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.