Toward the end of an exhausting cross-country flight, my six-year-old fell asleep with his sweaty head on my wife’s lap. Lisa was traveling alone with our two kids after a month-long visit with her parents. Feeling tranquil and looking forward to the comfort of her own bed, she gazed at the snoring boy and ran her fingers through his hair, brown and thick as a horse’s mane. That’s when she spotted movement at scalp level. Suppressing the urge to scream, she parted his hair for a closer look, hoping that her suspicion would prove wrong.

“I had no idea about lice,” she told me later. “I didn’t think it was possible that we would have lice. I’m pretty sure they were mating. They were huge, and I couldn’t believe it.”

During the battle that played out over the next three weeks, I began to admire the tenacity of our newfound adversaries and the tightness of their adaptations to living exclusively in jungles of human hair. At my kitchen counter one bright morning, I paused to observe a louse dangling on a detached strand of my son’s hair. It twirled and ascended the line on crab-like legs with the grace of a Cirque du Soleil performer. Lice stealthily navigate scalps, drawing blood unnoticed. The itching, if there is any, only comes later, brought on by the biologically active substances in louse saliva. I did some research and learned that when feeding lice inject a potent cocktail of vasodilators and anti-clotting agents to keep blood flowing. Their demands are modest: even the hugest gatherings of lice on one human host will consume a total of only about four teaspoons of blood in a month.

Lice and us, we have a lot of shared history, it turns out. Italian scientists recently identified head lice and pubic lice on the mummified corpse of a 15th century king, Ferdinand II of Naples. Lice and combs used to remove them have turned up in royal Egyptian tombs. The oldest direct evidence of our long association with head lice comes from 10,000-year-old human remains at an archaeological dig in Brazil. Archaeologists recovered a well-preserved louse egg, or nit, attached to a human hair. Genetic studies make clear that lice have been with us much farther back than recorded history. University of Utah researchers worked out a louse family tree by comparing the gene sequences of human head lice with those of lice that afflict chimpanzees. The lineage goes back 5.5 million years, to the last common ancestor of humans and chimps.

Others have used lice genetics to explore and verify milestones in human history. A scientist at the Florida Museum of Natural History and his colleagues analyzed lice genes to corroborate the “out of Africa” theory that the human species emerged from a small band of early humans who ventured from Africa more than 50,000 years ago. Another group used lice to come up with a date for the origin of clothing. Researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, tracked down the origin of body lice, a species related to head lice that split off to occupy a different niche: clothing. The researchers calculated that body lice originated about 72,000 years ago and no sooner than about 114,000 years ago — evidence that clothing may have been a surprisingly recent innovation in human history.

So my son was in the company of kings and pharaohs. We were still a bit freaked out and used insecticidal shampoos indiscriminately, lathering up both kids’ heads — and their mother’s — without bothering to check them for lice. I later learned that only those with live bugs in their hair merit such treatment.  We also wasted time on excessive housecleaning. Head lice, research has shown, can survive only for about six to 26 hours away from a human host’s warmth and readily available blood meals. Nits on detached hairs can remain viable for up to a week, but hatchlings die quickly if they can’t feed on blood. They mostly spread by direct head-to-head contact. Which means humanity is nowhere close to getting rid of them. Each year in the U.S., head lice infest something like six million schoolchildren. Over the past decade, head lice have evolved resistance to the most widely used chemical treatments in our arsenal: pyrethrins extracted from chrysanthemum flowers and the synthetic version called permethrin. Nit combing alone can eliminate an infestation — but only if repeated every day for two to three weeks.

We spotted many stunned but active survivors on my son’s head the morning after his insecticidal shampoo. Using a fine-toothed comb on my son’s tangled mop, I removed dozens of lice and nits, which the gravid females glue to individual hair shafts. I worked the teeth through a handful of his hair as close to the scalp as possible and drew the comb out to the ends, dipping it in a bowl of scalding water between passes. The process took about thirty minutes — an inconvenient chunk of time during the morning rush to get ready for work and school. But as the days passed, I began to feel a deep satisfaction during our grooming sessions. My hands drew the wet comb through my son’s hair with practiced ease and my mind drifted in a meditative fashion. I imagined a kinship with some million-year-old hominid ancestor pinching lice from the pelt of his beloved offspring.

[I had fun writing this last spring for a class in narrative nonfiction writing during my fellowship at MIT. The assignment was to work the exposition of facts into the narrative of a story.]

Image: a male head louse by Gilles San Martin from Namur, Belgium [CC-BY-SA-2.0], via Wikimedia Commons