The origin of life is one of the great outstanding mysteries of science. How did a non-living mixture of molecules transform themselves into a living organism? What sort of mechanism might be responsible?
A century and a half ago, Charles Darwin produced a convincing explanation for how life on Earth evolved from simple microbes to the complexity of the biosphere today, but he pointedly left out how life got started in the first place. “One might as well speculate about the origin of matter,” he said.
However, that did not stop generations of scientists from investigating the puzzle.
The problem is, whatever took place happened billions of years ago, and all traces long ago vanished — indeed, we may never have a blow-by-blow account of the process. Nevertheless we may still be able to answer the simpler question of whether life’s origin was a freak series of events that happened only once, or an almost inevitable outcome of intrinsically life-friendly laws. On that answer hinges the question of whether we are alone in the universe.
Most research into life’s murky origin has been carried out by chemists. They have tried a variety of approaches in their attempts to recreate the first steps on the road to life, but little progress has been made. Perhaps that is no surprise, given life’s stupendous complexity. Even the simplest bacterium is incomparably more complicated than any chemical brew ever studied.
However, a more fundamental obstacle stands in the way of attempts to cook up life in the chemistry lab. The language of chemistry simply does not mesh with that of biology. Chemistry is about substances and how they react, whereas biology appeals to concepts such as information and organization. Informational narratives permeate biology.
DNA is described as a genetic “database,” containing “instructions” on how to build an organism. The genetic “code” has to be “transcribed” and “translated” before it can act. And so on. If we cast the problem of life’s origin in computer jargon, attempts at chemical synthesis focus exclusively on the hardware — the chemical substrate of life — but ignore the software — the informational aspect. To explain how life began we need to understand how its unique management of information came about.
In the 1940s, the mathematician John von Neumann compared life to a mechanical constructor, and set out the logical structure required for a self-reproducing automaton to replicate both its hardware and software.
However, Von Neumann’s analysis remained a theoretical curiosity. Now a new perspective has emerged from the work of engineers, mathematicians and computer scientists, studying the way in which information flows through complex systems such as communication networks with feedback loops, logic modules and control processes. What is clear from their work is that the dynamics of information flow displays generic features that are independent of the specific hardware supporting the information.
Information theory has been extensively applied to biological systems at many levels, but rarely to the problem of how life actually began. Doing so opens up an entirely new perspective on the problem. Rather than the answer being buried in some baffling chemical transformation, the key to life’s origin lies instead with a transformation in the organization of information flow.
Sara Walker, a NASA astrobiologist working at Arizona State University, and I have proposed that the significant property of biological information is not its complexity, but the way it is organized hierarchically. In all physical systems there is a flow of information from the bottom upwards, in the sense that the components of a system serve to determine how the system as a whole behaves. Thus if a meteorologist wants to predict the weather, he may start with local information taken at various locations, and calculate how the weather system as a whole will change. In living organisms, this pattern of bottom-up information flow mingles with the inverse — top-down information flow — so that what happens at the local level can depend on the global environment, as well as vice versa.
To take a simple example: Whether a cell expresses a gene can depend on mechanical stresses or electric fields acting on the whole cell by its environment. Thus, a change in global information (a pattern of force) at the macroscopic level translates into a change in local information movement at the microscopic level (switching on a gene). More generally, a range of signals received from its environment help to dictate how a cell’s DNA is distributed and transcribed. Walker and I propose that the key transition on the road to life occurred when top-down information flow first predominated. Based on mathematical models, we think it may have happened suddenly, analogously to a heated gas abruptly bursting into flame.
There is a second distinctive way in which life handles information processing. The language of genes is digital, consisting of discrete bits, cast in the language of a four-letter alphabet. By contrast, chemical processes are continuous. Continuous variables can also process information — so-called analogue computers work that way — but less reliably than digital. Whatever chemical system spawned life, it had to feature a transition from analogue to digital.
The way life manages information involves a logical structure that differs fundamentally from mere complex chemistry. Therefore chemistry alone will not explain life’s origin, any more than a study of silicon, copper and plastic will explain how a computer works. Our work suggests that the answer will come from taking information seriously as a physical agency, with its own dynamics and causal relationships existing alongside those of the matter that embodies it — and that life’s origin can ultimately be explained by importing the language and concepts of biology into physics and chemistry, rather than the other way round.
Paul Davies is director of the Beyond Center for Fundamental Concepts in Science at Arizona State University.