Of lice and men

The loneliest Homo

When watching movies about hobbits, dwarves, and elves, I often think that our fascination with other human-like forms comes from our loneliness as a species—we are the sole living representatives of our genus. So we invent other species that might fit into our genus, creating companions for Homo sapiens.

Or…not quite that lonely

New research suggests that in our history we passed enough evenings with other members of our genus to exchange a few parasites—specifically lice—with them. Lice are very host-specific, and requires direct contact to transfer from organism to organism. Host-parasite specificity provides a tool to use the parasite to explore the evolutionary history of the host. This approach is especially handy in situations like the one we face with human evolution: little DNA data from our ancestors, but lots of information about the parasites that colonize us.

Before lice research, we used tapeworms, malaria protozoa, and human papilloma viruses to explore the contours of our family tree. All such studies agree with the fossil and genetic data we have demonstrating our origins in Africa. But the lice tell an even more thorough story with a surprise twist.

A research team that included a high-school student examined the genetics and morphology of the lice that colonize our heads and bodies. What they found was that this louse species—Pediculus humanus—has two lineages, one that colonizes both our heads and our bodies, and another that colonizes only our heads. The head-only louse is found only in the New World (the Americas), while the head-body louse occurs worldwide. The two lineages appeared to have diverged from one another 1.18 million years ago.

As the lice go, so go the Homo

It just so happens that Homo sapiens diverged from Homo erectus about…1.2 million years ago. Head-louse was an H. erectus parasite, and head-body louse was an H. sapiens parasite. When the Homo lineages went their separate ways, the lice co-evolved right along with them and formed two lineages.

They spent about a million years separated, but then something strange happened in the louse lines. They met up again on the same host, turning up on H. sapiens about 25,000 to 30,000 years ago. Head-only eventually made its way to the New World on the heads of H. sapiens.

Reunited…and it feels so…itchy

But how did this meeting of the lice occur? The only way it can: by direct contact between the two hosts. In other words, we found we were not alone. Whether or not we obtained the head-only lice via fighting, mating, or sharing clothing with H. erectus can’t be told. But for awhile there, we had company. Then pretty soon afterward, we didn’t, as H. erectus became extinct.

The lice seem to confirm one of two competing theories about our origins. One idea holds that H. sapiens emerged from Africa, spread around the world, and outcompeted other Homo species. The other theory is that H. sapiens ancestors emerged from Africa, spread around the world, and evolved into Homo sapiens while keeping genes flowing freely among populations. The lice appear to support the “out of Africa” or “replacement” school of thought. The head-body lice underwent the kind of genetic bottleneck that H. sapiens did at the same time in history, possibly because a relatively small group of humans emerged from Africa to find success through the rest of the planet, and took their lice with them.

Look to the pubes?

The research is not complete—there is still the question of how the transfer happened. Turns out, there’s another parasite that might clear up whether or not mating was the method: pubic lice. But we also seem to have a pattern of association with our generic brethren, including H. erectus and H. neanderthalensis: we meet them, and they become extinct. It’s no wonder that we’re alone now.

Flying drunk no problem for bats

Drunk New World bats fly fine under the influence

People can’t do it. When we drink, alcohol impairs all kinds of functions, including our ability to drive or walk a straight line. Bat researchers in work published in the online open-access journal PLoS ONE hypothesized that the same rule would apply to bats: the frugivorous (fruit-eating) types often encounter fermented fruits, meaning that frequently, a meal for a bat comes with the alcohol equivalent of a dry martini.

Sonar unaffected

And the humans–not for the last time–were wrong. Bats flying under the influence of a blood alcohol measuring three times the human legal limit maneuvered just fine in their human-imposed drunk tests. The test consisted of plastic chains suspended from the ceiling, requiring the bats to make their way around and through without a collision. Whether they’d imbibed sugar water or grain alcohol, the world’s only flying mammals performed equally well.

Bats may build up a tolerance

Not all bat species have this capacity. It seems that bats, like people, may vary in their alcohol tolerance. In addition, bat species like the New World bats in this study that encounter fermented fruits all the time may have a better tolerance for alcohol than bats who imbibe only occasionally. Old World bats, it appears, are less able to hold their liquor compared to their New World, daily imbibing cousins.

Alcohol: a previously unidentified force of natural selection?

Humans may have long been aware that alcohol can drive certain choices. And now, the bats may confirm that. According to the study authors, sensitivity to ethanol may have determined which bat species developed where. Just as types of fruit may have influenced the speciation of bats, the bat ability to tolerate–or not–ethanol may also have affected bat adaptive radiation.

Ideas for questions

Bats navigate by sonar, while humans rely primarily on inputs including vision to maintain balance and walk a straight line. Do you think that this difference might help explain why these New World bats don’t show the effects of alcohol in their navigation? Why or why not?

The paper refers to the bat “adaptive radiation.” What is an adaptive radiation, and what are the conditions that are required for one to occur? How did bat speciation exemplify this process?

Other frugivorous or omnivorous species encounter fermented foods, as well. One hypothesis, the Drunken Monkey hypothesis,  is that the smell of fermenting fruit drove primate evolution. Can you find other research describing the influence of ethanol on animals?

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