A new look at circadian clocks, cells and cognition

A botanist and a psychologist have put forward a highly novel theory of the circadian clock based on the integration of bioelectric time-sensing mechanisms in individual cells and parts of cells. 

A circadian clock, or circadian rhythm, refers to innate cycles observed in a broad array of organisms from bacteria to humans. These cycles seem to correspond to temporal cues, including the 24-hour cycle of the Earth’s rotation, as well as the lunar motion and the yearly revolution of the Earth. 

The new theory of circadian clocks was published Aug. 11 in the online journal BioEssays, written by František Baluška of the Institute of Cellular and Molecular Botany at the University of Bonn (Germany) and Arthur S. Reber of the Department of Psychology at the University of British Columbia (Canada).

At the root of the new theory is the authors’ earlier stated belief that sentience or consciousness was not a later development in evolutionary history, but had to be present at the very beginning of life, with the emergence of “the first autonomous self-replicating cell.” 

The most important feature at this early phase of cellular evolution, the authors argue, was the plasma membrane, “separating the outside (non-life) from the inside (life).”

They call this theory the Cellular Basis of Consciousness (CBC). 

“Besides allowing ordering of molecules (acting against the second law of thermodynamics) into biological macromolecules, the plasma membrane also represents a smart sensory border capable of handling energy and ion fluxes which enable its bioelectrical excitability.” 

Proceeding from this foundation, Baluska and Reber propose that these early cells must have possessed some means of recording and responding to external cues in order to survive. 

“Awareness of diverse cues from their environments was a central feature allowing the ancient vesicle-like proto-cells to evolve slowly into the first self-replicating and autonomous cells some 3.7 billion years ago,” they write.

In the more advanced eukaryotic cells, the authors posit, the circadian clocks of the simpler bacterial cells (such as the blue-green algae of plants and the mitochondria of animals) become “coordinated and integrated into the holobiont-like circadian clock of the whole eukaryotic cell.” 

Their view here is premised on the endosymbiotic theory of cell evolution, first put forth by biologist Lynn Margulis, in which the eukaryotes are a symbiotic combination of a prokaryote (bacterial) and a nucleated cell. She coined the term holobiont to indicate the combined organisms formed by symbiosis.

In multi-celled organisms, the unit prokaryote and eukaryote clocks then become integrated by a hypothesized whole-body mechanism. The authors cite recent studies of circadian rhythms in cyanobacterial cells, regulation of glucose metabolism in human red blood cells, and circadian oscillations of mitochondrial metabolism as suggestive models for how this might take place. 

The phenomenon known as reactive oxygen species (ROS) homeostasis is also considered. “Bioelectricity phenomena, based on redox homeostasis associated electron transfers in membranes and within protein complexes inserted in excitable membranes” are invoked as possible coordinating mechanisms for the supra-clocks. 

In the concluding portion of the essay the authors take issue with philosopher Daniel Dennett, a proponent of a strict adaptionist interpretation of Darwinism, also known as an outspoken opponent of creationist theory. Specifically challenging Dennett’s arguments against cellular sentience made in his 2017 book, From Bacteria to Bach and Back: The Evolution of Mind, they write:

“In his recent book Dennett, in an effort to argue against this fundamental principle, distinguished between ‘competence’ and ‘comprehension.’” 

“Primitive species,” they write, “including the unicellular bacteria of the book's title, have the former but lack the latter. They function effectively but do so without any subjective experiences, without sentience and without any internal representations of their actions.”

However, the authors argue, “Such a framework, despite Dennett's creative efforts, makes little sense.”

“A prokaryote without sentience,” they write, “one lacking valenced perceptions, one bereft of organic clocks, one that did not know which molecules to admit and which to block, would have been a Darwinian dead-end.”