Once we had tails and gills. Can we get them back? Some people say yes, because evolution can work in reverse. That's the idea underlying Dollo’s Law.
Louis Dollo was an engineer who made his money in supervising mining sites, but found his true calling in supervising paleontological digs. He became a paleobiologist, famous for excavating a group of iguanodon. After years of unearthing giants, Dollo had to have had a few questions, including the tantalizing one of why dinosaurs died out forever. Their descendants, he knew, had to walk the Earth. Those descendants had had plenty of years to evolve, once again, into terrible thunder lizards, and yet they never did. Dollo decided that they didn’t because they couldn’t. Evolution was irreversible. When a species jettisons any complex structures, they are gone forever.
Dollo’s Law was formulated in 1890, and it has not aged well. It's reasonable to believe that, once a species loses the ability to form a complex structure, it won’t easily pick up the knack again. Dollo believed that the mechanisms to develop a trait were lost along with the trait, so to reacquire it, the animal has to be exposed to the same conditions that caused it to evolve in the first place. In a world in which every other living creature is changing, and the climate and geography change too, conditions are not commonly reproduced. That being said, “it’s not likely,” is not a good basis for a law, especially one that tries to encompass the massive amount of species and conditions of life on this planet. Problems with Dollo’s Law stack up.
A group studying limpets – a species of sea snail that lost their coil and were presumed to be an evolutionary dead end – discovered that a specific species of limpet had recoiled and had lost their coil again before we got around to studying them in their present form. In 2003, it was proved that a winged species of insect had had, and then lost, wings once before in the past. The most recent Dollo defy-er is the dust mite. It’s been shown to have evolved into an independent creature that can survive on its own. Previously, it had been a parasite, and Dollo’s Law doesn’t allow for any parasite to regain its independence once it has evolved to be dependent on another organism.
But the primary nail in the coffin of Dollo’s Law is a creature that does not yet exist – the chickenosaurus. Scientists are getting a handle on expressing junk genes to turn birds back into their ancestors, the dinosaurs. This seems like a weak argument against Dollo’s Law. The animals aren’t evolving by themselves, they’re being sculpted by humans. But it strikes at the principle that supports the law. Dollo’s Law is based on the idea that the mechanisms for building a trait is lost along with the expression of the trait. The fact that chickens still carry the genes of their ancestors shows that the information is still there, and can still be exploited. We carry around information that shows that we can go back to what we once were.