The greatest achievement in Earth’s 2.8 billion years of evolution isn’t human DNA or even the emergence of life from lifeless molecules swirling in steaming pools of chemical-rich water around the fissures of geysers. Evolution’s greatest triumph is memory. Memory is what made life possible. This is clear enough. The antibodies in your immune system contain the memory of all the diseases confronted by the human race. A newborn baby fends off disease by relying on the immune system of its mother, which it has borrowed. Soon the baby’s own immunities will develop as the thymus gland, the repository of past battles against invading bacteria and viruses, starts to produce antibodies. The thymus gland expands as it reaches full functioning during adolescence and then shrinks as its task is completed around age twenty-one.
If we focus just on this one process, the role of memory runs deep. The genes of your family line determine which antibodies will appear in you. That’s just a twig on the branch of human evolution; that branch leads back to the trunk of the tree, which contains the memory of how to make antibodies in the first place. The roots of the tree are DNA’s ability to remember experiences and to encode them for future generations. So the next time you don’t catch the cold that is going around, you owe your immunity to the first molecule of DNA.
Epigenetics suggests that our cells can in a sense “remember” everything we have experienced. But a suggestion isn’t proof. There’s a big difference between remembering your tenth birthday party and a geneticist examining the genetic modifications that encode the memory. Imagine that you are a telegraph operator from decades ago as streams of dots and dashes come across the wire. You can hold the code in your hands and count all the punches in the paper tape, but if you don’t know English, the messages are unreadable. In present-day genetics, the code is in our hands, but they’re in a language infinitely more difficult than English, the very language of all human experiences.
It’s a terrible fate to be at the mercy of your memories, but that’s the situation almost everyone finds themselves in. Old fears, wounds, traumatic events, and accidents litter the mind, roaming at will and distorting how we view the present. If you are an agoraphobic, with a fear of open or public places, you can’t leave the house without suffering from anxiety. Your fear has made you a slave to memory. In small and large ways we are all enslaved by events that are dead and gone. To be fully alive, you must learn to use your memories, not the other way around.
This is a slightly uncomfortable exercise, but sit for a minute and let a bad memory return. It can be anything—the content doesn’t matter. Don’t reach for a freshly painful memory. Instead, go back to something that happened when you were a small child. It could be falling off a swing or getting separated from your mother in the grocery store. What do you notice? First, that the memory exists; second, that you can retrieve it. Depending on how deep the memory goes, you will also notice that it feels like real life repeating itself. The same part of the visual cortex that sees a train wreck or a battle scene comes into play when a person visualizes the wreck or the battle by recalling it.
Everything you are noticing is reflected in your epigenome. Let’s go a step further. When the children of the Dutch famine became vulnerable to obesity, diabetes, and heart disease, those memories could be traced to their mothers’ experience of near starvation. The children couldn’t see this experience in their mind’s eye, and yet they inherited a molecular memory nonetheless. A striking study published in the high-impact journal Nature Neuroscience in 2014 added new evidence about memory’s effect on DNA, only in this case the driver was not diet, but fear. In this study, scientists trained mice to fear the scent of the chemical acetophenone (which is pleasant, like orange blossoms and cherries) by giving them a mild electric shock whenever the smell was introduced.
The shocks produced a stress reaction in the mice, which could be observed in their nervous, shuddering behavior. After a while it wasn’t necessary to deliver the shocks. Merely the smell of acetophenone was enough to produce the stress reaction. A maker of horror movies can do much the same thing by showing a dark room with the sound of a creaking door. The heroine’s eyes dart in fear, and what’s happening in the audience? These harmless images and sounds produce the anticipation of something horrible about to happen. Evidence of the stress response will appear in most viewers.
But the mouse study that associated a harmless odor with an electrical shock went further. This acquired fear in adulthood was inherited by the mice’s offspring and even by the next generation. The children and grandchildren of the fear-conditioned mice had never experienced the fragrance of the acetophenone before, but they shuddered as soon as they smelled it, simply because their parents were conditioned to associate the odor with pain. The researchers then looked at the gene that makes the protein receptor needed to smell the chemical and found that it had been epigenetically modified by methylation.
Folk wisdom has known about this phenomenon forever, as expressed in a piece of cracker-barrel wisdom from Mark Twain: “If a cat sits on a hot stove, that cat won’t sit on a hot stove again. That cat won’t sit on a cold stove either.” In the same vein, the wisdom behind getting back on a horse after you fall off is based on the instinctive knowledge that fear can make a lasting impression unless you counter it as soon as possible. Of course, this type of conditioning is mediated by memories maintained by the neural networks in your brain. The same experiences can chemically modify your genome to create a parallel “molecular memory.”
We’ve repeated several times that DNA is responsible for both stability and change. Now we’ve arrived at a new wrinkle. How do our brain and genes determine the difference between real danger (a hot stove) and imagined danger (a cold stove)? Animals apparently don’t, as proved by studies of cattle trained by electric fences. The first step is to enclose cattle in a tight corral bounded by an electric fence that delivers a harmless shock if touched. The electrical current runs through one thin wire.
