Sunday, July 01, 2018

Evolution Is Cleverer Than You Are

theatlantic |  But chimeras are not just oddities. You surely know one. In pregnant women, fetal stem cells can cross the placenta to enter the mother’s bloodstream, where they may persist for years. If Mom gets pregnant again, the stem cells of her firstborn, still circulating in her blood, can cross the placenta in the other direction, commingling with those of the younger sibling. Heredity can thus flow “upstream,” from child to parent—and then over and down to future siblings.

The genome, Zimmer goes on to reveal, eludes tidy boundaries too. Forget the notion that your genome is just the DNA in your chromosomes. We have another genome, small but vital, in our cells’ mitochondria—the tiny powerhouses that supply energy to the cell. Though the mitochondrial genes are few, damage to them can lead to disorders of the brain, muscles, internal organs, sensory systems, and more. At fertilization, an embryo receives both chromosomes and mitochondria from the egg, and only chromosomes from the sperm. Mitochondrial heredity thus flows strictly through the maternal line; every boy is an evolutionary dead end, as far as mitochondria are concerned.

Beyond the genome are more surprises. Schoolchildren learn that Darwin’s predecessor, the great French naturalist Jean-Baptiste Lamarck, got heredity wrong when he suggested that traits acquired through experience—like the giraffe’s neck, elongated by straining and stretching to reach higher, perhaps tenderer, leaves—could be passed down. The biologist August Weismann famously gave the lie to such theories, which collectively are known as “soft” heredity. If Lamarckism were true, he said, chopping the tail off mice and breeding them, generation after generation, should eventually produce tailless mice. It didn’t. Lamarck wasn’t lurking in the details.

Recent research, however, is giving Lamarck a measure of redemption. A subtle regulatory system has been shown to silence or mute the effects of genes without changing the DNA itself. Environmental stresses such as heat, salt, toxins, and infection can trigger so-called epigenetic responses, turning genes on and off to stimulate or restrict growth, initiate immune reactions, and much more. These alterations in gene activity, which are reversible, can be passed down to offspring. They are hitchhikers on the chromosomes, riding along for a while, but able to hop on and off. Harnessing epigenetics, some speculate, could enable us to create Lamarckian crops, which would adapt to a disease in one or two generations and then pass the acquired resistance down to their offspring. If the disease left the area, so would the resistance.

All of these heredities—chromosomal, mitochondrial, epigenetic—still don’t add up to your entire you. Not even close. Every one of us carries a unique flora of hundreds if not thousands of microbes, each with its own genome, without which we cannot feel healthy—cannot be “us.” These too can be passed down from parent to child—but may also move from child to adult, child to child, stranger to stranger. Always a willing volunteer, Zimmer allowed a researcher to sample the microbes living in his belly-button lint. Zimmer’s “navelome” included 53 species of bacteria. One microbe had been known, until then, only from the Mariana Trench. “You, my friend,” the scientist said, “are a wonderland.” Indeed, we all are.

With this in mind, reconsider the ongoing effort to engineer heredity. The motto of the Second International Eugenics Congress, in 1921, was “Eugenics is the self-direction of human evolution.” Since then, controlling heredity has become technically much easier and philosophically more complicated. When, in the 1970s, the first genetic engineering made medical gene therapy feasible, many of its pioneers urged caution, lest some government try to create a genetic Fourth Reich. In particular, two taboos seemed commonsense: no enhancement, only therapy (thou shalt not create a master race); and no alterations in germ-line tissues, only in somatic cells (thou shalt not make heritable modifications).