Authors: Sam Kean
Because pheromones help guide social interactions, especially intimate encounters, shutting off the VNO in certain mammals can have awkward consequences. In 2007, scientists at Harvard University genetically rewired some female mice to disable their VNOs. When the mice were by themselves, not much changed—they acted normally. But when let loose on regular females, the altered mice treated them like the Romans did the Sabine women. They stormed and mounted the maidens, and despite lacking the right equipment, they began thrusting
their hips back and forth. The bizarro females even groaned like men, emitting ultrasonic squeals that, until then, were heard only from male mice at climax.
Humans rely less on scent than other mammals do; throughout our evolution we’ve lost or turned off six hundred common mammalian genes for smell. But that makes it all the more striking that our genes still build a VNO. Scientists have even detected nerves running from the fetal VNO to the brain, and have seen these nerves send signals back and forth. Yet for unknown reasons, despite going through the trouble of creating the organ and wiring it up, our bodies neglect this sixth sense, and after sixteen weeks it starts shriveling. By adulthood, it has retracted to the point that most scientists dispute whether humans even have a VNO, much less a functional one.
The debate about the human VNO fits into a larger and less-than-venerable historical debate over the supposed links between scent, sexuality, and behavior. One of Sigmund Freud’s nuttier friends, Dr. Wilhelm Fliess, classified the nose as the body’s most potent sex organ in the late 1800s. His “nasal reflux neurosis theory” was an unscientific hash of numerology, anecdotes about masturbation and menstruation, maps of hypothetical “genital spots” inside the nose, and experiments that involved dabbing cocaine on people’s mucus membranes and monitoring their libidos. His failure to actually explain anything about human sexuality didn’t lower Fliess’s standing; to the contrary, his work influenced Freud, and Freud allowed Fliess to treat his patients (and, some have speculated, Freud himself) for indulging in masturbation. Fliess’s ideas eventually died out, but pseudoscientific sexology never has. In recent decades, hucksters have sold perfumes and colognes enriched with pheromones, which supposedly make the scentee a sexual magnet. (Don’t hold your breath.) And in 1994 a U.S. military scientist requested $7.5 million from the air force to develop a pheromone-based “gay
bomb.” His application described the weapon as a “distasteful but completely non-lethal” form of warfare. The pheromones would be sprayed over the (mostly male) enemy troops, and the smell would somehow—the details were tellingly sketchy, at least outside the scientist’s fantasies—whip them into such a froth of randiness that they’d drop their weapons and make whoopee instead of war. Our soldiers, wearing gas masks, would simply have to round them up.
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Perfumes and gay bombs aside, some legitimate scientific work has revealed that pheromones can influence human behavior. Forty years ago, scientists determined that pheromones cause the menstrual cycles of women who live together to converge toward the same date. (That’s no urban legend.) And while we may resist reducing human love to the interaction of chemicals, evidence shows that raw human lust—or more demurely, attraction—has a strong olfactory component. Old anthropology books, not to mention Charles Darwin himself, used to marvel that in societies that never developed the custom of kissing, potential lovers often sniffed each other instead of smooching. More recently, Swedish doctors ran some experiments that echo the dramatic Harvard study with mice. The doctors exposed straight women, straight men, and homosexual men to a pheromone in male sweat. During this exposure, the brain scans of straight women and gay men—but not straight men—showed signs of mild arousal. The obvious follow-up experiment revealed that pheromones in female urine can arouse straight men and gay women, but not straight women. It seems the brains of people with different sexual orientations respond differently to odors from either sex. This doesn’t prove that humans have a functioning VNO, but it does suggest we’ve retained some of its pheromone-detecting ability, perhaps by genetically shifting its responsibilities to our regular nose.
