Lice genes provide clues to ancient human history

On our evolutionary journey from ape-like primates to bipedal apes and big-brained humans, we have been in the company of an extremely loyal companion: Pediculus humanus, also known as the human louse.

And in the meantime, the lice have recorded this journey in their genes. a new studyFor example, they discovered that some lice in the Americas are hybrids of those transported there by Native Americans and others transported across the Atlantic by European settlers.

“We humans don’t live in a bubble,” says 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 commonly reside on people’s heads, clinging to hair shafts, piercing the scalp and drinking blood. Since they cannot survive without human bodies, the parasites jump from one person to another. When people are crammed together in unsanitary conditions, such as in an overcrowded prison, lice can spread into clothing and feed on other parts of the body.

Other mammals and birds have lice themselves. Each species of the parasite has exquisite adaptations to its specific host, be it a penguin or a bat. This intimate association is centuries old. In Germany, paleontologists discovered a 44 million year old louse with bits of feathers preserved in its intestines.

Lice fossils are too rare to reveal much about their history. But their DNA contains many more clues. By analyzing genetic material from lice, entomologists can piece together their family trees and reveal which species are most closely related.

Often the close relative of a louse species lives on the close 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 that live on chimpanzees and even more distantly related to lice that live on monkeys. In other words, our lice have been following us on our evolutionary path for about 25 million years.

That doesn’t mean lice are completely loyal. Another species, Pthirus pubis (commonly known as crabs), lives only in human pubic hair. Crabs are not closely related to head lice. Instead, Dr. Reed and his colleagues discover that their closest cousins ​​are lice that live on gorillas. It’s possible that early human ancestors picked up crabs while sleeping in an ancient gorilla nest, or fed on gorilla carcasses.

In another provocative study, Dr. Reed and his colleagues collected human lice from different parts of the world. They looked at genetic material known as mitochondrial DNA, which is only passed from females to their offspring. The researchers found that many lice belonged to one of two genera. Remarkably, these lineages split from a female louse that lived perhaps a million years ago.

Dr. Reed and his colleagues speculated that this deep divide emerged as humans expanded out of Africa. Along with their own lice, they picked up lice from Neanderthals or another extinct group of humans.

More recently, lice researchers have turned their attention to the chromosomal DNA that lice inherit from both their mothers and fathers.

In 2010, Dr. Ascunce joins the team of Dr. Reed and led an effort to collect such DNA from a wider part of the world.

In the new study, published Wednesday in the journal PLOS ONE, Dr. Ascunce and her colleagues collected DNA from 274 lice from people in 25 places around the world, including Honduras, France, Rwanda and Mongolia.

The DNA revealed two geographic clusters of lice. One was present in Africa, Asia and America. Among these lice, the researchers found a close genetic link between Honduras and Mongolia. They suspect that this relationship is a sign that the Asian population that first spread to the Americas about 23,000 years ago brought lice with them.

The remaining lice formed a second cluster, which the researchers found in Europe, but also in the United States, Mexico and Argentina. The researchers also found 33 hybrids of the two clusters, 25 of which lived in the Americas.

Dr. Ascunce and her colleagues see these results as a chronicle of modern history: European settlers sailed to the New World and brought their lice with them. In America, the second cluster spread and sometimes landed on the heads of people who were already infected with lice from the first cluster.

But if these lice are indeed colonial hybrids, Dr. Ascunce and her colleagues are surprised that they have not found more. The rarity of the hybrids could be the result of some barrier to interbreeding. It’s possible that the two lice clusters were isolated from each other for so long that they acquired mutations that didn’t work well when they were mixed back together.

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 louse DNA. The next wave of research will examine the entire lice genome, and she expects this new data will provide more insights.

For example, it might be possible 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 typhoid. And researchers may be able to pinpoint exactly how our ancestors picked up the lice that still plague us today.

“The genetic information we see in today’s human lice can still tell us things about our human past,” says Dr. Ascunce.