Ssince the sequencing of the human genome in 2003, genetics has become one of the key frameworks for how we all think about ourselves. From worrying about our health to debating how schools can accommodate non-neurotypical pupils, we are searching for the idea that genes provide answers to intimate questions about people’s outcomes and identities.
Recent research corroborates this, showing that complex traits such as temperament, longevity, resilience to mental illness and even ideological leanings are all “hardwired” to some degree. Environment is of course also important for these qualities. Our upbringing and life experiences interact with genetic factors to create a fantastically complex matrix of influence.
But what if the issue of genetic inheritance was even more nuanced? What if the old polarized debate about the competing influences of nature and nurture was a 21st-century upgrade?
Scientists working in the emerging field of epigenetics have discovered the mechanism that makes it possible to transmit lived experience and acquired knowledge within one generation by changing the shape of a specific gene. This means that an individual’s life experience does not die with them, but lives on in genetic form. The impact of the famine your Dutch grandmother suffered during World War II, for example, or the trauma your grandfather suffered when he fled his home as a refugee, may shape your parents’ brains, their behavior, and ultimately yours.
Much of the early epigenetic work was performed in model organisms, including mice. My favorite study is one that rocked the neuroscience community when it was published in Nature Neuroscience, in 2014. Conducted by Prof Kerry Ressler at Emory University, Georgia, the study’s findings neatly analyze the way in which a person’s behavior is influenced by ancestral experience.
The study made use of mice’s love for cherries. Typically, when a whiff of sweet cherry scent reaches a mouse’s nose, a signal is sent to the nucleus accumbens, causing this pleasure zone to light up and motivating the mouse to scramble around in search of the treat. The scientists first exposed a group of mice to a cherry-like smell and then immediately to a mild electric shock. The mice quickly learned to freeze in anticipation every time they smelled cherries. They had cubs, and their cubs were left to live happy lives without electric shocks, but with no access to cherries. The cubs grew up and had their own offspring.
At this point, the scientists resumed the experiment. Could the acquired association of a shock with the sweet smell possibly be passed on to the third generation? It did. The grandchildren were very afraid of and more sensitive to the smell of cherries. How did it happen? The team discovered that the DNA in the grandfather mouse’s sperm had changed shape. This, in turn, changed the way the neuronal circuit was laid out in his pups and their pups, redirecting some nerve cells of the nose away from the pleasure and reward circuits and connecting to the amygdala, which is involved in fear. The gene for this odorant receptor has been demethylated (chemically marked), so that the circuits for its detection have been improved. Through a combination of these changes, the traumatic memories swept across generations to ensure that the young would gain the hard-won wisdom that cherries might smell great but were bad news.
The study’s authors wanted to rule out the possibility that learning through imitation could play a role. They then took some of the mice’s offspring and bred them. They also took the sperm from the original traumatized mice, used IVF to produce more pups and raised them away from their biological parents. The reared pups and those conceived through IVF still had increased sensitivity and different neural circuits for the perception of that particular smell. Just to top things off, mouse pups that didn’t experience the traumatic pairing of cherries with shocks didn’t show these changes, even when fostered by parents that did.
The most exciting thing of all occurred when the researchers began to investigate whether this effect could be reversed so that the mice could heal and other descendants could be spared this biological trauma. They took the grandparents and exposed them to the smell again, this time without any associated shocks. After a certain amount of repetition of the pain-free experience, the mice stopped being afraid of the smell. Anatomically, their neural circuits returned to their original format. The traumatic memory was crucial no longer transmitted in the behavior and brain structure of new generations.
Can the same thing apply to humans? Studies on Holocaust Survivors and Their Children conducted in 2020 by Prof. Rachel Yehuda at the Icahn School of Medicine at Mount Sinai Medical School, New York, revealed that the effects of parental trauma can indeed be transmitted in this way. Her first study showed that participants carried changes to a gene linked to levels of cortisol, which is involved in the stress response. In 2021, Yehuda and her team did more work to find expression changes in genes linked to immune system function. These changes weaken the white blood cell barrier, allowing the immune system to inappropriately engage the central nervous system. This interference has been linked to depression, anxiety, psychosis and autism. Since then, Ressler and Yehuda collaborated, with others, to reveal epigenetic tags in PTSD-afflicted warzone-exposed combatants. They hope this information can aid PTSD diagnosis or even preemptively screen for individuals who are more likely to develop the condition before entering the battlefield.
In all times and across all cultures, people have paid their dues to their ancestors and thought about the legacy they will leave for their descendants. Few of us believe anymore that biology is necessarily destiny or that our bloodline determines who we are. And yet, the more we learn about how our body and mind work together to shape our experience, the more we can see that our life story is woven into our biology. It is not only our body that keeps the score, but our own genes.
Can this new understanding increase our capacity for self-awareness and empathy? If we can grasp the potential impact of our ancestors’ experiences on our own behavior, might we be more understanding of others, who also carry the inherited weight of experience?
We are, as far as we know, the only animals capable of “cathedral thinking”, working on projects over many generations for the benefit of those who come after. This is an idealistic way of thinking about legacy, but without it we will struggle to tackle complex multigenerational challenges such as the climate and ecological emergencies. Our knowledge of epigenetics and its potential to massively speed up evolutionary adaptation can support us in doing all we can to be the ancestors our descendants need. Conflict, neglect and trauma cause unpredictable and far-reaching changes. But so is trust, curiosity and compassion. Doing the right thing today can indeed spill over generations.
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Dr Hannah Critchlow is a neuroscientist and author of The Science of Fate and Connected thinking (Hodder).
Further reading
The Epigenetics Revolution: How Modern Biology is Rewriting Our Understanding of Genetics, Disease and Heredity by Nessa Carey (icon, £11.99)
Genome: The Autobiography of Species in 23 Chapters by Matt Ridley (4th Estate, £10.99)
Blue print: How our childhood made us, makes us who we aree by Lucy Maddox (Robinson, £10.99)