Probably the most straightforward evidence that smells can influence human arousal comes from—and we’ve finally circled back to it—the MHC. Like it or not, your body advertises your MHC every time you lift your arm. Humans have a high concentration of sweat glands in the armpit, and mixed in with the excreted water, salt, and oil are pheromones that spell out exactly what MHC genes people have to protect them from disease. These MHC ads drift into your nose, where nasal cells can work out how much the MHC of another person differs from your own. That’s helpful in judging a mate because you can estimate the probable health of any children you’d have together. Remember that MHC genes don’t interfere with each other—they codominate. So if Mom and Dad have different MHCs, baby will inherit their combined disease resistance. The more genetic disease resistance, the better off baby will be.
This information trickles into our brains unconsciously but can make itself known when we suddenly find a stranger unaccountably sexy. It’s impossible to say for sure without testing, but when this happens, the odds are decent that his or her MHC is notably different from your own. In various studies, when women sniffed a T-shirt worn to bed by men they never saw or met, the women rated the men with wild MHCs (compared to their own) as the sexiest in the batch. To be sure, other studies indicate that, in places already high in genetic diversity, like parts of Africa, having a wildly different MHC doesn’t increase attraction. But the MHC-attraction link does seem to hold in more genetically homogeneous places, as studies in Utah have shown. This finding might also help explain why—because they have more similar MHCs than average—we find the thought of sex with our siblings repugnant.
Again, there’s no sense in reducing human love to chemicals; it’s waaaay more complex than that. But we’re not as far removed
from our fellow mammals as we might imagine. Chemicals do prime and propel love, and some of the most potent chemicals out there are the pheromones that advertise the MHC. If two people from a genetically homogeneous locale—take Hideo and Mayumi—came together, fell in love, and decided to have a child, then as far as we can ever explain these things biologically, their MHCs likely had something to do with it. Which makes it all the more poignant that the disappearance of that same MHC empowered the cancer that almost destroyed Emiko.
Almost. The survival rate for both mothers and infants with simultaneous cancer has remained abysmally low, despite great advances in medicine since 1866. But unlike her mother, Emiko responded to treatment well, partly because her doctors could tailor her chemotherapy to her tumor’s DNA. Emiko didn’t even need the excruciating bone-marrow transplants that most children with her type of cancer require. And as of today (touch wood) Emiko is alive, almost seven years old and living in Chiba.
We don’t think of cancer as a transmissible disease. Twins can nevertheless pass cancer to each other in the womb; transplanted organs can pass cancer to the organ recipient; and mothers can indeed pass cancer to their unborn children, despite the defenses of the placenta. Still, Emiko proves that catching an advanced cancer, even as a fetus, doesn’t have to be fatal. And cases like hers have expanded our view of the MHC’s role in cancer, and demonstrated that the placenta is more permeable than most scientists imagined. “I’m inclined to think that maybe cells get by [the placenta] in modest numbers all the time,” says a geneticist who worked with Emiko’s family. “You can learn a lot from very odd cases in medicine.”
In fact, other scientists have painstakingly determined that most if not all of us harbor thousands of clandestine cells from our mothers, stowaways from our fetal days that burrowed into our vital organs. Every mother has almost certainly secreted
away a few memento cells from each of her children inside her, too. Such discoveries are opening up fascinating new facets of our biology; as one scientist wondered, “What constitutes our psychological self if our brains are not entirely our own?” More personally, these findings show that even after the death of a mother or child, cells from one can live on in the other. It’s another facet of the mother-child connection that makes mammals special.
G
od knows the evolution of human beings didn’t stop with fur, mammary glands, and placentas. We’re also primates—although that was hardly something to brag about sixty million years ago. The first rudimentary primates probably didn’t crack one pound or live beyond six years. They probably lived in trees, hopped about instead of striding, hunted nothing bigger than insects, and crept out of their hovels only at night. But these milquetoast midnight bug-biters got lucky and kept evolving. Tens of millions of years later, some clever, opposable-thumbed, chest-beating primates arose in Africa, and members of one of those lines of primates literally rose onto two feet and began marching across the savannas. Scientists have studied this progression intensely, picking it apart for clues about the essence of humanity. And looking back on the whole picture—that
National Geographic
sequence of humans getting up off our knuckles, shedding our body hair, and renouncing our prognathous jaws—we can’t help but think about our emergence a little triumphantly.
