Dust mites are known as tiny scavengers, living on the vast pall of dead skin shed by the animal world—and while they clean up the world in their own little way, they do so at the expense of triggering allergies in up to 1.2 billion people worldwide.
But the microscopic dust mite also turns out to be a peculiar example of evolution “in reverse”—or more precisely, despecialization—according to a new study by University of Michigan biologists Pavel Klimov and Barry OConnor. And this counters a long-held assumption in evolutionary biology known as Dollo’s law (after the 19th-century Belgian paleontologist Louis Dollo), which stipulates that once you gain a complex trait, you can’t return to the simpler states of your distant ancestors.
This poses a conundrum in the mite world. The distant ancestors of dust mites evolved to be able to feed directly on a bird or mammal—to become parasitic and therefore more complex. So how did the free-living dust mites get to be free living, and thus less complex? There were 62 different hypotheses in the academic literature trying to answer this question.
The 63rd looks like it will settle the issue. Klimov and OConnor embarked on an epic quest to sequence the DNA of 700 of the world’s mite species chosen to produce a representative family tree. Sixty-four biologists from 19 countries collaborated. Finding one vital species of mite that happens to live only in the quill feathers of hummingbirds, took them, says Klimov, a month of fieldwork in Mexico.
But the result was clear. “The authors have made a compelling case that free-living dust mites evolved from a parasite, thus providing another counterexample to Dollo’s law,” says Jeff Gore, a biophysicist at the Massachusetts Institute of Technology who did groundbreaking work on bacterial evolution. As Klimov explains, “we found that very specialized organisms like parasites can drastically become despecialized.”
What about very specialized organisms like humans? If we look at the human “family tree,” says OConnor via email, “we see lots of evidence of what we term ‘regressive evolution.’” That we’re no longer covered in dense hair, like our primate relatives, is one example—as is our very small appendix. “In our strictly plant-eating relatives, this organ, called the caecum, is quite large and serves as a place where special bacteria break down tough plant fibers,” he explains. “When our ancestors added much more easily digested meat to our diet, a large caecum was no longer needed, so chance mutations that caused it to shrink were no longer a disadvantage.” In other words, we used to be more specialized at digesting vegetables, which explains a lot about our dietary preferences.
More to the point for modern humans, having a detailed family tree for mites could help us understand how their digestive enzymes, which end up in their waste, cause miserable allergies for so many people. Part of the problem, says OConnor, is that our two species haven’t spent enough evolutionary time together getting to know each other. We’re frenemies, but there’s plenty of evidence that the more complex organism in this relationship is slowly adapting.