Time always marches forward, of course, but does evolution?
It's certainly easy to impose a march of progress on the course of evolution. That's why the sequence of apes transforming into humans as they march from left to right is so universal. Of course, there are also pictures in which Homo sapiens, having risen up to noble, upright proportions, begins to crouch back down again, until he (never a she, I've noticed) is crouching in front of a computer or a television or facing some other ignoble end. As I wrote in Parasite Rex, this anxiety--an anxiety mostly about ourselves and not about nature--led biologists to come up with the concept of degeneration. While most life strove upwards towards more complexity, some backsliders slipped down again. Barnacles (once nimble crustaceans) were a classic example.
If a lineage could degenerate, could it then regenerate--could it recover the complexity its ancestors had lost? In 1893, the French biologist Luois Dollo declared absolutely not. It was too unlikely that evolution could retrace its steps so carefully to restore some lost trait.
Dollo's Law survived the rise of genetics and the modern synthesis of evolutionary biology, albeit it a very altered form. It was no longer an ironclad law, like the laws of physics, but instead a striking pattern that speaks to how evolution works. If a lineage of animals no longer needs some feature--eyes for example--the genes that build eyes gradually mutate, usually into dead pseudogenes. It would be a nearly impossible roll of the dice that would mutate all of those genes precisely back into the form they had before. Whales didn't' re-evolve fish fins, for example, but instead evolved paddles, for example.
To mark the centennial of Dollo's Law, the late Stephen Jay Gould wrote an influential paper in which he used coiled sea shells as evidence of the new and improved Dollo's Law. In some lineages of gastropods, the shell has uncoiled. Gould pointed out that an uncoiled shell allows a gastropods to grow flexibly around obstacles or to reach out towards sources of food. Gould made a careful study one of these groups and argued that none of its member species had ever managed to re-evolve a coiled shell. He suggested that the uncoiled gastropods had become so committed to their new way of life that natural selection could not return them back to their former coiled glory.
Now, on the 110 year anniversay of Dollo's Law, comes a fascinating report that challenges Gould. In a paper published online today in the Proceedings of the Royal Society of London, biologists Rachel Collin and Roberto Cipriani take a look at another group of gastropods--the Calyptraeidae, which includes slipper shell limpets, cup-and-saucer-limpets, and hat shells. Out of 200 species, just a dozen or so are coiled. It used to be thought that the coiled species branched off first, before the common ancestor of the remaining species lost its whorl.
But that's not what shook out when the biologists constructed a family tree for these gastropods by sequencing three different genes in 94 species. They discovered that the gastropods actually re-coiled on at least two separate occasions.
At first this seems hard to swallow. Fossils of these particular shells suggest that they had been uncoiled for anywhere between 20 and 100 million years before re-coiled species arose. How could the genes for coiling have survived all that time? Collin and Cipriani point to a study that came out earlier this year on stick insects. That study showed that some stick insects lost their wings, but that their descendants re-evolved them many times over. These re-coiled limpets may be rare, but they are not flukes.
The answer to these puzzles appears to lie in the genes that assemble these animals. In the case of stick insects, the genes for building wings were probably preserved because they continued to do something else in another part of the body--they built legs. Collins and Cipriani propose a related hypothesis for the shells. The re-coiled gastropods develop directly from eggs, but many other gastropods have distinct stages in their life cycle. As larvae, they develop one type of shell, and then as adults, they develop a completely new shell. Collin and Cipriani envision a lineage losing its coils in its adult shell, but still retaining them as larvae. So the coiling genes were still on active duty for millions of years. Then, in some lineages, these larval coiling genes were borrowed to build coiled adult shells. And finally, through other evolutionary changes, these gastropods lost their larval stage and simply developed uncoiled shells.
So evolution can, it seems, double back on itself sometimes. But only if it tucks away the secrets of the ancestors in a safe place.