Humans are extremely “plastic”. “Plastic” meaning that our development can be shaped by what goes on (or does not go in) in our developmental environment along with the environment outside of the womb. Many factors drive development, and if one factor changes then part of the developmental course for the organism changes as well. Thus, environment can and does drive development, with the addition (or subtraction) of different factors. In this article, I will discuss some of the factors that drive development and physical plasticity and what can change them.
Subsistence provides food while food provides nutrition. Nutrients, then, supply our bodies with energy and promote tissue growth—among other things. However, nutrient requirements vary across and between species, while all mammals need a mixture of macronutrients (carbs, fat, protein, water, and fiber) and micronutrients (vitamins and minerals). Biological variability in nutrient requirements and “the eventual degree of metabolic function that an individual can achieve for a particular intake level is determined to a greater or lesser extent by genetic variants in enzymes controlling the absorption, uptake, distribution, retention or utilization of the nutrient” (Molloy, 2004: 156). Thus, individuals who consume the same amount of micro and macronutrients—who also have different polymorphisms in genes coding for the metabolism of any nutrient (through hormones and enzymes)—can, and do, have differing physiological responses to same vitamin intake. Thus, differences in genetic polymorphisms between individuals can—and do—lead to different disease.
Next we have phenotypic plasticity. Phenotypic plasticity, simply put, is the ability for a genome to express a different phenotype in variable environments. For instance, people born in hotter environments—no matter their race or ethnicity—develop larger pores in order to sweat more, since sweating is needed for cooling the body (Lieberman, 2015). Phenotypic plasticity can be a problem, though, in environments with numerous environmental stressors that will stress the mother and, in turn, affect the baby’s development in the womb as well affecting post-birth events. An example of this is when food availability is low and exposure to infection is high (in-utero and post-birth), and when these stressors are removed, the organism in question shows “catch-up growth”, implying that these stressors impeded the development of the organism in question.
Maternal nutritional imbalance has been found—both in animal studies and epidemiological studies—and metabolic disturbances, during critical windows of development for the organism, have both a persistent effect on the health of the organism and can be transmitted epigenetically to future generations (Gallou-Kabani and Junien, 2005). Gallou-Kabani and Junien (2005) write:
Epigenetic chromatin marks may be propagated mitotically and, in some cases, meiotically, resulting in the stable inheritance of regulatory states. Transient nutritional stimuli occurring at critical ontogenic stages may have lasting influences on the expression of various genes by interacting with epigenetic mechanisms and altering chromatin conformation and transcription factor accessibility (11).
Thus, metabolic syndrome can show transgenerational effects by way of incomplete erasure of the epigenetic factors carried by grandparents and parents. (See also Treretola et al, 2005.) Epigenetic regulation was extremely important during our evolution and especially during the development of the human organism, and is how and why we are so phenotypically plastic.
Epigenetic regulation during fetal reprogramming of the individual in preparation for the environment they expect to enter is likely to be a response to seasonal energy imbalance; changes that favour the metabolic efficiency are likely to be adaptive in such circumstances. Removal of seasonal energy stress, as has taken place in contemporary industrialized societies, may turn efficiency toward pathology. Humans thus have evolved an animal model that can respond genetically (through natural selection), phenotypically (through developmental plasticity) and epigenetically (by a balance of both). (Ulijaszek, Mann, and Elton, 2013: 19)
This seems to be a fundamental response to the human organism in-utero, responding to the lack of food in its environment and growing accordingly (low birth weight, susceptibilities to differing disease), which are still a problem for much of the developed world. Though this can be maladaptive in the developed, industrialized world, since poor early-life environments can lead to epigenetic changes which then spell out bad consequences for the low-birth-weight baby who was exposed to a slew of negative nutritional factors during conception (and post-birth).
It has already been established that nutrition can alter the genome and epigenome (Niculescu and Lupu, 2011; Niculescu, 2012; Anderson, Sant, and Dolinoy, 2012). So if differing nutritional effects can alter the genome and epigenome and these effects are transgenerationally inherited by future generations, then famines change the expression of the genome and epigenome which can then inherited by future generations if the epigenetic factors carried by the grandparents and parents are not erased (and there is mounting evidence for this claim, see Yang, Liu, and Sun, 2017).
