Through hot and cold: the survival of the toughest -Thinking out loud

“One general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die.”

― Charles Darwin, The Origin of Species

The evolutionary theory first proposed by Darwin has served as a thinking tool that has revolutionized the way scientists view the way organisms change over time. The theory of evolution is so versatile with its predictive and explanatory capabilities it has been adapted to fields ranging from social sciences to geology. This versatility or adaptability of evolutionary theory has captured the imagination of many brilliant thinkers starting with Darwin and Wallace, Erwin Schrodinger, and Richard Dawkins to name just a few. My thinking was completely opened wide when I listened to an online lecture series by Dr. Robert Hazen on biogeochemistry. The fundamental concept was that evolutionary thinking including Darwinian evolution could be viewed as a just portion of forward feedback loops in fundamental physical, chemical, and ultimately biogeochemical processes.

The evolutionary process can be considered as a part of a forward feedback loop. The other part is the process of adaptation, interactions that produce environment-dependent change. Forward feedback loops are processes that repeatedly cycle to transform the objects involved in a constructive process. The main concept is that these loops make things that are differentially stable. When loop products interact, the stable product is the one that retains most of its relative structure and persists through the interaction. We see this play out in nature as dominant and recessive genes where recessive gene loses most of their relative ability to produce gene products that impact the functioning of the organism. Thus feedback loops that produce products of differential persistence subject them to Darwinian-like processes where there is a "struggle" for persistence and each "struggle" produce a different product that is fed into the next feedback loop.

The curious point is that each "struggle" is also a loop that combines two products into new products at the end of the loop. The first key point in this model of feedback loops is that the products of multiple feedback loops "compete" or interact with each other. The second key point is that it must be iterative, if the feedback loop or all feedback loops are terminated, the corresponding adaptive/evolutionary process is also terminated. The continuity of feedback loops and their products through time is the fundamental characteristic of evolutionary processes.

The positive feedback loops of interest here are ubiquitous throughout our world and can be described through mathematical expressions (far beyond my mathematical skills). However, what is the fundamental nature of these positive feedback loops? The simplified answer is that these loops can all be reduced to cycles of high movement to low movement. Effectively cycles between high temperatures and low temperature or heating and cooling. This builds off the fundamental physical concept that heat is movement, or that movement can be viewed in terms of heat (measured as a temperature). The strength in this line of thought is that while Darwinian evolution is currently restricted to systems of molecular inheritance such as DNA, forward feedback loops by definition build off the products of their previous loop which is a form of inheritance. Thus there is no "need" for inheritance within this model because it is built into the mathematical definition of forward feedback loops. The idea of positive feedback loops being the backbone of evolutionary thinking is not new. However, identifying the essential/irreducible characteristics that are involved within these feedback loops has not been fully articulated in the field of evolutionary thinking.

Evolution is driven by a selection of natural variation. Why is heating and cooling the essential characteristic of forward feedback loops in evolutionary processes?

Heating and cooling is the only physical characteristic that captures all systems from subatomic particles to social networks. Heat/movement can also embody variation, or chaos depending on the context. Before we can apply this modified evolutionary theory to all complex systems, we need to confirm that it still functions in classical biological contexts. The book "What is Life?" by Erwin Schrödinger provides a fundamental perspective into the physics of heating and cooling in regard to biological systems. "Natural variation" describes variation in DNA code and the following change in organism function that occurs from high energy (heating) events that result in breaks within the DNA's structure. In this example DNA code experiences a high energy (heating) event that breaks its interactions with neighboring units of code, with its bonds broken, it flops around, moving more freely. These DNA breaks are then fixed through events where DNA is either capped or joined together with neighboring codes. This restricts its relative movement and can be considered a form of cooling. With energy transfer, DNA was heated and thus increased the amount of the DNA code's physical movement. It was "free" to interact with many other molecules including proteins that facilitated its repair. The repair is a cooling event, where the movement or interaction of the DNA code is being restricted, however, depending on how the repair proceeds, the repair may result in altered or removed DNA code. This is called a mutation event, where DNA is "heated" and the repair process "cools" the system in a way that code is altered. This is a general summation of how mutations occur from a physical context. The natural variation caused by DNA mutations that the modern synthesis of Darwinian evolution relies on is the product of heating and cooling the molecules that make up the DNA code.

