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Why skinks that lost their legs evolved new ones

A team of researchers affiliated with Clark University, the Museum of Natural History, Yale University and the University of Massachusetts Amherst, has developed a theory to explain why skinks living in the Philippines lost their legs over many years of evolution, and then regained them many years later. In their paper published in Proceedings of the Royal Society B, the group describes their study of the tiny lizard and what they learned about it.

One of the rules of thumb in evolutionary theory is that when a creature loses a complicated structure (like its legs) over many years, it is very unlikely that its descendants will express them later. In this new effort, the researchers have found a possible reason for an exception to the rule—Brachymeles lizards, more commonly known as skinks, which lost their legs and then grew them back again.

Prior research had suggested that some of the skinks native to the Philippines had once lost their legs due to evolution and then for unknown reasons, their legs returned many years later. Other skinks living in the vicinity had also lost their legs but they did not grow them back. In this new effort, the researchers sought to understand why it had happened with one group but not the other.

The work involved putting captured skink specimens (both those that had re-evolved legs and those that did not) through a variety of physical tasks, such as running around a tiny racecourse. The idea was to find out what sorts of conditions might be better suited to use of legs and which might be better suited to snake-like mobility.

The researchers discovered that skinks with no legs were better able to move around in dry conditions—without legs, they were better at burrowing, for example. On the other hand, skinks with legs (and feet) were better able to move around under wetter conditions. Their legs allowed them to navigate over wet soil. The researchers suggest that legs in skinks appear during long periods of wet conditions and then disappear during long dry spells.