After only a day, and sometimes just an hour, the zapped cows have learned to avoid the fence. They can then be released into a grazing area that is fenced in by a single wire. Even though the cattle could easily break through this barrier, training them with an electrified wire keeps then inside. Thus the old principle of physically hemming the cows in with barriers like rail fences is exchanged for a psychological barrier. It’s difficult for old ranchers to accept that a psychological fence can be more powerful than a physical one, but in experiments in which hungry cows were separated from a bale of hay by a single strand of electric wire, they would not break through it to get at the food.
Is this form of psychological training inheritable? So it appears, as evidenced once again by cattle. To keep cattle from wandering down a road, ranchers install grids, usually steel railing, with gaps between the rails. Yet it appears that actual cattle grids aren’t necessary. They can be fooled by phony grids, as described by Rupert Sheldrake, a British biologist famed for adventurous thought and investigation. (This trait has made him a groundbreaking thinker, an audacious rebel, an outlier from mainstream biology, or someone far too credulous about mysterious phenomena, depending on the view of him you take. We greatly appreciate his daring.) In a New Scientist article from 1988, Sheldrake writes:
Ranchers throughout the American West have found that they can save money on cattle grids by using fake grids instead, consisting of stripes painted across the road….Real cattle grids make it physically impossible for cattle to walk across them. However, cattle do not usually try to cross them; they avoid them. The illusory grids work just like real ones. When cattle approach them, they “put on brakes with all four feet,” as one rancher expressed it to me.
Although Sheldrake picked up on this phenomenon from American friends he was visiting in Nevada, the implications resonated with him. For decades Sheldrake had been almost a lone voice proposing that memories can be passed down from one generation to the next. Undaunted by ridicule from orthodox geneticists—this was long before the advent of epigenetics—he wrote farseeing books like A New Science of Life (1981) and The Presence of the Past (1988) to amass the mounting evidence that inheritance across the generations was real. These are still among the most fascinating and eye-opening books on the subject of memory as the major force in evolution. As Sheldrake explains:
According to my hypothesis…organisms inherit habits from previous members of their species. This collective memory, I suggest, is inherent in fields, called morphic fields, and is transmitted through both time and space….From this point of view, cattle confronted for the first time by grids, or by things that look like grids, would tend to avoid them because of [inheritance] from other cattle that had learnt by experience not to try to cross them.
A skeptic would protest that other, more conventional explanations must be at work. It could be that cows don’t inherit an avoidance of cattle grids but acquire it individually through painful exposure to real grids, or else they somehow pick it up from more experienced members of the herd.
Sheldrake responds:
This does not seem to be the case. Ranchers have told me that herds not previously exposed to real cattle grids will avoid the fake ones. This has also been found by researchers in the departments of animal science at Colorado State University and Texas Agricultural and Mechanical University, with whom I have been in correspondence. Ted Friend, of Texas A&M, has tested the response of several hundred head of cattle to painted grids, and has found that naïve animals avoid them just as much as those previously exposed to real grids.
Is this also a possibility among humans? Inheriting a behavioral trait might explain why Mohawk Indians have worked for generations on the construction of New York skyscrapers—they walk on the beams hundreds of feet in the air, apparently with no fear of falling. Did they inherit this trait? Does the same kind of inheritance account for why Russian chess players have won the world championship many times over?
Yet the effect of memory inherited across generations is soft enough that it can be reversed, at least in animals. Writing about the cattle who shy away from phony grids, Sheldrake says:
Nevertheless, the spell of a fake grid can be broken. If cows are driven towards one under pressure, or if food is placed on the other side, a few will jump it; but sometimes one will examine it closely and then simply walk across. If one member of a herd does this, then the others soon follow. Thereafter, the phony grid ceases to act as a barrier.
At least some sheep and horses also show an innate aversion to crossing painted grids. By contrast, in perhaps the only experiment of this kind ever carried out with pigs, the animals ran up to the painted grid, sniffed it, and started to lick it up. The Texas researchers had used a washable water-based paint with a flour-and-egg base.
Noticing these aspects of memory comes easily. We are all expert time travelers in our minds. But as skillful as we are at storing a memory and recalling it, we are much worse at erasing bad memories. Memories are sticky. Years of therapy can fail to undo the power of old traumas. Drugs and alcohol only mask them temporarily. Denial pushes a bad memory under the carpet, but there’s no guarantee that it will stay there.
Genetics tells us that any past experience, good or bad, is sticky because it has taken its place, using chemical bonds, deep inside the cell, in the nucleus where DNA resides. In a molecule of salt, atoms of sodium and chlorine are tightly bound together. A lot depends on their remaining stuck, because if you poured out some salt and it separated into its components, the release of chlorine gas would be poisonous. Likewise, it’s necessary for DNA’s bonds to remain secure or life would vanish into a cloud of atoms.
Life is about the persistence of memory. Until recently, the only memories available to geneticists were the rungs that connect the double helix of DNA, and these were fixed in place long, long ago in evolutionary time. However, epigenetics now uses chemistry to create genetic memories of past experiences, which are much more recent and intimate than the 2.8-billion-year-old memories that originally built the DNA molecule.