The second example of heating and cooling will be in regard to organism development. We often say that babies or children "can be anything when they grow up". This sentimental statement captures a concept that at birth or early childhood a child possesses the ability to take up whatever role or profession in society that they want to. We also recognize that as the child gets older that this becomes less and less likely. A tone-deaf child with excellent communication skills is more likely to become a manager of a fortune 500 HR department than the next Beyoncé. And a child with limited mathematical training and a killer throwing arm is more likely to become a pitcher for the New York Yankees than an engineer that works on particle accelerators. However, this line of thinking can extend back to the beginning of the child. When an egg is fertilized the DNA code from the mother and the father are recombined. This is a high-energy event where DNA code from each parent can be swapped and mixed between chromosomes (large continuous bundles of DNA code). Once this stops, the DNA molecules will not do this again (at this frequency) until they produce their own reproductive cells later in life. In this sense, the set of DNA code (a chromosome) cools down because it no longer has rampant swapping and mixing. Thus chromosomes mix/swap (heat) and cool (integrate) DNA code resulting in a cycle of chromosomal heating a cooling coincident with the beginning of each new generation of life.

This concept of heating and particularly cooling chromosomes can be extended into development. As the cells divide and start to form the parts that will eventually become the embryo they tightly pack and seal away more and more DNA code. Some of this code will never be used again in the child's life. Importantly, some cells start to specialize, by activating and/or sealing away different parts of DNA code. They maintain specific sets of DNA code while the bulk majority of the DNA code that was readable is now sealed away. The use of DNA code requires the movement of proteins that facilitate code reading and interaction of the short-term messenger code. The more DNA code is read/used the more it moves. Thus DNA code could be considered to have a temperature in regard to its relative movements based on how often the DNA code is read. As the embryo develops, more and more general code is sealed away and specific code is activated. This continues on until most of the cells in the body have their final sets of instructions. From here on out, they will only operate using a limited set of DNA code to do specific functions. Even the legendary stem cells only possess the ability to become a few different cells with a few different functions. Important to this concept of cooling, cells (even stem cells) no longer have the ability to unseal most of the DNA code that has been packed away. All in all the DNA code is cooled and each set of cells are only able to do a limited set of functions. DNA code now has on average, less movement in each cell across the body. The system has cooled. This cooling process will continue slowly throughout the child's life and continue into adulthood. In this framework, old-age-associated disease and death can be considered as the system cooling so much that things can't even move enough to repair broken structures.

These two examples consider the process of cooling in both DNA code and the chromosomes/cells/organism. While it is simple to see cooling as a linear process, it is not, and we must remember that as the next generation is made, the system is reheated again. While as individual humans we may be inexorably headed towards a frigid end, we give the spark of life to the next generation. Thus as a system of interconnected people, we go through repeated events of heating followed by cooling. This is the Darwinian evolutionary theory put in the context of heating and cooling.

Footnote: Much of this could have been written in the context of entropy. However, conceptually even scientists have difficulty grasping the definition of entropy. Heating and cooling or high energy and low energy states are much more simple to understand than the "number of microscopic states" a molecule can have. To further confuse many scientists, biology is about reducing entropy in the system by exporting entropy into the environment. Thus we would have to continually reference what scale is being described and whether we are referring to the environment & organism or just the individual organism.

Disclaimer: These ideas are not entirely new. They have been iteratively processed throughout the scientific community first by physicists who have made forays into biology and developmental biologists who view development as the establishment of patterns based on cooling of the system. We are not a random pool of molecules, we just haven't quite figured out the all governing principles of why we are what we are.