Notes on Engineering Health, September 2022

Nature, Nurture, and Good Luck

“One of the deepest, one of the most general functions of living organisms is to look ahead, to produce future,” according to the late François Jacob (citing Paul Valéry).

The role of scientists, and biologists in particular, has been to describe possible futures, predict the likeliest path to get there (through deterministic models, for example), confront the reality of where those paths actually lead, then revise their hypotheses and carry on. Amidst the array of what is possible, some events are necessary, and some are contingent. Jacques Monod, in his essay Chance & Necessity, argued that any commitment to objectivity requires a central role being accorded to chance. The ability to establish how much of the observable world is deterministic and how much comes down to chance — either through quantum indeterminacy or stochastic uncertainty — via well-designed experiments should allow us to describe the world not through averaging large numbers across some distribution, but to understand what makes individual events, cells, organisms unique. 

The study of individual behavior is one where there should be a particular emphasis on going beyond simple averages and distributions to understand what underpins the sense of individuality. The age-old debate between nature (the contribution of genes) and nurture (the environment, parenting, food, etc.) seems to have missed an influential factor, though: noise. It is well known that individual animals with identical genomes, reared identically, nevertheless exhibit behavioral differences. Some recent research suggests the role of this randomness in shaping who we are could be far more significant than measurable environmental factors and, in some cases, as much as genetic ones. To understand the contributions of genes and environments, scientists typically rely on two kinds of studies:

1. The comparison of identical twins with non-identical twins. In each case, upbringing is assumed to be similar, but identical twins share nearly all of their DNA while non-identical twins share only half; and 
2. The comparison of pairs of biological siblings with pairs of siblings where one was adopted at birth: once again, similar upbringing (to some extent), but half the DNA is shared in one case and none in the other.

Decades of studies concluded that DNA generally accounts for about half of the difference between individuals. If the other half is attributable to a vaguely described “environment,” understanding better what this black box entails is essential. The de Bivort lab at Harvard University, which seeks to understand the role of the environment on phenotypic outcomes, subdivided non-heritable effects into those which can be predicted from measurable variables and those that cannot. These latter effects are stochastic. Using Drosophila melanogaster as a model organism, the lab demonstrated that genetically identical animals raised in conditions that are as similar as possible could have slightly different brain wiring. For instance, they found that fruit flies have an innate, long-lasting preference for turning to the left or right while walking around, controlled by the wiring pattern of brain cells called columnar neurons. Fly offspring didn’t inherit this tendency from their parents. They concluded that the cause was intrinsic random events at the cellular level. The low number of cells involved in these processes and a well-furnished genetic toolkit (including neuronal silencing/activation, gene over-expression/silencing, optophysiological recording) allowed the group to identify what stochastic events influenced cell fate and gave the organism its idiosyncratic biases. To further understand how infinitely minor differences in gene expression between two cells can lead to significant phenotypic differences, new tools were developed to assess the role of feedback loops in stochastic fluctuation and its amplification. 

Saying that each individual is unique sounds like an unnecessary truism, but studying stochastic factors leading to such diversity highlights the unpredictable nature of nature. And hopefully over time will lead to better understanding of why people behave the way they do. 

– Jonathan Friedlander, PhD & Geoffrey W. Smith