Ecological Inheritance

FOR SIX CONSECUTIVE SEMESTERS in the early 1990s, I taught a seminar on Charles Darwin to nonscience majors at an urban community college. We read Darwin’s writings closely — often out loud to each other — along with commentary by scholars. We looked at the evidence that Darwin amassed for his theory of natural selection, and we looked at the evidence amassed in subsequent years.

At the beginning and end of each semester, I asked students if they themselves accepted Darwin’s ideas, and every semester, predictably, about half said they did, and half said they did not — a ratio that did not budge much over the course of the term. Mostly, those who had come into the class believing that humans had evolved continued to so believe, and those who came in hewing to a biblical account of the origins of life still hewed to it when they left.

One hundred fifty years have now gone by since the 1859 publication of On the Origin of Species by Means of Natural Selection, an anniversary that has prompted new scholarly reflections on Darwin’s legacy. Many of these speeches and papers have focused on the bombshell elements of his theory — how it blew the human race away from the center of creation, generating psychic aftershocks that reverberated for decades. Even the Victorian novelist Thomas Hardy marveled at the consequences to ethics and altruism posed by “the establishment” of a common origin of species.

That was 1910. And yet there are still plenty of people blithely walking around in a pre-Darwinian world, admitting shared origins with no one whose last name is not sapiens. According to the Pew Research Center, polls conducted over twenty years reveal little movement in the percentage of the public who accept evolution. In a one-to-one ratio that echoes my own classroom findings, about 40 to 50 percent of Americans say they believe in it, and a slightly smaller percentage say they do not. Those who believe that natural selection is the driver of evolution (Darwin’s keynote point) are firmly in the minority at 14 to 26 percent.

With numbers like these, I am unsurprised that the findings emerging from an obscure field of study called epigenetics have not yet rocked the world. They are rocking my world, though, and they are also mounting a profound challenge to the traditional systems of environmental regulation, which presume that toxic chemical exposures create health risks primarily through the accumulation of genetic damage (mutations) and that people can be categorized as inherently vulnerable or resistant. (“Genetics loads the gun; environment pulls the trigger.”) Moreover, in the way that it upends our understanding of heredity, epigenetics offers a whole new way of appreciating Darwin.

Epigenetics is the study of gene expression. Genes, of course, are made of DNA and are strung like beads along the chains of our chromosomes. Each cell in our bodies has a complete set. We humans have more than two hundred distinct kinds of cells, and they all contain the same number of genes. What makes a prostate gland cell so different in form and function from, say, a salivary gland cell is not the genes contained within them but the activity of those genes. During prenatal life and infancy — and again in puberty — immature cells become differentiated when long strings of genes that are not needed for the specific tasks of, say, semen production or saliva production are silenced. The rest are allowed to express themselves. Epigenetic regulation of the genome is what makes development possible.

Unlike the human genome, which has been exhaustively sequenced and mapped, plans to decode the human epigenome are still in the planning stage. What we know about it now is that the epigenome exists, in part, within various bobbles attached to our chromosomes. Previously ignored by cell biologists, these ornaments play a key role in regulating genetic activity. Some are simple methyl groups and others are proteins called histones. Together, they hush the genes whose messages are not needed at the moment. Methyl groups and histones are highly sensitive to messages streaming in from the outside world. In other words, the epigenome guides the genome and, in turn, responds to environmental signals.

Consider this: identical twins are epigenetically unique; attached to their identical chromosomes are nonidentical patterns of methyl groups and histones. Moreover, in a phenomenon called epigenetic drift, twins become more different with time. As revealed in a 2005 study, younger twins are more alike than older twins. As twins age and have different environmental experiences, their genetic expression diverges. Twins who spend more of their lives together in the same environment have gene-expression portraits that are more similar than twins who go their separate ways.

As an adoptee, I can’t help wondering if the reverse process might also be true. Growing up together in the same environment, do adopted siblings experience epigenetic convergence? Is this why, as girls, my genetically unrelated sister and I suffered from the same allergies, developed identical digestive problems, and wore the same eyeglass prescription? More generally, might it be possible that the longer people share a common environment, the more their genes act like each other? Do we carry on our chromosomes a kind of extra-genetic memory of all of our past habitats?

There is reason to think so. Environmental epigenetics examines how environmental exposures influence gene expression. What the results of this nascent field of study reveal is the vulnerability of early life. When epigenetic regulation is disrupted early on, the process of differentiation can be thrown off course in ways that may raise the risk for many diseases, including cancer. We already know that Inuit people in Greenland who acquire high body burdens of persistent pollutants have fewer methyl groups attached to their chromosomes than their lesser-exposed compatriots. This is not good. In the laboratory, hypomethylation is associated with chromosomal instability. We know from lab experiments that certain chemical exposures in prenatal life can alter developmental pathways and lead to altered architecture of adult structures (such as breasts). But our current system of environmental regulation — with its narrow focus on identifying chemicals that cause mutations — does not screen for chemicals that trigger changes in development. And our current system of genetic testing — with its narrow focus on identifying carriers of certain genes that bestow notably higher cancer risks — does not consider the regulation of genes by environmentally mediated signals either.

