What is Life?
Can quantum biology help us to understand what distinguishes a bunch of molecules from a living organism?
In the beginning, during the first billion years (Gyr) after the Earth was formed about 4.5 Gyr ago, intense meteor bombardment left little remains of the original crustal rocks. High energy collisions with meteorites up to 500 km in diameter periodically disintegrated newly formed continental crust and vapourised early oceans, annihilating primitive life forms possibly existing near the surface.
Gene sequencing suggests that the most primitive known domains of life, namely bacteria and archaea, have been evolving separately for as long as 4 Gyr. Deep-sea volcanic vents are candidates for supporting such early life. The oldest fossils of single-celled microbes are dated at around 3.5 Gyr, while the oldest evidence of the more complex multicellular precursors of plants and animals is more recent, at around 2 Gyr. Structurally, it appears that this complexity in fact arose as the happy endosymbiotic result of the invasion of the archaean cell by bacteria!
In the meantime, life has become so prolific and so complex that human life forms have taken up the study of life and its origins. But are we any closer to an answer? Can we state exactly what it is that distinguishes a bunch of molecules from a living organism?
An intriguing experiment by Miller and Urey in 1953 showed that electrical activity in a gaseous mixture of methane, ammonia, water and hydrogen can produce amino acids: the building blocks of proteins. In spite of more recent and remarkable work in viable DNA design, no experiment has yet been able to synthesise from basic components an object that has the characteristics of a living cell.
All living systems are made up of molecules, and the properties of molecules are given by quantum mechanics, our most successful and fundamental theory to date. Living systems are necessarily open systems constantly exchanging energy and matter with the environment in order to maintain the non-equilibrium state synonymous with living. While living systems are therefore fundamentally open quantum systems, the level of complexity typical of biological systems poses a huge computational challenge to such a fundamental description. Furthermore, many of the processes associated with life are sufficiently described by Newtonian physics.
Quantum biology is the applied science of open quantum systems to those aspects of biology where a description in terms of Newtonian physics is insufficient. An important question is whether quantum theory can add anything to biology: We know that molecules are ultimately described by quantum chemistry, but can such a description help us to understand life itself?
The most well-established area in quantum biology is the study of aspects of one of life's oldest processes: photosynthesis. While evidence of quantum mechanical tunneling in electron transfer in purple bacteria was first reported almost half a century ago, more recently the detection of quantum coherence in energy transfer in green sulphur bacteria and marine algae has contributed to a revival of interest in the possibility that the optimality of some biological processes is due to a sustenance of quantum effects in the warm, wet and noisy environments typical of living systems.
As theorists, we are working hard to keep up. Our research in Durban, with collaborators in Singapore and Amsterdam, has involved the application of open quantum systems models of energy transfer to the photosynthetic process, showing how interaction with an environment can in fact enhance transport efficiency. More recently, we have proposed that quantum spin plays a direct role in reducing the yield of potentially destructive statesduring charge transfer in photosynthesis, constituting a new example of a quantum mechanical protective mechanism in a living organism.
Given that the simplest living systems exhibit functional complexity of a quantum nature when probed at the limits of our instrumentation, that far more complex animals are able to sense subtle changes in their environments with an accuracy described by quantum mechanics, should come as no surprise. The proposal that navigation in the Earth's magnetic field, as well as our senses of vision and smell, and also our cognition, require quantum mechanical description, are exciting developing areas of quantum biology.
The highest achievement of quantum biology would be a contribution to a scientific understanding of what distinguishes a living system from the inanimate matter from which it is constructed, i.e. a theory of life. The test of such a theory would be the synthesis of life itself. In the absence of such a theory and its confirmation, outside of famous works of fiction, quantum biology will, for now, have to fulfill a more practical role.
The primary importance of the field of quantum biology, in its present state, lies in the identification and mimicry of the ingenious feats of engineering taking place in systems ranging from bacteria to birds. If non-trivial quantum effects on a macroscopic scale play a role in getting the job done better in certain processes perfected over billions of years at physiological temperatures and in immensely complex systems, then there exists before our very eyes a wealth of information in the biological world from which to draw inspiration for our own technologies.
Synthetic biology is gathering momentum to become the next big thing in science, with biologically-inspired quantum artificial photosynthetic systemspromising to contribute to the development of the kind of renewable energy technologies essential for our continued existence on this planet (and perhaps others!), and this is just the beginning.
As far as understanding what life is, however, we are limited by a lack of precise knowledge of the conditions under which life emerged on Earth, in a possibly singular event. Barring the sudden discovery of evidence of life on Mars by the Curiosity rover or a roving Mars One colonist, for now, we will have to be satisfied with a definition of life as the continual state of change preceding death, and with the knowledge that the rabbit hole goes as least as deep as we are prepared to venture.