On the way to the airport, I realized that the chimp sperm created something of a dilemma. I had the vials in my day pack, the only bag I had brought for my short trip to Arizona. If I wanted to carry the bag with me on the plane, I would have to pass it through security, and surely the screeners would question the liquid in my vials. What would I say? It was hair conditioner? Packed in laboratory vials? If I told the truth, would they think I was a modern Ilya Ivanovich Ivanov, the Russian scientist who tried to breed a “humanzee”? But if I checked my small day pack as luggage, would they suspect that I was a drug smuggler or some such, and escalate to a search and a humiliating outing?
I gambled that the security checkpoint was a higher risk, and I checked my day pack at the ticket counter. My bet paid off. Before I knew it, I was back in San Diego, sperm in hand, at a late-night rendezvous with Gagneux.
People tend to think of sperm as cylindrical, but they are actually paddle-shaped, Gagneux told me. “When they move around, they resemble a surfboard tumbling around in the waves,” he said. He prepared some of the sperm I’d flown in, placing it on a microscope slide. The microscope was connected to a computer screen, so I could watch in real time. The sperm did not resemble surfboards tumbling in the waves so much as bugs ﬂittering about on the top of a pond. “Wow, look at that,” said Gagneux. “It’s pretty sweet, huh? There’s nowhere near that many in humans’.”
Gagneux’s lab space was adjacent to that of his collaborator Ajit Varki, who had helped uncover the functioning of the sugars, known as sialic acids, on cell surfaces. The sialic acids on the surfaces of human and chimp sperm have become the focus of Gagneux’s work, too. Humans, as Varki discovered, have lost the ability to make one sialic acid, Neu5Gc, and Gagneux suspected that Neu5Gc played a role in fertilization. He hypothesized that Neu5Gc helped female chimpanzees, in a process called “cryptic female choice,” get the benefit of the most-compatible, highest-quality sperm. The sugar acted like the fuzzy part of Velcro and attached to barbs formed by sugar-binding proteins on the surface of the cells in the uterus or fallopian tubes. Neu5Gc, as Gagneux imagined it, might “sweet-talk” the female reproductive system.
Gagneux’s Neu5Gc ideas had a critical implication for human fertility. Although we have lost the ability to synthesize Neu5Gc, we ingest the sugar when we eat meat and dairy products, and it, in turn, can then be incorporated into our cells. Does Neu5Gc coat the surface of human sperm? Is it found more readily on the sperm of men who eat lots of animal products? Does the extremely foreign Neu5Gc then trigger in women an immune response that selects against the survival of the sperm? “It could be that men who eat loads of meat pass a threshold and become infertile,” suggested Gagneux.
I left Gagneux shortly after midnight, and he was cranking away on the fresh chimp- and human-sperm samples he had received during the day. Science has few “Eureka!” moments, and Gagneux did not solve any great mysteries that night. But profound insights time and again come from asking simple questions that, once raised, seem abundantly obvious. Do the different sugars on the surfaces of chimp and human sperm impact fertility? is one of those obvious, beautiful questions. And it may just lead to an inobvious explanation for one of the more vexing problems that modern humans face.