Still, while the rise of human beings was indeed precious, our DNA—like the slave in Roman times who followed a triumphant general around—whispers in our ears,
Remember, thou art mortal.
In reality the transition from apelike ancestor to modern human being was more fraught than we appreciate. Evidence tattooed into our genes suggests that the human line almost went extinct, multiple times; nature almost wiped us out like so many mastodons and dodos, with nary a care for our big plans. And it’s doubly humbling to see how closely our DNA sequence still resembles that of so-called lower primates, a likeness that conflicts with our inborn feeling of preordainment—that we somehow sit superior to other creatures.
One strong piece of evidence for that inborn feeling is the revulsion we feel over the very idea of mixing human tissues with tissues from another creature. But serious scientists throughout history have attempted to make human-animal chimeras, most recently by adulterating our DNA. Probably the all-time five-alarmer in this realm took place in the 1920s, when a Russian biologist named Ilya Ivanovich Ivanov tried to unite human genes with chimpanzee genes in some hair-raising experiments that won the approval of Joseph Stalin himself.
Ivanov started his scientific career around 1900, and worked with physiologist Ivan Pavlov (he of the drooling dogs) before branching out to become the world’s expert in barnyard insemination, especially with horses. Ivanov crafted his own instruments for the work, a special sponge to sop up semen and rubber catheters to deliver it deep inside the mares. For a decade, he worked with the Department for State Stud-Farming, an official bureau that supplied the ruling Romanov government with pretty horses. Given those political priorities, it’s not hard to imagine why the Romanovs were overthrown in 1917, and when the Bolsheviks took over and founded the Soviet Union, Ivanov found himself unemployed.
It didn’t help Ivanov’s prospects that most people at the time considered artificial insemination shameful, a corruption of natural copulation. Even those who championed the technique went to ridiculous lengths to preserve an aura of organic sex. One prominent doctor would wait outside a barren couple’s room, listening at the keyhole while they went at it, then rush in with a baster of sperm, practically push the husband aside, and spurt it into the woman—all to trick her egg cells into thinking that insemination had happened during the course of intercourse. The Vatican banned artificial insemination for Catholics in 1897, and Russia’s Greek Orthodox Church similarly condemned anyone, like Ivanov, who practiced it.
But the religious snit eventually helped Ivanov’s career. Even while mired in the barnyard, Ivanov had always seen his work in grander terms—not just a way to produce better cows and goats, but a way to probe Darwin’s and Mendel’s fundamental theories of biology, by mixing embryos from different species. After all, his sponges and catheters removed the main barrier to such work, coaxing random animals to conjugate. Ivanov had been chewing over the ultimate test of Darwinian evolution, humanzees, since 1910, and he finally (after consulting with Hermann Muller, the Soviet-loving
Drosophila
scientist) screwed up the courage to request a research grant in the early 1920s.
Ivanov applied to the people’s commissar of enlightenment, the official who controlled Soviet scientific funding. The commissar, a theater and art expert in his former life, let the proposal languish, but other top Bolsheviks saw something promising in Ivanov’s idea: a chance to insult religion, the Soviet Union’s avowed enemy. These farsighted men argued that breeding humanzees would be vital “in our propaganda and in our struggle for the liberation of the working people from the power of the Church.” Ostensibly for this reason, in September 1925—just months after the Scopes trial in the United States—the
Soviet government granted Ivanov $10,000 ($130,000 today) to get started.
Ivanov had good scientific reasons to think the work could succeed. Scientists knew at the time that human and primate blood showed a remarkable degree of similarity. Even more exciting, a Russian-born colleague, Serge Voronoff, was wrapping up a series of sensational and supposedly successful experiments to restore the virility of elderly men by transplanting primate glands and testicles into them. (Rumors spread that Irish poet William Butler Yeats had undergone this procedure. He hadn’t, but the fact that people didn’t dismiss the rumor as rubbish says a lot about Yeats.) Voronoff’s transplants seemed to show that, at least physiologically, little separated lower primates and humans.