Perhaps most astonishing of all, epigenetic changes can be inherited. This means that the environmental exposures we experienced as children can have consequences not just for us but also for our descendants. More philosophically, it means that, contrary to current biological dogma, the nineteenth-century idea that acquired traits can be passed down the generations may not be so wrong-headed after all. And this brings us back to Darwin, who developed his ideas before we had a working understanding of genes and who was agnostic on the subject of the heritability of acquired characteristics. The reality of epigenetic inheritance hardly overturns natural selection — indeed it shows us another route by which species can adapt. Finally, it shines a spotlight on one of Darwin’s lesser-appreciated insights: that all of life is interrelated — not only by our common origins but also by our common ecology.

Sandra Steingraber is the author of Living Downstream and several other books about climate change, ecology, and the links between human health and the environment. She was an Orion columnist for six years. Author photo: Laura Kozlowski.


  1. Yet again we find that everything is more complicated than we thought. J. B. S. Haldane (geneticist, 1892-1964) put it this way: “Now my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose.”

  2. Facinating and intrigueing. Professor EO Wilsons teatise on biophillia becomes even more relevant. The interelationship of of life and the environment is enescapeable.

  3. very interesting story and very well explained by Sandra S. Darwin of course is the epitome of Man’s ability to overthrow his acquired environmental education and beliefs, as it happened in his very life with the Beagle Journey. I note here that Carl G. Jung added a lot to this with his studies and his ongoing relationship with the natural world and “primitive” peoples he joined during his long and amazing life. I also think mankind is still way behind in the mysterious evolutionary process: we still have a hard time in accepting wild laws, rights of nature and the fact we are absolutely NOT at the centre of the universe. Interelationship is the key: otherwise we’re doomed to eternal blindess.

  4. What dynamite this is. But does anyone think that those who doubt the ecological perspective would be swayed? Our blindness, like all perceptual phenomena, depends on the assumptions we cling to — or that cling to us like suction cups. We can’t even see evidence that doesn’t fit our assumptions. How are we going to get a knife-blade under the edge to break the suction?

  5. I read of a recent study that explained why populations of Native Americans typically have higher rates of alcoholism: their bodies produce less of an enzyme that assists in the metabolism of alcohol. Without this enzyme, that Europeans generally have, alcohol stays in their system much longer. Thus, it takes less alcohol for Native Americans to become intoxicated and stay drunk longer. Reading this article, I wonder if our chromosomes are not so different, and Europeans being exposed to alcohol for several millennia, simply adapted to alcohol in their diet by activating the genes that produce more of this enzyme?

  6. The vindication of Lamarck, whose theory of evolution preceded Darwin’s, and which posited that aquired traits were inheritable.

    What the author failed to note is the most radical ramification of this epigentic understanding: that there is nothing but a conceptual separation between organism and environment. All evolves together. The totality of the “living” and “non-living” world are one evolving entity – GAIA – in which we each are but cells.

    If we were to truly comprehend this truth, then not only are we not the center of the universe, we are not even individuals. We are but part of a much larger whole in which the greatest illusion is the individuated ego.

  7. As a North American indigenous person, I recall the ecological philosophy of our tribal nation which encompasses the concepts: “we are the land” and ” look after the earth in the best manner and the earth will return the care to you.” These concepts may be expressed simply but many millenia of observation and experience have led our people to apply these concepts in our relationships with the natural world.
    It is heart-breaking that the dominant culture is only just beginning to study these relationships after they have already made their life- threatening changes and assaults on the natural world.

  8. Unlike some other respondents, I do not find the natural world queer at all, but rather orderly, logical,and so remarkable in its capacity to be, among other things, sustainable and self-repairing. This is what would be expected from an orderly and logical Creator who absolutely must be behind it all. No process within our natural world holds a whisper of an answer to how our unimaginably complex yet wonderfully functioning world came to be. It needs to be from an intelligence from another dimension.
    And as to whether a human being with our perceived individuated ego is any different from a meteorite fragment, I find that to be a mind game from nowhere.
    Perhaps someone can direct us all to a chat room where meteorite fragments are discussing their existence.

  9. Do I understand from this that natural selection’s explanation may have to be enlarged to include additional triggers carrying environmentally influenced methyl groups and histone proteins through the DNA/RNA reticulation process? As well as modifications made during cell regeneration that are environmentally triggered? I really love this complexity and discovery process accelerating so rapidly in my lifetime.

  10. I 1st discovered epigenetics in mid ’09 & couldn’t believe what I was reading. As a grduate student who couldn’t decide between psychology & sociology & had always been suspicious of the rigid division between genes & the environment, this was welcome news indeed. As educated people don’t accept evolutionary theory 150 yrs after Darwin proposed it, I wonder how long it will take for epigenetics to be accepted by the general public.

  11. Epigenetic conditioning to group mores, environmental milieu, and acquired resistance to viral diseases has been observed over more than a century. It wasn’t named such in past decades, but is part of the Darwinian evolutionary matrix –the natural selective process of inherited genetic tendencies and heritable, acquired habits and proclivities.

    I try to discuss this in part in my study, “Gaia and of Midas, Univ. Press of America, April 2009.

    -Richrd Krooth

  12. This explains a lot; evolution by itself, not enough! Survival of fittest after random occurrences does not adequately explain what we are experiencing. Changes in genetic expression, alternatively, are proactively adaptive, even within one lifetime. In response to Schonfield’s question: Once the two theories are presented cohesively to the general public, this will help!

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