Phototropism

Even on a day when ninety-five degree temperatures have me and the dogs hiding inside with the shades drawn, I want to give a shout out to the glories of sunlight. It's been dealt an unfairly bad rap in the last few decades, and I say this despite the fact that my mom nearly lost her life to melanoma.

I've been reading a terrific book by Spencer Wells, Explorer-in-Residence (there's a great oxymoron) at the National Geographic Society and point man for the "Genographic Project," whose given mission is to map the early demographic expansion of homo sapiens through analysis of contemporary DNA. In Pandora's Seed: The Unforeseen Cost of Civilization, Spencer considers the many ways that our very recent (by anthropological standards) cultural transition from semi-nomadic hunting and gathering to settled agriculture has reverberated in our health and the health of the planet. The idea that we are living at odds with our genetic heritage is not a new one, but like Sarah Blaffer Hrdy, Spencer has genuinely original and evocative insights into this disconnect. (His focus is much broader, his evidence less exhaustive, but he's admirably coherent and generally careful not to overreach. His chapter on climate change is the only one I found thin and comparatively rote.)

Early in the book, Wells recounts an exchange with Jonathan Pritchard, an evolutionary geneticist who's been analyzing recent flux in the human genome, functional "hot spots" on the chromosome where change has been unusually sudden, suggesting dire environmental pressure and a corresponding urgency in our adaptation. According to Pritchard, the most dramatic metamorphoses in recent millennia (he's focused on the last ten thousand years) have involved the genes controlling pigmentation. As Wells notes, this doesn't come as much of a surprise; pigmentation is the most obvious of the differences that mark us out from each other, obvious enough that it unfortunately obscures how recently and closely we are all related.

For Wells, it is simply confirmation of the known, and "consistent with what anthropologists had long argued: that humans evolved originally in Africa with dark skin. It was only as we moved out of the tropics and into higher latitudes, with their lower levels of ultraviolet light, that we had to lose some of our dark pigmentation in order to allow the deeper layers of our skin to synthesize enough vitamin D-- something they only do when exposed to enough UV light. The reason Europeans have pale skin-- and part of the reason some of us have fair hair-- is that our ancient ancestors needed to make enough vitamin D for their bones to survive the rigors of northern life thousands of years ago."

Old news, as he says. And yet this ho-hum observation seems only in the last few years to have trickled over into considerations of UV light and human health. Even Wells himself makes offhand reference many chapters later to the "myth... of a healthy tan," without acknowledging that it's been replaced by the myth of a healthy pallor (which is potentially much more dangerous). Dermatologists have run amok in their single-minded terror of cancer and other less threatening forms of skin damage. In so doing, they have dismissed as an irrelevance one of the skin's most vital and extraordinary functions: the manufacture of vitamin D, which supports much more than the health of our bones. Maladies as diverse as arthritis and autism may be caused in part by vitamin D deficiencies. Their incidence skyrockets as we slather ourselves in sunscreen. (And that sunscreen is itself potentially carcinogenic. When? Wait for it... when interacting with strong sunlight! That, my dear Alanis, is ironic.) Likewise, the racial health gap, while no doubt exacerbated by socioeconomic stratification, may persist in part due to the excess of protection that dark skin provides at high latitude.

Medical research remains inconclusive on many of these repercussions, but the genomic record makes it plain: we need the sun. We got rid of pigment at an evolutionary sprint as we moved north-- not because it's generally good to be white, but because we depend so heavily on the agency of ultraviolet light.

Granted, with a longer average life span comes a higher risk of troublesome or fatal damage to the glorious and hardworking organ that "covers me from head to toe, except a couple tiny holes and openings" (thanks, David Byrne), and this should teach us moderation in UV exposure as in all things. Our increased mobility as a species also means that many people of varying colors are living at latitudes ill-matched to their pigmentation. But my instincts as a phototropic San Diego kid were basically sound (and need to be honored more consciously now that I've migrated north): I love the sun and the sun loves me. At least a little.

P.S. I had always assumed that our production of vitamin D operated on a kind of "pay as you go" economy: whatever you got on a given day, or maybe week, would have to be used pretty much immediately, or it would somehow decay (or be filtered out by the kidneys like the excess vitamins we take as supplements that turn our pee a violent shade of green). But I've recently read (in secondhand sources whose reliability is open to question) that, in climes with big seasonal variations in usable UV radiation, we stock up in the summer for our supply in the winter, when the sun is too low to do us any but psychological good.

Only about five more weeks 'til the fall equinox, my brethren and sistren (pink and brown!), so get that sunlight while you can! Just think of it as a fine whiskey, served neat-- sip and savor.