When Charles Darwin published On the Origin of Species in 1859, he gave a convincing account of how life has evolved over billions of years from simple microbes to the complexity of the Earth's biosphere today. But he pointedly left out how life got started. One might as well speculate about the origin of matter, he quipped. Today scientists have a good idea of how matter originated in the big bang, but the origin of life remains shrouded in mystery.
Although Darwin refused to be drawn on how life began, he conjectured in a letter to a friend about "a warm little pond" in which various substances would accumulate. Driven by the energy of sunlight, these chemicals might become increasingly complex, until a living cell formed spontaneously. Darwin's idle speculation became the basis of the "primordial soup" theory of biogenesis, and was adopted by researchers eager to recreate the crucial steps in the laboratory. But this approach hasn't got very far. The problem is that even the simplest known organism is incredibly complex. Textbooks vaguely describe the pathway from non-living chemicals to primitive life in terms of some unspecified "molecular self-assembly".
The problem lies with 19th-century thinking, when life was regarded as some sort of magic matter, fostering the belief that it could be cooked up in a test tube if only one knew the recipe. Today many scientists view the living cell as a type of supercomputer - an information-processing and replicating system of extraordinary fidelity. DNA is a database, and a complex encrypted algorithm converts its instructions into molecular products.
Viewed this way, the problem of life's origin is switched from hardware to software. The game of life is about replicating information. Throw in variation and selection, and the great Darwinian experiment can begin. The bits of information have to be physically embodied in matter somehow, but the actual stuff of life is of secondary importance. There is no reason to suppose the original information was attached to anything like the highly customised and evolved molecules found in today's living cells.
The rapid convergence of nanotechnology, biotechnology and computer technology has opened up new possibilities for processing information on ever-smaller scales. The goal of this race to the bottom is quantum computation, in which information is attached to atomic/ subatomic states and manipulated using the rules of quantum physics. If life is formed by trial and error, speed is the key. This suggests life may have emerged from the quantum realm directly, without the need for chemical complexity.
All it takes to get life started is a quantum replicator - a process that clones bits of information attached to quantum systems by allowing them to interact with other quantum systems in a specific way. The actual system could be anything at all - the spin of an electron, a meta-stable atomic state, or a molecule that can flip between two conformations. The uncertainty inherent in quantum mechanics provides an in-built mechanism for generating variations.
How, then, did life arise? We can gain a clue from modern computers. Quantum systems may be fast, but they are very fragile. Computers routinely transfer important data for safekeeping from speedy yet vulnerable microchips to slow and bulky hard disks or CDs. Perhaps quantum life began using large organic molecules for more stable data storage. At some stage these complex molecules took on a life of their own, trading speed for robustness and versatility. The way then lay open for hardy chemical life to go forth and inherit the Earth.
· Paul Davies is a physicist at the Australian Centre for Astrobiology and the author of The Origin of Life