A laboratory, of sorts: The author sampling boiling fumaroles in the crater of Mt. Mutnowski, in Kamchatka, Russia
The systems would have to exhibit the two primary functions of life: growth and reproduction. Cells grow by taking in nutrients–simple molecules from the environment. They use energy to link those molecules into the polymers that we call proteins and nucleic acids. Reproduction requires a mechanism by which genetic information can be stored and then replicated, so that the information, in the form of genes, can be passed on. But the transfer of information is necessarily imperfect. A certain number of errors–mutations–must occur to produce variations in the population such as those that enabled primitive life to explore different niches and begin evolving toward the magnificent biosphere of today’s Earth.
We’re talking about forms of life much simpler than even the most primitive bacteria that exist now. Still, how could cells of any kind spontaneously appear out of random mixtures of simple organic molecules? The prospect is so mind-boggling that a few scientists state flatly that we will never understand how it came about. I’m more optimistic. But attempting to discover how life began is hard work, with no certainty that we will ever find answers. We need to formulate and test hypotheses, and be willing to venture into vast unexplored territory. I will briefly describe some of the milestones on this journey. As we acknowledge them, we can begin to assemble a logical cage that constrains speculation and guides us toward answers.
First milestone: A Source of Organic Monomers
The four basic varieties of biomolecules are amino acids and proteins, carbohydrates, nucleic acids, and lipids. There is little doubt that similar–even identical–organic compounds were present in the prebiotic environment. That was the conclusion of Stanley Miller’s famous experiment in the early 1950s in which he exposed mixtures of ordinary gases to electrical discharges and observed the formation of amino acids. Since that time, virtually all the primary carbon compounds of life have been synthesized in prebiotic simulations.
The results of simulation studies were strongly supported when a remarkable meteorite fell to Earth near Murchison, Australia, in September 1969. It was clear that the meteorite contained organic material, because a strange smell emerged from the hot surfaces. Forty years later, when I grind Murchison samples in the lab, the same odor wafts up from the mortar–simultaneously dusty, oily, and sour. This is an ancient odor, older than Earth itself, preserved for five billion years in a comet or asteroid from which the original meteorite was derived.
There are thousands of organic compounds present in the Murchison meteorite and other carbonaceous meteorites that have been analyzed–confirming Miller’s experiment by showing that biologically relevant organic compounds are in fact produced by nonbiological processes. This makes it plausible that organic compounds were present on prebiotic Earth, either synthesized by geochemical processes or delivered as meteoritic and cometary infall more than four billion years ago.
Second Milestone: Self-Assembly of Compartments and Protocells
The unit of all life today is the cell. My research background is in membrane biophysics, and I began my career by studying the lipid membranes that are the essential boundaries defining living cells. Twenty years ago, when I obtained a golf-ball-size stone from the Murchison meteorite, I wanted to see whether anything resembling lipids was present in the mix of meteoritic organics, perhaps shedding light on how life became cellular.
In earlier research I had often used a mixture of chloroform and methanol to extract lipids from a variety of biological materials, such as red blood cells, chloroplasts, mitochondria, and even egg yolks–the last being a rich source of a phospholipid called lecithin. And in 1975 I had spent a sabbatical leave in the lab of Alec Bangham, who discovered in the 1960s, working at a research institute near Cambridge, England, that lecithin could spontaneously self-assemble into membranous sacs, or vesicles, that have come to be called liposomes. I now employed the chloroform-methanol mixture to isolate compounds from samples of the Murchison stone, then used a chromatographic procedure to purify those that might be capable of self-assembly into membranes. The left panel in the figure on page 71 shows what happened when a few micrograms of the extract were dried on a microscope slide and exposed to water to simulate the cycles of wetting and drying that would have been frequent on the early Earth. The results were very exciting. Not only were lipidlike molecules present in the mix, but they readily self-assembled into cell-size vesicles.