A FIREBALL OVER AUSTRALIA

In the summer of 1981, a small black stone wrapped in aluminum foil changed the course of my life. About the size of a marble and indistinguishable from any other rock that might be found on a beach, this stone had traveled from southeastern Australia to NASA Ames Research Center in Mountain View, California, where researcher Sherwood Chang showed me the specimen and gave me a small sample to test. But the stone had traveled even farther. It was a piece of a meteor that had lit up the night sky over the town of Murchison, Australia, in September 1969. The fall began with a bright orange fireball and rolling thunder, followed minutes later by a shower of black stones strewn over five square miles. During the next few weeks, townspeople and scientists collected more than 100 kilograms of meteorites ranging in size from marbles to bricks.

Why would a marble-sized rock be so significant that it can change one's life? The reason is that before traveling 7,000 miles from Australia to California, this tiny bit of rock traveled to Earth from the asteroid belt between Mars and Jupiter, a distance of 250 million miles, or even billions of miles if we take into account all of the rock's orbits around the sun before reaching Earth. The rock was produced when a smaller asteroid happened to collide with a larger one, knocking off fragments of the surface and leaving behind a crater resembling those that we can see in photographs of asteroid surfaces (Figure 1).

I held in my hand a genuine rock from outer space. But there was more to this meteorite than just the mineral content typical of other stony meteorites. When the original boulder-sized object exploded over Murchison, the surfaces of the smaller fragments were heated by atmospheric friction to white-hot temperatures. In a few seconds, the friction slowed the fragments from an initial velocity of 20 kilometers per second, and they finally fell to the ground at the same speed they would reach if they were dropped from an airplane. The first stones to be discovered were still emitting a smoky smell from their hot surfaces, a distinctive aroma that I would later notice while extracting organic compounds from the meteorite. Whenever I give talks about this work, I evaporate a drop of Murchison extract into a wine glass and pass it around for audience members to sniff, joking that 4,570,000,000 BCE was a very good year.

That age, 4.57 billion years, is the reason why certain meteorites have changed scientific lives. The Murchison meteorite belongs to a relatively rare group of meteorites called carbonaceous chondrites. Their aroma is produced by organic compounds older than Earth itself, some of which were present in the vast molecular cloud of interstellar dust and gas that gave rise to our solar system 4.57 billion years ago. Most of the organic material—nearly 2% of the total mass of a typical Murchison sample—is in the form of a tarlike polymer called kerogen, but there are also hundreds of different compounds that sound like a chemist's laboratory: oily hydrocarbons, fluorescent polycyclic aromatic hydrocarbons (PAHs), organic acids, alcohols, ketones, ureas, purines, simple sugars, phosphonates, sulfonates, and the list goes on. Where did all this stuff come from? Did it have anything to do with the origin of life?

THE MURCHISON METEORITE AND SELF-ASSEMBLY

With a sample of a carbonaceous meteorite in hand, I was ready to do an experiment I had been dreaming about. Ten years earlier, shortly after the Murchison event, Keith Kvenvolden and a group of researchers at NASA Ames had analyzed a sample of the meteorite and convincingly demonstrated that amino acids, one of the essential organic compounds composing all life on Earth, were present in the meteorite. And these were not just the kinds of amino acids found on Earth (which might have been contamination), but more than 70 other kinds that were clearly alien to biology as we know it. This study, and many that followed, established that amino acids, the fundamental building blocks of proteins, can be synthesized by a nonbiological process. From this, it seems reasonable to think that amino acids, at least, would have been available on prebiotic Earth.

I had spent much of my earlier research career studying lipids, which, along with proteins, nucleic acids, and carbohydrates, represent the four major kinds of molecules that compose living organisms. "Lipid" is a catch-all word for compounds like fat, cholesterol, and lecithin that are soluble in organic solvents. In earlier research I had extracted triglycerides (fat) from the livers of rats, phospholipids such as lecithin from egg yolks, and chlorophyll from spinach leaves. All of these procedures used an organic solvent mixture of chloroform and methanol to dissolve the lipids, and I wanted to try the same thing with the Murchison material. The surface of the meteorite certainly had surface contamination from being exposed to the laboratory atmosphere, so I broke it into smaller pieces and carefully obtained an interior sample weighing about 1 gram. Then I ground the sample in a clean mortar and pestle with a mixture of chloroform and methanol as the solvent, and decanted the clear solvent from the heavier black mineral powder. The chloroform solvent had a yellow tint, which meant that it had dissolved some of the organic material in the meteorite. I dried a drop of the solution on a microscope slide, added water, and then examined it at 400× magnification. It was an extraordinary sight. Lipidlike molecules had been extracted from the meteorite and were assembling into cell-sized membranous vesicles resembling microscopic soap bubbles (Figure 2). Could it be that similar compartments were present when the first liquid water appeared on Earth more than four billion years ago? Maybe, just maybe, if we studied the Murchison meteorite we might know what kinds of molecules made up the membranous boundaries of the first cellular life.

But a huge question remained: Where did the stuff come from? For that matter, where does anything come from? To set the stage for telling that story, let's begin with stars, where everything begins.

WHERE DOES EVERYTHING COME FROM? THE LIFE AND DEATH OF STARS

Forty years ago, when a boulder-sized meteorite blazed through the skies above Murchison, Australia, we had only a few speculations about when our universe began and how galaxies, stars, and planets come to be. Now, within a single lifetime, we have definite answers to fundamental questions that have been pondered throughout recorded history. The...