molecules.

Similar meteorites and comets would have blanketed Earth with organic chemicals from the time it was born about 4.5 billion years ago until the era when life appeared, a few hundred million years later. Maybe this is how Earth became a living world. Maybe the same thing has happened in many other places as well. “The processes that made these materials and dumped them on our planet are universal. They should happen anywhere you make stars and planets,” Sandford says.

The first persuasive hints of life’s possible cosmic ancestry came in 1953, courtesy of a renowned experiment devised by chemists Stanley Miller and Harold Urey. From studies of ancient rocks, geologists had a rough sense of our planet’s original chemical composition. Biologists, meanwhile, had uncovered the amazingly complex organic molecules that allow living cells to survive. Miller and Urey wanted to see if pure chemistry could help explain how the former transformed into the latter.

The two researchers prepared a closed system of glass flasks and tubes and injected a gaseous mixture of methane, ammonia, hydrogen, and water—four basic compounds thought to be abundant in Earth’s primitive atmosphere. Then Miller and Urey applied an electric current to simulate the energy unleashed by lightning strikes. Within a week their concoction had produced several intriguing prebiotic compounds. Many scientists interpreted this as hard experimental evidence that the building blocks of life could have emerged on Earth from nonbiological reactions.

In many ways, though, the experiment supported the opposite view. Even the simplest life forms incorporate two amazingly complex types of organic molecules: proteins and nucleic acids. Proteins perform the basic tasks of metabolism. Nucleic acids (specifically RNA and DNA) encode genetic information and pass it along from one generation to the next. Although the Miller-Urey experiment produced amino acids, the fundamental units of proteins, it never came close to manufacturing nucleobases, the molecular building blocks of DNA and RNA. Furthermore, it is likely that Miller and Urey erred by simulating Earth’s early atmosphere with gases containing hydrogen, which reacts easily, as opposed to carbon dioxide, a gas that is far less reactive but was probably far more plentiful at the time. “Interesting chemicals could not have been made as easily as the experiment made it seem,” says astrobiologist Douglas Whittet of Rensselaer Polytechnic Institute in upstate New York.