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Dollo's law states that evolution is a one-way street, a never-ending march toward progress. It is considered that, once an animal develops specialized traits, it cannot go back to living the lifestyle of its ancestors. According to a study recently conducted by researchers from the University of Michigan, Ann Arbor, that is not quite true. Their study of the common house dust mite - the bane of existence for 1.2 billon people - has indicated that evolution is, in fact, reversible.

The researchers studied dust mites because they are at once common and poorly understood. Mites are related to spiders and are one of the most diverse species in the world. Still, it is not known how the speck-sized animal originated. In fact, there are about 62 current hypotheses that speculate on the origins of the creature, believing them to either be descendants of free-living creatures, like themselves, or of parasites that feed off hosts.

The study evaluated all of these hypotheses. It was quite an ordeal, requiring 64 researchers in 19 countries to obtain specimens of 700 mite species, no small feat considering that many mite species are associated with rare mammals and birds. Then researchers analyzed five nuclear genes that were present in each species. From there, they created detailed evolutionary trees and statistical analyses.

In what might seem like a win for the parasite branch of dust mite evolution, researchers found that house dust mites' closest ancestor was the skin mite, which lives on human skin and in the ears of cats and dogs. This mite is a full-time parasite. However, that dust mite's ancestor's ancestor was a free-living animal.

"Parasites can quickly evolve highly sophisticated mechanisms for host exploitation and can lose their ability to function away from the host body," Pavel Klimov said in a statement. "They often experience degradation or loss of many genes because their functions are no longer required in a rich environment where hosts provide both living space and nutrients. Many researchers in the field perceive such specialization as evolutionarily irreversible."

The study was published in the journal Systematic Biology.