Evolution Makes Us Flexible Because Life Is Unpredictable

“Unbending rigor is the mate of death/ And wielding softness the company of life: Unbending soldiers get no victories/ The stiffest tree is readiest for the axe.” (Tao Te Ching: 76)“Genes, unlike gods, are conditional. They are exquisitely good at simple if-then logic: if in a certain environment, then develop in a certain way… So here is the first moral of the tale: Don’t be frightened of genes. They are not gods; they are cogs.” (Matt Ridley, 2003: 250)Part 1 of this essay looked at some of the ways that people have thought about nature and nurture in the past, underscoring some reasons to be skeptical of the notion that we are hard-wired in our biology and behavior. One problem with this topic is that we often remain stuck in dichotomous “nature versus nurture” thinking. Some of us lean more toward one side than another, but as biologist Anne Fausto-Sterling warned, that view is too simplistic. There is no seesaw between nature and nurture (and there is no spoon).[quoted]Most people have only one model to think about how human traits develop. Whether they call themselves determinists or constructionists, nativists or empiricists; whether they emphasize genes or environment, nature or nurture, or think of important traits as hardwired or plastic, they envision a seesaw. Sometimes nature (genes, wiring, etc.) is heavier, and the seesaw bounces onto nature’s ground. Sometimes the environment (culture, nurture) is heavier and weighs down that end of the seesaw. Frustratingly, even though a process approach to understanding motor development is pretty much settled science, in most other arenas (sex differences, intelligence, athletic and musical ability, and more), people can’t stop arguing—adding a little weight to one side or there to the other—and the seesaw teeters back and forth with each new research finding.[/quoted][quoted]But there is no seesaw. Human behaviors are not “things” more or less influenced by one or another source. They are processes that are simultaneously embodied and shaped by experience. The behavior of walking, as just one example, differs from one individual to the next depending on health history, body size and shape (for example, limb-to-trunk length ratios), past physical experience, the character of the walking surface, clothing, and many other factors.[/quoted]An emphasis on development as a process underscores how interwoven nature and nurture are. Perhaps this concept doesn’t come easily to us, in part, because time is invisible. But it helps to reflect on the obvious fact that we aren’t born fully-formed. Rather, all organisms grow into maturity, responding to stimuli and challenges, while exchanging matter and energy with the world around us. And as humans, we can take a rather long time to develop into maturity.As we grow, we incorporate our environmental circumstances. Researchers have given this idea different names such as “embodiment” (Krieger 2005), experiences getting “under the skin,” racial inequalities “becoming biology” (Gravlee 2009), or culture becoming “en-brained” throughout our lives (Downey and Lende 2012: 32). And though I have to admit that I never quite got what Pierre Bourdieu was trying to say, I do remember liking his phrase “the internalization of externality.” In subtle and not-so-subtle ways, our outsides get in our insides. And, sometimes it can seem like everything in our inside wants to be on our outside. Insides and outsides may be in more intimate contact than we are sometimes aware.On a personal note, when I look at my own children now, of course, I see their current selves. Even as a father who has been present nearly every day of their lives, it can be hard to remember how much they’ve changed in size, appearance, and behavior in only a few years. Their present selves might be even more different, had their past selves contracted a nasty infection, lived in a different neighborhood, attended a different school, etc. Each transient state impacts the next, with the earliest ones having a disparate effect on the trajectory of subsequent ones.Some have emphasized the importance of the first 1,000 days as a critical period of life. There’s nothing magical about a thousand days (we seem to have an affinity for nice round numbers; maybe there’s a gene for that). Still, this is a good approximation of when our development is most vulnerable, and when our developmental trajectory is most alterable, depending on what part of our biology we’re talking about.Of course, events later in life can also have profound effects. Rachel Yehuda and Linda Bierer (2009) have referred to the development of post-traumatic stress disorder as akin to an ‘existential transformation.’ In a way, falling in love is also transformational for people and for prairie voles. There are cultural examples too. Someone who has undergone a rite of passage – an initiation, marriage, a rise in rank – is often conferred a new status and set of rights and responsibilities (van Gennep 1960). A person (or vole) who has fallen in love, been initiated into a society as an adult, or diagnosed with PTSD is obviously still the same person they were before. Yet, in another sense, they can also be seen as qualitatively different.Hard-wired for PlasticityOver at The Mermaid’s Tale, anthropological geneticist Ken Weiss once wrote that “we are hard-wired not to be hard-wired” for our behavioral repertoire. Bill Leonard and colleagues made a similar claim about our diet: humans evolved to be flexible omnivores, rather than to have a highly specialized dietary niche (Leonard et al 2010). As Matt Ridley succinctly put it, plasticity is evolution’s “masterstroke” (2003:174).This brings us to a fundamental question: how does a genome accommodate so much unpredictable environmental complexity? In a nutshell, plasticity. Variants of genes (alleles) that thrive in given set of ecological conditions will be passed on at higher rates than others. But this is not the only force affecting allele frequencies; nor is it the only way to adapt. I once heard the biological anthropologist R. Brooke Thomas say that, without a doubt, natural selection was an essential mechanism in shaping our biology. However, he also felt it was “too clunky” to try to explain all variation and adaptation. And he was right.Genetic adaptations certainly exist. But a genome that can respond to environmental feedback and operate in many possible, unpredictable conditions would be even more likely to survive and reproduce than a rigid one, hardwired to just do its thing regardless of circumstances. “The stiffest tree is readiest for the axe.” Sometimes those responses are due to developmental constraints, sometimes they result in pathologies, and other times they may be truly adaptive. However, it is not always easy to differentiate between them (Ellison and Jasienska 2007).Take body temperature regulation, for example. There do seem to be genetic adaptations for this across human populations. Near the equator, body builds tend to be thinner and longer-limbed – on average – in order to dissipate heat more easily. But this has always been a very imperfect correlation, and there are exceptions to this trend. The strength of this association has also weakened in recent decades, as globalization and changes in diet and activity patterns have affected patterns of body mass around the world (Katzmarzyk and Leonard 1998). Rather than relying solely on body build to regulate temperature, there are other levels of adaptation (Frisancho 1993):

• Behavioral: Is it hot out? Go find some shade.
• Cultural: Are you cold? Build a fire. Put on a parka.
• Physiological: Sweating, shivering. Vasodilation/vasconstriction.
• Developmental: Older Quechua adults in high-altitude Peru are more resistant to cold than young Quechua, as they had more time to develop under cold conditions (Little et al 1971).

All of these adaptations take place on different time scales. Whereas natural selection takes generations, by definition, behaviors can take just a few seconds. Developmental adaptations occur while the organism is growing, and may take years, but this is still within a single lifetime. Finally, there may be another, intermediate level of adaptation, which Chris Kuzawa referred to as “phenotypic inertia” (Kuzawa 2005).It’s an intriguing idea. Kuzawa hypothesized that: “As a mode of adaptation, phenotypic inertia may help the organism cope with ecologic trends too gradual to be tracked by conventional developmental plasticity, but too rapid to be tracked by natural selection.” In other words, the experiences of parental generations can carry over, perhaps by developmental constraint, perhaps epigenetically (yes, I realize that word is overused).Final example. Stewart et al (1980) conducted an experiment where one group of rats was given a low protein diet for ten to twelve generations, while a control group was given a normal diet. At the end of that time, the poor-diet group weighed about half of what