My little series of posts (see here) on quantum biology was missing a review of Johnjoe McFadden’s book of a couple of years ago, Quantum Evolution. Below, I take a look at this speculative but well-written and detailed account of how quantum effects may be responsible for distinctive features of life and mind.
McFadden is a professor of molecular genetics who wrote this book for a popular audience back in 2000. Excerpts of the book appear here (evidently with the author’s permission). McFadden begins with a discussion of what defines life. He gives a brief history beginning with Aristotle and progressing through the triumphs of reductionist biochemistry over believers in vitalism. But after discussing the famously difficult problem of providing a precise definition of life, he concludes that “directed action” is a key notion. This is something analogous to the appearance of “will” in humans or higher animals. Moreover this directed action takes place all the way down to the microscopic level within organisms. Organisms are characterized by order via directed action at scales large and small (unsurprisingly, for a book on this subject, Erwin Schrödinger’s What is Life? is quoted several times, including is statement that life is “order from order”).
Prior to presenting the core arguments for quantum effects in life, McFadden reviews evolution and DNA replication. He presents the case that quantum-tunneling effects are one of the significant sources of mutation (in itself, I think this is generally accepted). He then discusses whether this could be responsible for some of the remaining challenges in understanding the workings of DNA evolution. He mentions the very controversial theory that adaptive mutations may occur at a frequency greater than chance. He will return to this subject later in the book.
Next is a discussion of the biggest mystery of biology, the origin of life. He discusses the inability of researchers to create primordial pre-cellular replicators in the laboratory. He reviews and criticizes some of the ideas on the origin of life that have been put forward: ideas from complexity theory; models of an ‘RNA’ world; and the invoking of the anthropic principle.
On his way toward providing his own answer, McFadden next takes a closer look at biochemistry, showing that as you drill down into particular biological functions you find they are driven by directed movements of individual protons or electrons via the electromagnetic force. This puts us squarely in the domain of physics.
So next comes a physics overview. He does a good job discussing thermodynamics and arguing why modeling biology in thermodynamic terms cannot tell the whole story. While order can emerge via energy flow in a thermodynamic context, this happens when random behavior at the micro-level leads to macro-level order. In biology, order exists all the way down to the atomic and sub-atomic realm.
Of course, the physical theory of the atomic and sub-atomic realm is quantum mechanics (QM). McFadden presents his own very readable summary of QM, leaning heavily on the two-slit experiment as a heuristic device. His strategy is to show that quantum measurements are happening at the micro-level in living systems. He gives an example of an enzyme action that ultimately depends on a single proton, which we know must be in a superposition of states absent measurement. So, a living system must be measuring itself. His view is that the classical world depends generally on continual measurement for its manifestation. This discussion leads to the next key tool McFadden wants to use: the quantum Zeno effect (and inverse Zeno effect). This, he speculates, is what is responsible for directed action at the micro-level.
With the review of QM in hand, he returns to a discussion of the origin of life and the question of how the first replicator was assembled (given the extreme improbability of it happening by chance). He theorizes that quantum superpositions could allow exploration of a large space of possibilities at the scale of an amino acid peptide chain. But the chances still seem small of making the self-replicator. However, harnessing the (inverse) Zeno effect could increase the probability. And, once you have a self-replicator, can we assume natural selection can do the rest of the job? No, there is still a big challenge here in getting a simple replicator to build the complex machinery of a cell. Moreover, in computer simulations, replicators tend to generate simpler systems, not more complex ones.
McFadden speculates that if a system on the edge of the classical frontier repeatedly fell back into quantum superposition and took advantage of the inverse quantum Zeno effect, this could have added complexity. Still, we haven’t been able to do anything like this in the lab.
And yet, the case seems relatively more compelling that non-trivial quantum effects are being exhibited in living cells (even if they are difficult or impossible to directly detect). To give credence to the existence of these effects one can estimate that decoherence times would be lengthy enough for them to occur in the relevant context. Also, important to note is that it is only coherent systems are sensitive enough to be affected by the weak electromagnetic fields which are known to exist in the cellular realm. McFadden concludes the quantum/classical barrier exists at the sub-cellular level of biology, and that organisms are comprised of “quantum cells”.
Getting back once again to the definition of life, McFadden says the cell’s ability to “capture” low entropy states to maintain order at the microscopic level via (internal) quantum measurements and the quantum Zeno effect is responsible for the distinctive directed action which characterizes life.
In the final chapters, McFadden first reprises the discussion of the role of quantum effects in DNA mutation and adaptive evolution. Then, he closes with a theory of how quantum effects in the brain may be linked to human will and consciousness. While structures in the brain (ion channels) are of the appropriate scale to invoke QM, the binding problem of how activities in the warm, wet brain would be correlated across large-scale neuronal assemblies is a problem. McFadden’s solution is that coherent quantum systems are coordinated by an electromagnetic field. Indeed, his model of the EM field as a solution to the binding problem can be decoupled from the quantum biology discussion. To save space in this post, let me refer the reader to this link for a review of this idea at Conscious Entities.
On the one hand, this book consists of speculation stacked on speculation. On the other hand, each step progresses from features of physics or biochemistry that we know to be true. Between the spheres of quantum physics and the human mind lies the world of biology: I continue to look for arguments and evidence that biological systems have features that can bridge these realms. This book was a fine effort along this line.