Symbiosis and Synthetic Biology
When I first began to hear the term astrobiology a few years ago, it sounded strangely discordant. And then another new discipline appeared that was even more of a stretch: synthetic biology. But this is how science progresses–by a kind of symbiosis between seemingly unrelated disciplines, in which traditional biology and chemistry become biochemistry, and biology and physics become biophysics. I began my career doing traditional biophysical studies on membranes, but now some of my research is funded by NASA’s astrobiology program, and many of our experiments could be described as synthetic biology: the application of engineering techniques to design or redesign biological functions and systems.
The field of synthetic biology is hot just now, because its methods are potentially very powerful. Synthetic biologists know enough about living systems to alter genetic programs in useful ways, the way expert computer programmers alter software. But what does such high-tech science have to do with volcanoes and the origin of life? Louis Pasteur once commented that chance favors the prepared mind; very often, even the most basic research produces an undreamed-of application. For example, one of the most powerful tools of molecular biology is the polymerase chain reaction (PCR), which is used to amplify DNA–that is, to make multiple copies of a given sequence. In PCR, cycles of heating and cooling combine with DNA synthesis by a polymerase, an enzyme that catalyzes the building of large molecules (polymers) from small molecules (monomers). Kary Mullis came up with the idea in 1983, first using a polymerase from ordinary E. coli bacteria, but a polymerase was needed that could survive near-boiling temperatures. In 1965–in completely unrelated research–Thomas Brock discovered a primitive bacterium, which he named Thermus aquaticus, living in the volcanic hot springs of Yellowstone National Park. This organism is the original source of the heat-resistant Taq polymerase now used in all commercial PCR devices.
If we follow Pasteur’s advice, we can increase the chances for more such serendipitous discoveries. In particular, we can prepare our minds by broadening the scope of synthetic biology to encompass studies of the origin of life. I will begin by describing nature’s version of synthetic biology; then I will show how our growing understanding of life’s molecular mechanisms suggests a way to reproduce the origin of life in the laboratory.
First Life: Synthetic Biology in the Wild
To take on the question of life’s origin, we need to have some idea of what Earth was like four billion years ago. There is good evidence that oceans were already present, predating life by several hundred million years. The oceans were salty, probably somewhat acidic, with volcanic land masses rising above sea level. Precipitation onto those islands produced freshwater ponds, so a marine environment is not the only one in which life could have begun. The atmosphere was a mixture of carbon dioxide and nitrogen, with little or no oxygen, and the average global temperature was 60 to 70 °C, much higher than today’s 15 °C. Thus the first forms of life probably resembled the thermophilic bacteria that inhabit hot springs today.
How could life begin in such an unpromising environment? Charles Darwin occasionally wondered about that, though he was too conservative to speculate in public about the origin of life. In a private letter to his friend Joseph Hooker, he wrote: “But if (and Oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.” And his great book On the Origin of Species touches on the question in a single sentence: “Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure, how, it has been asked, could the first steps in the advancement or differentiation of parts have arisen?”
Less eloquently, what would be required for the evolution of life to begin? First of all, evolution works on populations, not single organisms, so we need to find a way to generate large numbers of molecular systems in the prebiotic environment. Furthermore, there must be great variation in their properties. The requirement of variation within a population means that the first life forms capable of evolution could not be random mixtures of replicating molecules unable to assemble into discrete entities; instead, they would be systems of interacting molecules encapsulated in something like a cell.