In the summer of 1981, a colleague at the NASA Ames Research Center in Mountainview, California, gave me a small black stone wrapped in aluminum foil that 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, the stone was a piece of a meteorite that entered the Earth’s atmosphere over southern Australia in September 1969. It began with a bright fireball that broke up into several pieces, followed by claps of thunder, and minutes later a shower of black stones fell over several square miles near the small town of Murchison.
When the original boulder-sized object broke apart into smaller pieces, the surfaces of the fragments were heated by atmospheric friction to white-hot temperatures. In a few seconds the friction slowed them from their initial velocity and they finally fell to the ground at the same speed they would reach if they were dropped from an airplane. When the first stones were found they were still warm and had a smoky, aromatic smell. During the next few weeks townspeople and scientists collected over a hundred kilograms of meteorites ranging in size from marbles to bricks.
The smoky aroma hinted that there was more to this meteorite than just the mineral content typical of other stony meteorites. The Murchison belongs to a relatively rare group of meteorites called carbonaceous chondrites. The odor is produced by organic compounds older than the 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 the Murchison, is in the form of a coal-like polymer called kerogen, but there are also hundreds of different compounds that sound like a chemist’s laboratory: oily hydrocarbons, fluorescent polycyclic aromatic hydrocarbons (PAH), 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?
In 1953, a young graduate student named Stanley Miller published a short paper in Science that suggested an answer. Miller’s mentor was Nobelist Harold Urey at the University of Chicago, who had proposed that the early Earth’s atmosphere was likely to be a mixture of hydrogen, methane, ammonia and water. This is called a reducing atmosphere, with no free oxygen.
Miller decided to expose such an atmosphere to an electrical discharge to simulate lightning. The result was astonishing. because several amino acids were produced, along with hundreds of other compounds that resembled the mix of organic compounds in the Murchison. And then sixteen years later, Keith Kvenvolden and a group of researchers at NASA Ames analyzed a sample of the Murchison meteorite and convincingly demonstrated to everyone’s satisfaction that amino acids were present among the organic compounds. Not just the amino acids associated with life on the Earth, which might have been contamination, but more than 70 others that were clearly alien to biology as we know it. This study confirmed Stanley Miller’s conclusion that amino acids, the fundamental building blocks of proteins, can be synthesized by a non-biological process.
From this, it seemed reasonable to think that amino acids and other organic compounds would have been available on the prebiotic Earth, either delivered by meteoritic infall or synthesized by geochemical processing of atmospheric gases.
With a sample of a carbonaceous meteorite in hand, I was ready to do an experiment I had been dreaming about. 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 surfaces of the meteorite stone had surface contamination from being exposed to sheep pastures in Australia and the fingers of everyone who handled it, so I broke it into smaller pieces and carefully obtained an interior sample weighing about one gram. I pulverized 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, then examined it at 400X magnification. It was an extraordinary sight. Lipid-like molecules had been extracted from the meteorite and were assembling into the cell-sized membranous vesicles.
Could it be that similar compartments were present when the first liquid water appeared on the Earth over 4 billion years ago?
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?
In next week’s column I will describe a remarkable tale of stellar nucleosynthesis, scientific insight, and a missed Nobel prize.
I want to thank Bente Lilja, Patrick, Gerhard, and Michael for their thoughtful comments regarding the previous two columns. I would like to respond individually, but like most academics I must reply to a hundred or so emails during the work day. I am gathering them as we go along and once a month will set aside some column space to answer as best I can. Now, back to my story.