Lice Are Evolving Faster Than Humans

About six million years ago there was a grand split in the fates chimps and humans. Leaving behind the last common ancestor between the two, humans went on to become the weird beings overrunning the Earth today, and chimps became the weird beings that humans put funny hats on. Both varieties of creature are known to harbor lice, grody insects that live parasitically off of the skin and blood of a variety of mammals. One interesting feature of lice is that different genetic strains of the creatures tend to be specific to different varieties of mammals. Chimps have chimp lice and humans have human lice and never the twain shall meet, basically.

This makes for a particularly convenient natural laboratory to study genetic mutation. Because human lice evolve in concert with humans, it’s reasonable to look at the two species relative to one another. They’re partners in evolution. You wouldn’t be able to do the same thing with humans and most any other thing that isn’t dependent on a human body through its entire life-cycle. The neat fact of evolutionary divergence between humans and chimps and human lice and chimp lice gives us interesting vantage points from which to measure molecular evolution.

“Humans are chimps’ closest relatives and chimps are humans’ closest relatives—and their lice are each other’s’ closest relatives,” said Kevin Johnson, the lead author of a new study comparing the rates of this molecular evolution. Said study set out to discover if evolution moves at a constant rate between different creatures, or if the pace by which portions of the genetic code are swapped out in different sorts of life varies among species. Using a wide swath of 1,534 genes shared by the primates and their primate lice partners, Johnson et al found that indeed, the rate of mutation is quite different. And the lice are winning.

“For every single gene we looked at, the lice had more differences (between them) than (were found) between humans and chimps. On average, the parasites had almost 15 times more changes,” Johnson said in a press release.

It doesn’t seem very fair. Despite the quick rate of molecular evolution, lice are still pretty crappy animals and humans have smart-phones and pants (and parasites, interestingly). Lice are switching up their genetic code at a rate 15 times that of humans, but human lice haven’t made a whole lot of progress from their chimp-dwelling days. The reason, it seems, is that a much larger proportion of the genetic changes in human beings target protein structure, while the changes in lice tend to be silent or, worse, harmful. And of course harmful mutations are weeded out by natural selection.

“Any difference that we see between species at the morphological level almost certainly has a genetic basis, so understanding how different genes are different from each other helps us understand why different species are different from each other,” Johnson said. “Fundamentally, we want to know which genetic differences matter, which don’t, and why certain genes might change faster than others, leading to those differences.”

In any case, lice may be evolving faster, but humans are still evolving better. Or at least they have been over the past six or seven million years. Give humans another million or so years and lice might be winning on both counts.