Since the sequencing of the human genome in 2003, genetics has become one of the key frameworks within which we all see ourselves. Whether it’s worrying about our health or debating how schools can accommodate non-neurotypical students, we embrace the idea that genes provide answers to intimate questions about outcomes and identity of people.
Recent research backs this up, showing that complex traits such as temperament, longevity, resilience to mental health issues, and even ideological leanings are all, to some extent, “hardwired.” Of course, the environment also counts for these qualities. Our upbringing and life experiences interact with genetic factors to create an incredibly complex matrix of influence.
But what if the question of genetic heritage was even more nuanced? What if the old polarized debate about the competing influences of nature and nurture needs to be improved in the 21st century?
Scientists working in the emerging field of epigenetics have discovered the mechanism for passing on lived experience and acquired knowledge within a generation by changing the shape of a particular gene. This means that an individual’s life experience does not die with him or her but lives on in genetic form. The impact of the famine your Dutch grandmother suffered during World War II, for example, or the trauma inflicted on your grandfather when he fled his home as a refugee, could shape the brains of your parents, their behavior and possibly yours.
Much of the early epigenetic work was done in model organisms, including mice. My favorite study is the one that shook the neuroscience community when it was published in Nature Neuroscience in 2014. Carried out by Professor Kerry Ressler of Emory University in Georgia, the study results carefully dissect how of which a person’s behaviors are affected by ancestral influences. experience.
The study used mice’s love of cherries. Typically, when a whiff of cherry scent hits a mouse’s nose, a signal is sent to the nucleus accumbens, causing this pleasure zone to light up and motivating the mouse to scurry in search of the treat . Scientists exposed a group of mice first to a cherry smell and then immediately to a mild electric shock. The mice quickly learned to freeze in anticipation every time they smelled cherries. They had puppies, and their puppies were able to live happy lives without electric shocks, but without access to cherries. The puppies grew up and had their own offspring.
At this point, the scientists resumed the experiment. Could the association acquired from a shock with the sweet smell have been passed on to the third generation? This has. The grandparents were very fearful and more sensitive to the smell of cherries. How had this happened? The team discovered that the DNA from the grandfather mouse’s sperm had changed shape. This in turn changed the way the neural circuitry was established in her and her puppies, diverting some nerve cells in the nose away from pleasure and reward circuits and connecting them to the amygdala, which is involved in fear. The gene for this olfactory receptor had been demethylated (chemically marked), so the detection circuitry was improved. Through a combination of these changes, the traumatic memories were carried through the generations to ensure that the pups would acquire the hard-won wisdom that cherries might smell delicious, but were bad news.
The study authors wanted to rule out the possibility that learning by imitation could play a role. So they took some of the mice’s descendants and put them into foster homes. They also took sperm from the original traumatized mice, used IVF to conceive other pups, and raised them away from their biological parents. Adopted puppies and those conceived through IVF always had increased sensitivity and different neural circuits for the perception of this particular odor. Just to confirm things, mouse pups that hadn’t experienced the traumatic link of cherries to shocks didn’t show these changes even though they were raised by parents who did.
The most exciting thing of all happened when researchers began to study whether this effect could be reversed so that the mice could heal and other offspring would be spared this biological trauma. They took the grandparents away and re-exposed them to the smell, this time without any shock. After a number of repetitions of the experiment without pain, the mice stopped being afraid of the odor. Anatomically, their neural circuits returned to their original format. Basically, the traumatic memory was no longer transmitted in the behavior and brain structure of new generations.
Could the same thing apply to humans? Studies of Holocaust survivors and their children conducted in 2020 by Professor Rachel Yehuda of the Icahn School of Medicine at Mount Sinai Medical School in New York found that the effects of parental trauma can indeed be passed down in this way. His first study showed that participants had changes in a gene linked to cortisol levels, which is involved in the stress response. In 2021, Yehuda and his team conducted more work to find expression changes in genes related to immune system function. These changes weaken the white blood cell barrier, allowing the immune system to become inappropriately involved in the central nervous system. This interference has been linked to depression, anxiety, psychosis and autism. Since then, Ressler and Yehuda have collaborated, with others, to reveal epigenetic tags in combatants exposed to war zones and suffering from PTSD. They hope this information could make it easier to diagnose PTSD or even preemptively screen people who might be more likely to develop the condition before entering the battlefield.
Throughout history and in all cultures, people have paid their dues to their ancestors and considered the legacy they would leave to their descendants. Few of us still believe that biology is necessarily destiny or that our lineage determines who we are. And yet, the more we learn about how our bodies and minds work together to shape our experience, the more we see that our life story is intertwined with our biology. It’s not just our bodies that keep score, but also our genes.
Could 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, to our knowledge, the only animals capable of “cathedral thinking”, working on projects over several generations for the benefit of those who will follow. This is an idealistic way of thinking about legacy, but without it we will struggle to address complex multigenerational challenges such as climate and ecological emergencies. Our knowledge of epigenetics and its potential to dramatically accelerate evolutionary adaptation could help us do all we can to be the ancestors our descendants need. Conflict, neglect and trauma cause unpredictable and profound changes. But so do trust, curiosity and compassion. Doing the right thing today could indeed impact generations.
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Dr. Hannah Critchlow is a neuroscientist and author of The Science of Fate and Joined-Up Thinking (Hodder).
Further reading
The epigenetic revolution: how modern biology is rewriting our understanding of genetics, disease and inheritance by Nessa Carey (Icon, £11.99)
Genome: The Autobiography of Species in 23 Chapters by Matt Ridley (4th Estate, £10.99)
Outline: How our childhood made us who we aree by Lucy Maddox (Robinson, £10.99)