We loped naked out of Africa, dressed in little more than parasites. Early humans radiated into Europe and Asia clad in trematodes, tapeworms, bacteria, mites, and lice. We have come a long way from those cave-dwelling, howling, grunting, stone-pounding days. We have built skyscrapers where there were plains. We have landed on the moon. But through it all, lice have held tight.  In 1998, 1 in 4 American children had head lice, more than were infected a hundred years ago, and probably as many as our naked progenitors. Lice are gaining on us. If they have a say (and it appears they do), man is less an island than a ship, coming together to exchange stowaways in the night. The story of human lice is really a story about us, where we go, and where we have been. It is a story that begins when the bodies of the continents were still fused.

120 million years ago, as North America and South America began to split along their bellies, early bird lice eeked out a living between feathers and skin. It was not a good life, but the view was good and the ride was thrilling. Lice, being lice, of course, knew not where they had been, nor where they were going. They hung on. When full of blood they mated, laid eggs, and died, still clutching their one necessity. It is a doggedness I can’t help but admire. On two occasions, lost lice made the transition from birds onto grubby, ground-hugging mammals. Over millennia some of those ship-jumpers would become the Anoplura, the mammal-sucking lice. A few of those lice would go on to become primate sucking lice, our familiar foe.

Lice seldom switch host species and so the evolutionary story of lice parallels the evolutionary history of hosts. Louse evolution tells a rough version of the story of vertebrate evolution. Men and women in two competing labs are currently working furiously to understand the evolutionary story of primate lice, that we might also understand something more about our own story. But what we already know about the evolution of primate lice is telling. Thousands of lice have been collected from zoo primates and less often wild, animals. Based on their morphology and genes, our lice appear most closely related to those of two of our closest living relatives, gorillas and chimps. The closest living relatives of our head lice and pubic lice suck on chimpanzees and gorillas respectively. From this evidence alone we can infer that Neanderthals probably had lice, as did Lucy, and each ancestor before her, stretching back in time to the first awkward primates, hiding in branches of ancient trees.

Lice thus came with us from the beginning, down from the trees, across the savannas, out of Africa, and on, and it seems they are likely to follow us wherever else we may go (whether that be new planets, or as seems more likely, extinction). Our ancestors carried their lice to Europe, Asia, the New World, and to each remote outpost of humanity. Lice have now been found on Peruvian and Egyptian mummies and in nearly every other archaeological site where anthropologists care to parse ancient hairs. There are two species of human lice; head lice and pubic lice (Pediculus humanus, and Pthirus pubis respectively). Body lice live on clothing and are a subspecies of head lice that appear to have evolved with the advent of the first shirt, when there was enough fur on the average human back to make the trip from head down comfortably. Body and head lice remain kissing cousins. Body lice like to live and breed in clothes, and head lice in hair, but on lonely nights the two will occasionally meet on a furry neck to rendezvous. Historically, body lice were responsible for carrying relapsing fever, typhus, trench fever, as well as a variety of other nasties that catch a ride in the lice gut to their hosts’ bloodstream. Head lice do not appear to transmit any of these disease, though no one is sure quite why. With some luck (bad I suppose), you may spy either head or pubic lice on your own tangling hairs. If you do (and it is better to have both for comparison), note the thick legs on pubic lice, evolved to grasp the flat, relatively thick pubic hairs, and the wider grippers of head lice intended for your own round locks. Head lice can straddle two hairs at once, climbing, as one might climb between two trees.

Because human body temperature and blood changes little with the environment, lice probably notice little difference between Cambridge and the Kalahari. Their geography is more anatomical, that of nose, neck, ears, and genitals. Our smooth stomachs, and napes are the louse equivalent of the Sahara; dry and inhospitable, exposed to the elements. A head louse can no more survive on your neck than on the moon. Lice are isolated in our hair islands, until we come together. When our parts do meet, lice move quickly from body to body. They have to. In a schoolroom head lice can pass over several heads in an hour. If you painted the head lice on your child’s head and then looked at them at the end of the school day (this experiment has actually been done), they will have dispersed with vigor to any head her head touched. Pubic lice are more sluggish, but cross the brief bridges of hair from one human to another at opportune moments. It is all in the timing.

Because lice travel almost exclusively hair to hair (lice can survive in hats and on toilet seats, but only for a short while), the status of lice tells us much about our personal history and the history of our species. We touch each other. Lice count on it. Where lice prosper appears to be where humans have the most contact. Our children are most likely to get head lice because they push each other into piles, hold hands, play tag, pull hair, and bump heads. Girls, who typically have more physical contact with one another than do boys, also get more lice. As adults we have relatively little physical contact beyond our partners and closest loved ones. Who was the last person whose hair touched yours long enough for a louse to run across? In other cultures and in other times, the patterns are reversed, children seldom get head lice and parents often do.

Lice provide insight into our social interactions and also into the way we deal with pests. Most human cultures have developed ways of fighting lice. In Western society, we often remove lice chemically, which on the whole has lead to chemically resistant lice and an increase in louse populations. Often, as in the case of applying DDT to one’s scalp, it has resulted in more damage to the host than to the parasite. Our ancestors dealt with lice more simply―they groomed, with fingers, and combs, brushing nits and adults away. Grooming for lice has a long tradition. Chimps groom one another and pick out lousy bits, as do bonobos, baboons, gorillas, and most other primates (Orangutans have shed their itch and swing branch to branch unencumbered). Ninety eight percent of macaque grooming is for lice. Grooming in primates plays a variety of social roles including establishing hierarchies, and placating aggressors. It has even been proposed that grooming was a precursor to human language. What is clear is that grooming is a socially important ritual. Were our ancestors not compelled to groom by lice, ticks, and the other great infesters, humans and our social interactions might have evolved much differently than they have. We might never have left Africa.

Regardless, the immediate effect is to remove lice, ticks, dirt, and other bits, and to release endorphins in both the groomer and the groomie. We inherited this circuitry. Grooming has the same effects on us as on the great apes, both in removing lice and in triggering the pleasure sensors. Grooming might not always get rid of lice (there is even anecdotal evidence that grooming helps lice spread), but it pleases both those being groomed and those grooming. If you weren’t pleased picking lice out of your screaming son’s hair, it may have had more to do with a cultural aversion to creepy crawlies than to what the pleasure sensors were saying.

We are unlikely to ever be pleased by lice, but we might be interested in what it is they teach us. We share not only our immune systems, but also the things that hairy them. The lice on your child’s head are the most recent addition to a lineage that has gone from head to head to head, back to when those heads were furrier, when they were in trees even. It is a lineage that should remind us of our kinship, but the lice know not. They walk from one head (or in the case of pubic lice, one sex part) to another. They have hopped ship to ship for millions of years, sucking blood, breeding, laying eggs, and evading chemicals and nimble fingers. As lice insert their intricate mouthparts into our blood vessels, they are performing a ritual more ancient than our kind. Take solace in losing the lousy battle. It is a battle long unwon, but a loss that measures our coming together, head to head, skin to skin, and most importantly for the lice, hair to hair.

Originally appeared in BBC Wildlife Magazine