SSince the sequencing of the human genome in 2003, genetics has become one of the main frameworks for how we all think about ourselves. From worrying about our health to debating how schools can accommodate non-neurotypical students, we come to the idea that genes provide answers to intimate questions about people’s outcomes and identity.
Recent research supports this, showing that complex traits such as temperament, longevity, resilience to mental illness, and even ideological leanings are all, to some degree, «connected.» The environment also matters for these qualities, of course. Our upbringing and life experiences interact with genetic factors to create a fantastically complex matrix of influence.
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 due for a 21st century improvement?
Scientists working in the emerging field of epigenetics have discovered the mechanism that allows lived experience and acquired knowledge to be passed down within a generation, changing the shape of a particular 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 caused to your grandfather when he fled his home as a refugee, may continue to shape your parents’ brains, their behaviors, and eventually yours.
Much of the early epigenetic work was done in model organisms, including mice. My favorite study is one that shocked 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 carefully analyze how a person’s behaviors are influenced from the ancestors experience.
The study used mice’s love for cherries. Typically, when a sweet cherry scent reaches the rat’s nose, a signal is sent to the nucleus accumbens, causing this pleasure area to light up and motivating the rat to run in search of the treat. The scientists exposed a group of mice first to a cherry-like odor and then immediately to a mild electric shock. The mice quickly learned to freeze in anticipation whenever they smelled the cherries. They had puppies and their puppies were allowed to lead happy lives without electric shocks, albeit without access to cherries. The little ones grew up and had offspring of their own.
At this point, the scientists started the experiment again. Could a friend’s acquired association with the sweet smell be passed down to the third generation? There was. Grandfathers were very afraid and more sensitive to the smell of cherries. How had this happened? The team found that the DNA in the rat’s grandfather’s sperm had changed shape. This in turn changed the way the neuronal circuitry was laid out in his pups and their pups, redirecting some neurons from 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 tagged), so the circuits for its detection had been improved. Through a combination of these changes, traumatic memories were passed down through the generations to ensure that the little ones would gain the hard-won wisdom that cherries might smell delicious, but they were bad news.
The study authors wanted to rule out the possibility that learning by imitation might have played a role. So they took some of the rat offspring and kept them outside. They also took sperm from the original traumatized rats, used IVF to conceive more pups, and raised them away from their biological parents. Puppies fostered and those conceived through IVF STILL there was increased sensitivity and different neural circuitry for perceiving that particular scent. Just to make matters worse, rat pups that had not experienced the traumatic association of cherries with shock did not show these changes even if they had been fed by the parents that had.
The most exciting thing of all happened when the researchers set out to investigate whether this effect could be reversed so that the mice could recover and the other offspring could be spared this biological trauma. They took the grandparents and exposed them to the wind again, this time with no accompanying shock. After a certain amount of repetition of the pain-free experience, the rats stopped being afraid of the smell. Anatomically, their neural circuits returned to their original format. More importantly, the traumatic memory was no longer transmitted to the behavior and brain structure of the younger generations.
Could the same be true for humans? Studies of Holocaust survivors and their children conducted in 2020 by Prof Rachel Yehuda at the Icahn School of Medicine at Mount Sinai School of Medicine, New York, found that the effects of parental trauma can indeed be transmitted in this way. Her first study showed that the participants had changes in a gene linked to cortisol levels, which is involved in the stress response. In 2021, Yehuda and her team conducted more work to find expression changes in genes related to immune system function. These changes weaken the white blood cell barrier, which allows the immune system to inappropriately engage the central nervous system. This intervention has been linked to depression, anxiety, psychosis and autism. Since then, Ressler and Yehuda have collaborated, with others, to uncover epigenetic tags in PTSD-affected combatants exposed to the war zone. They hope this information can help diagnose PTSD or even pre-screen for individuals who may be more prone to developing the condition before entering the battlefield.
In all times and 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 defines 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 just our body that keeps score, but our genes themselves.
Could this new understanding increase our capacity for awareness and empathy? If we can understand the possible influence of our ancestors’ experiences on our behavior, can 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. It’s an idealistic way of thinking about heritage, but without it we will struggle to tackle complex multi-generational challenges like climate and ecological emergencies. Our knowledge of epigenetics and its potential to massively accelerate evolutionary adaptation can help us do all we can to be the ancestors our descendants need. Conflict, neglect and trauma cause unpredictable and far-reaching changes. But so is faith, curiosity and compassion. Doing the right thing today really can be passed down through the 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 a Species in 23 Chapters by Matt Ridley (4th estate, £10.99)
Project: How our childhood made us who we aree by Lucy Maddox (Robinson, £10.99)
#big #idea #inherit #memories #ancestors
Image Source : www.theguardian.com