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Throughout our evolutionary journey from ape-like primates to bipedal apes to large-brained humans, we have had the company of an extraordinarily loyal companion: Pediculus humanus, also known as the human louse.
And meanwhile, the lice have engraved this journey into their genes. TO new studyFor example, he discovered that some lice in America are hybrids of those brought there by Native Americans and others brought there by European settlers across the Atlantic.
“Humans don’t live in a bubble,” said Marina Ascunce, an evolutionary geneticist at the U.S. Department of Agriculture and author of the new study. “Lice are part of our lives and our history.”
Lice usually live on people’s heads, clinging to hair shafts, piercing the scalp and drinking blood. Unable to survive away from human bodies, parasites jump from one person to another. If people are crowded together in unsanitary conditions (such as in an overcrowded prison), lice can spread to clothing and feed on other parts of the body.
Other mammals and birds have their own lice. Each species of parasite has exquisite adaptations to its particular host, whether it be a penguin or a bat. This intimate association is ancient. In Germany, paleontologists discovered a 44 million year old louse with pieces of feathers preserved in its belly.
Lice fossils are too rare to reveal much about their history. But your DNA contains many additional clues. By analyzing the genetic material of lice, entomologists can build their family trees, revealing which species are most closely related.
Often, the closest relative of a louse species lives on the closest relative of its host. For example, in the early 2000s, David Reed of the University of Florida and his colleagues discovered that human lice are most closely related to lice living on chimpanzees and more distantly related to lice living on monkeys. . In other words, for about 25 million years, our lice have been following us along our evolutionary path.
That’s not to say that lice are completely loyal. Another species, Pthirus pubis (better known as crabs), inhabits only human pubic hair. Crabs are not closely related to lice. Instead, Dr. Reed and his colleagues discovered, his closest cousins are the lice that live in gorillas. Early human ancestors may have collected crabs while sleeping in an old gorilla nest or feeding on gorilla carcasses.
In another provocative study, Dr. Reed and his colleagues compared human lice from different parts of the world. They looked at genetic material known as mitochondrial DNA, which is passed only from females to their offspring. The researchers found that many lice belonged to one of two lineages. Surprisingly, those lineages diverged from a female louse that lived perhaps a million years ago.
Dr. Reed and his colleagues speculated that this deep division occurred when humans expanded out of Africa. Along with their own lice, they contracted lice from Neanderthals or some other extinct group of humans.
More recently, lice researchers have focused their attention on the chromosomal DNA that lice inherit from both their mothers and fathers.
In 2010, Dr. Ascunce joined Dr. Reed’s team and led an effort to collect such DNA from a broader area of the world.
In the new study, published Wednesday in the journal PLOS ONE, Dr. Ascunce and her colleagues analyzed the DNA of 274 lice collected from people in 25 locations around the world, including Honduras, France, Rwanda and Mongolia.
The DNA revealed two geographic groups of lice. One was present in Africa, Asia and America. Among these lice, researchers found a close genetic link between Honduras and Mongolia. They suspect this relationship is a sign that the Asians who first spread to the Americas about 23,000 years ago brought lice with them.
The remaining lice formed a second group, which the researchers found in Europe, as well as the United States, Mexico and Argentina. The researchers also found 33 hybrids from the two groups, 25 of which lived in America.
Dr. Ascunce and her colleagues see these results as a chronicle of modern history: European settlers sailed to the New World, bringing their lice with them. In the Americas, the second group spread and sometimes ended up on the heads of people who were already infected with lice from the first group.
But if these lice are actually colonial hybrids, Dr. Ascunce and her colleagues are baffled that they haven’t found more. The rarity of hybrids could be the result of some type of barrier to interbreeding. It’s possible that the two groups of lice were isolated from each other for so long that they acquired mutations that didn’t work well when mixed again.
Dr. Ascunce said lice researchers are just beginning their work. In the new study, she and her colleagues looked at just 16 small regions of the louse’s DNA. The next wave of research will examine the entire genome of the louse, and she hopes this new data will provide more insight.
It may be possible, for example, to understand how human lice evolved the ability to move from the head to the body and why only body lice carry microbes that can cause diseases such as typhus. And researchers may be able to pin down exactly how our ancestors contracted the lice that still plague us today.
“The genetic information we are seeing in today’s human lice can still tell us things about our human past,” Dr. Ascunce said.