The genetic code embedded inside your DNA is not only the sole result of biological inheritance, as it was previously thought. Scientists now know that environmentally induced changes in the way certain genes are expressed can sometimes persist for several generations. To put things in more relatable terms, all of this means that a person's diet, exposure to stress, and chemicals can affect the composition of germline non-DNA molecules, and in doing so, this historical exposure may also carry through to descendants.
This process is known as epigenetic inheritance and studies on animals have shown that early exposure to certain chemical pollutants, stress and poor diet can shape anything from brain development to hair color.
The most illustrative example of epigenetics is the tragic Dutch Hunger Winter, which lasted from the start of November 1944 to the late spring of 1945. During this time, the western part of The Netherlands was still under German control. A German blockade resulted in a catastrophic drop in the availability of food to the Dutch population. At one point, people were surviving on only about 30 percent of their normal daily calorie intake. They ate anything they could get their hands on; grass, tulip bulbs, even book covers. By the time Holland was liberated in May 1945, some 20,000 people had died of starvation.
Epidemiologists have been able to follow the long-term effects of the famine, but what they found completely blew their minds at the time.
Mothers who were well-fed around the time of conception, but malnourished only for the last few months of pregnancy, gave birth to smaller babies, on average. On the other hand, mothers who were malnourished for the first three months of pregnancy, but later had access to enough calories when the blockade was lifted were more likely to birth normal-size babies. Over the course of the decades that doctors have been following the babies, they found those who were born small stayed small all their lives, with lower obesity rates than the general population, despite having access to as much food as they wanted. That’s not all. The children of the mothers who had been malnourished only early in their pregnancies had higher obesity rates than normal. Then, some of the same effects were observed, to a lesser degree, in the children of those who had been born in those troubled times, that is to say, the grandchildren of the malnourished grandmothers, even though the mothers themselves never went through malnourishment.
Other studies found, for instance, how holocaust survivors, passed down genetic changes associated with stress disorder to their children. Now, a new study found that people who were conceived during the Great Depression (1929-1939) show signs of accelerated aging, adding to a robust body of evidence supporting inheritable epigenetics.
The epigenetics of harsh times
Lauren Schmitz from the University of Wisconsin-Madison and Valentina Duque from the School of Public Affairs at the American University used a 'quasi-experimental strategy', whereby they compared biological markers of aging in about 800 people born during the Great Depression -- the worst economic downturn ever experienced by the USA -- in different states, which experienced different levels of unemployment and wage cuts.
Those who were born in states with the greatest economic hardship exhibited a pattern of biomarkers typically seen in cells that should be older, but this effect was diminished in the participants who were born in states that were better off.
It's not clear whether diet, stress, or some other factor led to this pattern of accelerated aging, but the researchers think that the prenatal period specifically may be "a sensitive window for the development of later-life disparities in aging."
The fact that the environment can drive important changes in our biology is not contested, but the notion that some of these changes can be passed down to offspring has proven controversial. But in light of recent evidence such as this, an increasing number of scientists support epigenetic inheritance.
Epigenetics refers to the modification of a phenotype without a change to the DNA sequence itself. For instance, a specific molecule could turn a gene on or off without changing the DNA sequence. In sperm and oocytes, these epigenetic molecules can be inherited at fertilization and thereby affect fetal organ development by modifying patterns of gene expression. These germline epigenetic factors are potentially altered based on an individual’s experiences and exposures, and in this manner, epigenetic abnormalities in sperm and oocytes can have significant effects on a descendant’s risk of disease.
An important implication is that disparities in individuals can begin to develop well before we're even born, not just based on the genetic inheritance from our ancestors, but also due to the lifestyle choices and circumstances our parents and grandparents lived through. Bearing this in mind, the authors of the new study make the case for strong social safety-net programs that support families, particularly during hard economic times.
The new findings were reported in the journal PNAS.