the controls did by adulthood. When the researchers tried to rehabilitate the poor-diet lineage by re-introducing a normal diet, some interesting patterns emerged.First, timing was important, and the earlier the rehabilitation, the better the results. Rats given good diets after weaning did not fare as well in terms of physical growth as those who were given good diets in infancy (‘cross-fostered’ to control mothers at birth) or in utero. Second, full recovery took two to three generations in both of the groups that were rehabilitated postnatally. One generation was not sufficient. By comparison, the prenatally rehabilitated group actually overshot the growth seen in the control group, after which they came back to “normal” by generation three.Finally, the low-protein group also suffered in terms of learning (a Lashley jumping platform test). Many never achieved complete proficiency on the various tests, which consisted of choosing between and jumping through one of two doors. By contrast, 100% of the control group did so, and in many fewer trials (average of 170 vs. 230). For the rehabbed offspring, those well-fed after weaning showed no improvement (230 trials), while the early postnatal (190) and prenatal groups (210) fared slightly better, though they never caught up completely, at least not within three generations.So where does this leave us? Stewart et al. hedged somewhat against making superficial comparisons between their study on rats and humans, but they also suggested there could be parallels with deprived populations, such as in developing countries facing generations of poverty. Recovery might not happen right away in terms of growth or learning, even if nutritional quality (or other factors) change radically.We can say, then, that of course genes matter tremendously, but it would be a major oversight to disregard the effects of the environment. And “the environment” does not begin in kindergarten, at birth, or even prenatally. The experiences of previous generations may also leave a mark. In truth, it all counts, and for fundamental biological reasons. I’ll end with Matt Ridley:[quoted]It is genes that allow the human mind to learn, to remember, to imitate, to imprint, to absorb culture, and to express instincts. Genes are not puppet masters or blueprints. Nor are they just the carriers of heredity. They are active during life; they switch on and off; they respond to the environment…They are both cause and consequence of our actions. Somehow the adherents of the “nurture” side of the argument have scared themselves silly at the power and inevitability of genes and missed the greatest lesson of all: the genes are on their side. (2003: 6)[/quoted]ReferencesDowney G, Lende DH. 2012. Neuroanthropology and the encultured brain. In Lende DH, Downey G (eds.) The Encultured Brain p. 23-65. Cambridge: MIT. LinkEllison PT, Jasienska G. 2007. Constraint, pathology, and adaptation: how can we tell them apart? Am J Hum Biol. 19(5):622-30. LinkFrisancho AR. 1993. Human Adaptation and Accommodation. U Michigan. LinkGravlee CC. 2009. How race becomes biology: embodiment of social inequality. Am J Phys Anthropol. 139(1):47-57. LinkKatzmarzyk PT1, Leonard WR. 1998. Climatic influences on human body size and proportions: ecological adaptations and secular trends. Am J Phys Anthropol. 106(4):483-503. LinkKrieger N. 2005. Embodiment: a conceptual glossary for epidemiology. J Epidemiol Community Health 59(5): 350–355. LinkKuzawa CW. 2005. Fetal origins of developmental plasticity: are fetal cues reliable predictors of future nutritional environments? Am J Hum Biol 17: 5-21. LinkLeonard WR, Stock JT, Valeggia CR. 2010. Evolutionary perspectives on human diet and nutrition. Evolutionary Anthropology 19:85–86. LinkLittle MA, Thomas RB, Mazess RB, Baker PT. 1971. Population differences and developmental changes in extremity temperature responses to cold among Andean Indians. Human Biology 43(1): 70-91. LinkStewart RJC, Sheppard H, Preece R, Waterlow JC. 1980. The effect of rehabilitation at different stages of development of rats marginally malnourished for ten to twelve generations. Brit J of Nutr 43: 403-11. Linkvan Gennep A. 1960. The Rites of Passage. Psychology Press. LinkWest-Eberhard MJ. 2003. Developmental Plasticity and Evolution. Oxford LinkYehuda R, Bierer LM. 2009. The relevance of epigenetics to PTSD: implications for the DSM-V. J Trauma Stress. 22(5):427-34. LinkImage: A tree incorporating a bicycle over the years on Vashon Island. 2nd Order Effect, Flickr.