Future Tense

Could the Experiences of Our Ancestors Be “Seared Into Our Cells”?

A science journalist responds to Carmen Maria Machado’s short story “A Brief and Fearful Star.”

Illustration: Baby rats nurse from their mother, while a squiggle in the approximate location of their brains suggests that knowledge is passed down through their genetic line.
Doris Liou

Science journalist Erika Hayasaki responds to Carmen Maria Machado’s short story “A Brief and Fearful Star.

Our memories are made up of the stories we come to believe about our past, about how we got here and who we are, a running inner-narrative of scenes, summary, and anecdotes colored with bits of truth and speculation. We tend to define our lives through largely made-up memories, to decipher what makes us resilient, or what makes us weak.

There’s something seductive in believing we could also inherit memories in a biological sense, too. An ancestor passing down the experience of endurance or trauma, for example, transmitting traces of a distant past that does not belong to us and yet might be built into us before we are born. A coding that primes descendants to fear, to cope with, to prepare for, or to survive through the same perils. It makes for an uncomfortable solace, thinking that the memories of generations before may reside within our genes. It gives us explanations.

I think of stories I’ve been told about my grandmother in 1945 Japan: how she scooped up her two young children, one in each arm, and fled bombs dropping over her city during World War II, while my grandfather was stationed away in the fight. It feels plausible, and poetic, to think that the horrors she endured, the courage she summoned in the face of these challenges while protecting my aunt and uncle, were passed onto my father when he was born, and then to me, becoming “seared” into my own being at a kind of molecular level, as Carmen Maria Machado alludes to in her haunting short story, “A Brief and Fearful Star.” My grit may be linked to my grandmother’s experiences, or my angst.

It’s this kind of psychoanalytical thinking that makes a nascent, uneven, and controversial scientific field known as epigenetic inheritance so alluring. Though it deals more narrowly with exploring whether experience-specific changes to the ways a parent’s genes are expressed can be passed down to offspring—and how such inherited changes might impact the health and behavior of descendants—it has the ring of experiential memory stored and passed down through the generations.

Early research, such as studies suggesting the existence of epigenetic imprints of trauma in the descendants of Holocaust survivors and famine victims, have already gripped the public with questions and possibilities. Could certain epigenetic “memories” of slavery, genocide, poverty, or abuse be inherited too? What kinds of traits might humans be passing down that—instead of coming through nurture, luck, or random mutations—arise as a result of personal histories the descendants may never know? This science of epigenetic inheritance suggests that the kind of lives we live today, shaped by environmental factors like stress or diet, will alter the biological instructions within our children and grandchildren. Just as ours may have been altered by ancestors long before.

Machado’s story thematically unwinds these ideas in a way that science, so far, cannot. Memories lived by a mother reverberate within her daughter’s body, even as they remain unknowable to her. “I know it feels like we are the first people on this land,” Machado’s protagonist daughter tells the reader, echoing these ideas, “but we have been preceded by monsters and men alike.” The child speaks of enduring experiences that occurred before she was born, “as if their very existence was seared into my cells.” It’s powerful, stirring prose, even though in our own reality, science has not proven that a daughter’s genes can bear the vestiges of experiences lived by her forebears, or that cellular memory in humans transmits to future generations.

“The idea that potentially some of what makes you you could be due to what your ancestors did or experienced is almost a philosophical concept,” says Jamie Hackett, who led research in mice out of the University of Cambridge in 2013 showing that some epigenetic markers could be passed on to future generations in mammals. With the epigenome, as it is called, a network of molecules bonds to the gene like fog on glass. These molecules orchestrate gene activity. Exposures to the exterior world can change the epigenome, which may in turn affect our susceptibility to disease or behavior. One area of transgenerational epigenetic inheritance research involves the study of various etchings on our genes—dynamic instructions that can influence particular traits—and how these patterns may be transmitted through successive lineages.

Hackett’s research is significant because many scientists used to assume that changes to these markers caused would be reset almost completely in the mammalian genome instead of passed down to the offspring. Hackett, now at the European Molecular Biology Laboratory in Rome, is continuing to study epigenetic reprogramming and inheritance using CRISPR gene-editing technologies. “As with many very exciting scientific ideas,” he says, “the excitement over epigenetics pushes beyond the actual scientific understanding.”

Scientists can agree upon some findings of inherited epigenetics in plants and worms, which do transmit memories and experiences to generations beyond. For example, researchers who genetically engineered Caenorhabditis elegans (or C. elegans) roundworms to fluoresce discovered that exposing the creatures to warmer temperatures, even temporarily, triggered changes in their gene expression that caused them to glow brighter. What’s more, those roundworms then passed down the warm-environment changes to multiple generations of offspring, causing them, too, to glow brighter than other worms, despite having never been exposed to their parents or to the warmth that first triggered the change.

But the existing research dealing with humans, and even in our fellow mammals, is far more preliminary—and often overstated because of how captivating many find the concept, says John Greally, a professor of genetics, medicine, and pediatrics at the Albert Einstein College of Medicine in New York. (That study about “inherited trauma” in Holocaust survivors, for example? Largely disputed.) The sample sizes are small, the scientific replication lacking. There is too much reliance on correlation, not causation. Though the gist of epigenetic inheritance makes for beautiful fiction, the collective proof that we humans inherit our ancestor’s experiences is a story that science is still figuring out, Greally says. “Give us time.”

