David’s Tree of Lice

Note: I am at the National Science Writer’s Association meeting in Gainesville, FL! Today was the New Horizon’s sessions featuring researchers in different scientific fields. This post is about one of those talks.

David Reed paces the stage, microphone clipped to his tie, flipping through slide after wordless slide, like he’s giving a TED talk. With great enthusiasm, he tells us about human evolution and how much we’ve learned about our ancestral lines in the past few decades.

He showed a lot of pictures like this:

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And this:

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Then he starts talking about pubic lice. Apparently, the evolution of lice tells a partial story of the evolution of man. Lice cospeciate with us, meaning that we are so closely linked they evolve along side us. Their lineage line, showing when a a line splits into two genetically distinct species, mirrors ours. Lice are our evolutionary pals.

Humans have two species of lice: Pediculus Humanus, which covers head and clothing, and Pthirus Pubis, which covers, you guessed it, the pubis—crabs. Closely related in the evolutionary trees is Pthirus Gorillae, pubic lice for gorillas. In fact, Reed has shown that human and gorilla pubic lice come from the same lineage. He noted science writers headlined their stories with “Humans Got Pubic Lice From Gorillas” even though this exchanged happened about 3 million years ago.

To test the louse lineage, he had to run the genome, which meant getting samples.

“Getting pubic lice is pretty easy,” he said. The crowd giggled at this seemingly Freudian slip. “But getting pubic lice off a gorilla is pretty hard.” The crowd roared.

Shared crabs isn’t the only thing he’s learned from our lice friends. Looking at where a species splits can give us valuable evolutionary timeline information. For example, clothing lice and head lice shared a common ancestor. The split gives us an estimate on when lice traveled from our bodies to the new furry things wrapped around our shoulders—our very first clothes. The split happened between 80 to 170 thousand years ago, so Reed estimates that sometime in there humans started wearing clothing (his work estimates around 100k). This time line also corresponds to when humans were moving north to cooler climates and our relatively non-hairy skin just wasn’t cutting it.

Humans evolve pretty slowly, so it’s hard to determine our “recent” history. What Reed is calling recent is actually about 15 thousand years ago, when humans first crossed the ice bridge over to the Americas. By “studying the passengers that took that road trip with us” we can discover something about ourselves.

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