Approximately five hundred years ago, the North African polymath, Ibn-Khaldun, observed that “the goal of civilization is sedentary culture and luxury. When civilization reaches that goal, it turns towards corruption and starts being senile, as happens in the natural life of living beings.” The analogous natural history of socially constructed civilizations and the biologically constructed individuals that comprise them might have been purely imaginative five hundred years ago, but in retrospect, perspicacious. As Geoffrey West describes in Scale, there appear to be a common conceptual framework underlying phenomena ranging from biological organisms to corporations and cities. The process from birth to growth and peak and then to decline and death are subject to generic laws and it is because of these laws, that we see a consistent pattern across such varied and outwardly unrelated phenomena. “Birth, growth, and death are all governed by the same underlying principles driven by metabolic rate and encapsulated by the dynamics and structure of networks.” (Geoffrey West)
The term ontogenesis describes the developmental process that occurs within an individual during growth that starts with fertilization, through birth, and up to maturity. An organism requires a continuous supply of increasing metabolic energy that broadly speaking, must be allocated between repair and maintenance on the one hand, and growth on the other hand. Due to network structure and dynamics that were molded over millions of years of natural selection, as an organism increases in size, energy supply becomes more efficient. Thereby larger organisms require less energy per unit of size. For every four orders of magnitude of increase in mass, metabolism increases only by three (3/4 exponent). This relationship is called Kleiber’s Law or more generally known as sublinear scaling and economies of scale. Like most systems of optimization, sublinear scaling comes with tradeoffs. Although energy supply becomes more efficient, it serves as a constraint to growth. The rate at which energy is needed for maintenance and repair of existing cells increases faster than the rate at which metabolic energy can be supplied.
Once organisms have achieved their peak growth, the next steps in their natural history is death and its largest risk factor – senescence. At a high level, this process of aging and death follow a predetermined pattern as well. On average, the aging phenotype not only follows a linear decline but surprisingly (and maybe depressingly) begins almost immediately after maturity at the age of twenty. Measures of organ function such as vital capacity and glomerular filtration rate all begin a gradual and linear down-sloping. The linearity is due to the fact that on average, damage is a function of time and associated with the transportation of metabolic energy through the networks servicing cells, mitochondria, respiratory complexes, and genomes. As the flow of cars and trucks causes a gradual wear and tear on the roads, the flow of metabolism in our body networks relentlessly degrades the body. On average, tissue function declines gradually over many decades and eventually falls below the threshold required for normal function. We become progressively less robust and unable to maintain functionality in the face of change or disturbance. This is borne in the data as survival curves show an exponential increase in disease and mortality beginning at the age of sixty.
Over millions of years of evolution, natural selection has led to minimizing energy loss and maximizing metabolic capacity by maximizing surface areas across through which resources and energy are transported. Within these dynamics lie the relatively standardized patterns of growth, aging, and death across biological and social paradigms. Escaping these patterns has been the obsession of humanity for time immemorial. Dreams of immortality, goals of a longer life, and objectives of a healthier life fascinate, inspire, and preoccupy us. However, Russian playwright, Anton Chekhov captured the essence of the second law of thermodynamics in his statement – “only entropy comes easy.” While we might or might not reach the escape velocity to break free of the bonds of entropy, what Ibn Khaldun failed to realize is that “sedentary culture and luxury” is not a binary choice or a monolithic goal, but one that is continuous and heterogeneous. Robustness and hormesis might hold the key to to delaying senility and corruption that accompanies with aging. [Next Essay]