This lack of clear findings also has to do with how complex we humans are. Scientists still don’t fully understand the extent to which the environment impacts our epigenome. There’s also no consensus over whether changes can be passed on in mammals, and which might make it through a built-in expunging process the epigenome undergoes during conception and reproduction. Some, perhaps, scientists such as Hackett say, but that does not ensure they also survive into the next generation. And even if some residue of epigenetic instructions endures, it still may not be enough to noticeably impact the expressed traits of the offspring. “There is certainly no consensus among scientists, but perhaps a general feeling of cautious optimism is now emerging,” Hackett says. Scientists can only keep working on answers to such questions. “The mechanism [for how this might happen] is what all scientists are chasing in this field,” he says, “and until we really understand how this information is being transmitted, we’re uncomfortable with really saying this is transgenerational inheritance.”

For now, we have stories, which help us imagine what may be.

“Though my mother never said, this was what had happened to my father, I knew,” Machado writes. “The river wrapped hungry fingers through his trousers and shirt and took him under in half a breath.” Written like an episodic memory, a cinematic moment the daughter could see playing out in front of her; she was never there and did not see. But something within her seemed to know.

We cannot time-travel into our grandparents’ minds and re-experience their most pivotal or private moments the way we can recall and relive our own. But, even though the research on inherited epigenetics is so preliminary, we may be seeing emerging hints from other species that shadows of particular memories, like fear, may be passed on.

One of the many poignant scenes in Machado’s story refers indirectly to this process: The daughter observes her mother trapping, drowning, and killing mice that emerge from the crevices of their house. Drawn out by a bitter-scented oil slathered on the baseboards, some escape to conceive more pups. Later, the daughter notices how the offspring scatter at the mere detection of the oil, “as if the scent carried some terrible story.”

This moment nods to a real-life study conducted by Brian Dias and Kerry Ressler at Emory University in Atlanta. In it, researchers pumped acetophenone—an aromatic chemical that isn’t bitter-smelling at all, but instead emits a scent similar to orange blossoms, cherries, or almonds—into the enclosures of a group of male mice. As they did this, the pair also sent small electrical shocks to the mice’s feet until the creatures began to associate the smell with the discomfort. When the researchers introduced the acetophone smell to the next two generations of mice, the offspring of the shocked ancestors reacted with jitters when sniffing it, too—even though these descendants had never been exposed to their fathers, to the smell, or to the electrical shocks before. It was as if they inherited a fear of the scent. (Though, as Dias pointed out, it was probably an inherent sensitivity to the smell that was passed down, “Whether this odor elicits a fearful response or not is something we haven’t explicitly tested.”)

Though Ressler and Dias didn’t definitively identify how this reaction could have been passed down, they, like many other scientists, are looking at a markup process known as DNA methylation as a possible epigenetic explanation of the inheritance effect. Methylation, a reversible modification in which chemicals (specifically, methyl groups) become attached to DNA molecules, doesn’t alter the DNA sequence itself. But it can silence or activate particular gene activity. Scientists wonder, if these same methylation markings appear in the same genetic locations of offspring, is it possible that the offspring might inheriting these patterns too? Or could some other environmental trigger have caused such marks, perhaps in the womb?

Some environmental experiences, like advanced paternal age, do influence epigenetic marks that could then potentially influence future offspring, Dias says. “But when you start talking about more ephemeral experiences—like stress in a mouse, social defeat in a mouse, dietary changes, the Holocaust, those kind of experiences—not something tangible, per se—then I think we really need to understand how is it that an experience we can’t actually quantity can find its way into the germ line and give us a genotype?” That is the “biggest black box,” he says. “How is the puppeteering happening?”

Other researchers, like Larry Feig, a professor of developmental, molecular and chemical biology at Tufts University School of Medicine, are intrigued by microRNAs, which can also produce gene-silencing effects. “New work implicates microRNAs as modifying whole sets of genes,” he says, “and the changes in the expression of these genes may be passed on across generations.”

In one recent study from Feig’s lab, researchers examined the microRNAs in the sperm of men who reported experiencing abuse or neglect in childhood, and found concentrations of two particular types of microRNAs were much lower in their samples. When the researchers then exposed male mice to early life stress, they found the mice had lower levels of the same two microRNAs in their mouse sperm, too.

When these mice mated with females who had not been exposed to the same stress, the changes were present in the early embryos of their offspring and in the sperm of their male offspring, meaning the effect seemed to stick for at least a couple generations after. But to what behavioral or physical effect, Feig can’t answer. Though his team did observe that female offspring of the stressed male mice seemed antisocial and anxious, and exhibited certain stress-related characteristics, “We don’t know yet whether these embryo changes account for the elevated anxiety and defective sociability in the female offspring of stressed males display,” he says.

Still, he explains, it’s exciting work, and the lab is preparing to carry out a larger study in humans and mice to further test these findings. If such changes are being transmitted in mice or men, that means they are being passed down in germ line. It raises controversial questions that could force historical rethinking about life on earth. “If it is a stable change in the germ line, in the sperm and eggs, it could be passed on forever,” Feig tells me. “That has big impacts on ideas about evolution.”

Machado’s story expresses the strange feeling we’ve all had before: of sensing the tip of some secret we cannot articulate. “I did not belong here, on this land. The way was paved for me and though I did not pave it, I followed it nonetheless. How did I never know? Had I always known?”

I would like to believe that is the same place where scientists who are studying epigenetics now find themselves: within close reach of knowing something revolutionary, which could one day not only explain parts of who we are—but also help us treat our ugliest diseases and disorders. It’s also profound to consider that, maybe, my genes are inscribed with fragments of moments lived by my grandmother.

But when it comes to our own epigenetic inheritance, as Hackett tells me: “The truth is, we just don’t know yet.”