There is evidence of phenotypic plasticity regarding the lack of nutrition when it comes to humans, in-utero, and the evidence comes from the Dutch Family Studies (see Lumey et al, 2007 for an overview of the project). Individuals who were prenatally exposed to the Dutch winter famine of 1944-45, six decades later, had less DNA methylation of the IGF2 (insulin-like growth factor 2) gene than same-sex siblings who were not exposed to the winter famine (Heijmns et al, 2008). The IGF2 gene plays an essential role of the development of the fetus before birth. The gene is highly active during fetal development, but much less so after birth. (It should be noted that the loss of imprinting on the IGF2 gene can promote prostate cancer; Fenner, 2017 and loss of imprinting on IGF2 can also promote other types of cancer as well; Livingstone, 2013).
Stein et al (2009) concluded that “famine exposure prior to conception is associated with poorer self-reported mental health and a higher level of depressive symptoms.” Tobi et al (2009) write that their data “support the hypothesis that associations between early developmental conditions and health outcomes later in life may be mediated by changes in the epigenetic information layer.” Tobi et al (2014) also show that the “Epigenetic modulation of pathways by prenatal malnutrition may promote an adverse metabolic phenotype in later life.” The prenatal—and neonatal—periods of development are of utmost importance in order for the organism to develop normally, any deviation outside of these measures can—and does—affect the genome and epigenome (Hajj et al, 2014).
Another strong example that these responses are adaptive to the organism in question is the fact that people who were exposed to nutritional imbalances in the womb showed a higher chance of becoming obese later in life (Roseboom, de Rooji, and Painter, 2006). Their study has implications for babies born in developing countries (since famines mirror, in a way, developing countries). Roseboom, de Rooji, and Painter (2006: 489) write:
This may imply that adaptations that enable the fetus to continue to grow may nevertheless have adverse consequences for health in later life.
Roseboom, de Rooji, and Painter (2006: 490) also write:
The nutritional experience of babies who were exposed to famine in early gestation may resemble that of babies in developing countries whose mothers are undernourished in early pregnancy and receive supplementation later on, but also of babies in developed countries whose mothers suffer from severe morning sickness.
So on-going studies, such as the Dutch Famine Study, have the chance to elucidate the mechanisms of low birth weight, and it can also show us how and why those exposed to adverse conditions in the womb show so many negative symptoms which are not present in kin who were not exposed to such malnutrition in the womb. These findings also suggest that nutrition before—and after—pregnancy can play a role in disease acquisition later in life. The fact that those exposed to famines have a higher chance of becoming obese later in life (Abeleen et al, 2012; Meng et al, 2017) shows that this adaptive response of the organism in the womb was very important in our evolution; the babe exposed to low maternal nutrition in the womb can, after birth, consume enough energy to become overweight, which would have been an adaptive evolutionary response to low maternal caloric energy.
Babies who are exposed to maternal under-nutrition in the womb—when exposed to an environment with ample foodstuffs—are at heightened risk of becoming type II diabetics and acquiring metabolic syndromes (Robinson, Buchholz, and Mazurak, 2007). This seems to be an adaptive, plastic response of the organism: since nutrients/energy were in low quantity in the womb, low nutrients/energy in the womb changed the epigenome of the organism, and so when (if) the organism is exposed to an environment with ample amounts of food energy, they will then have a higher susceptibility to metabolic syndromes and weight gains, due to their uterine environment. (Diet also has an effect on brain plasticity in both animals and humans, in the womb and out of it; see Murphy, Dias, and Thuret, 2014.)
In sum, phenotypic plasticity, which is driven in part by epigenetics, was extremely important in our evolution. This epigenetic regulation that occurs in the womb prepared the individual in question to be able to respond to the energy imbalance of the environment the organism was born in. The plasticity of humans, and animals, in regard to what occurs (or does not occur) in the environment, is how we were able to survive in new environments (not ancestral to our species). Epigenetic changes that occur in the grandparental and parental generations, when not completely erased during the meiotic division of cells, can affect future generations of progeny in a negative way.
The implications of the data are clear: under-nutrition (and malnutrition) affect the genome and epigenome in ways that are inherited through the generations, which is due to the physical plasticity of the human in-utero as well as post-birth when the baby developing. These epigenetic changes then lead to the one who experienced the adverse uterine environment to have a higher chance of becoming obese later in life, which suggests that this is an adaptive response to low amounts of nutrients/caloric energy in the uterine environment.