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Which came first, the lizard or the egg? In the case of at least one lizard, we have an answer: the live-bearing lizard came first and only later evolved the ability to lay eggs. It鈥檚 a rare example of a species re-evolving a complex trait that had been lost.

The common lizard is just that. It is found across a broad swathe of Eurasia, from Ireland in the west to Japan in the east. Its name means 鈥渓ive-bearing鈥� in both Greek and Latin, and as you might expect it gives birth to its young.

But there are exceptions. Two small populations on the edge of the common lizard鈥檚 range lay eggs. One of these subspecies is found around the border between Spain and France, and the other in the southern Alps.

Biologists had assumed these subspecies were the last remnants of an egg-laying ancestral population from which the live-bearers evolved 鈥� something that seems to have happened over 100 times in reptiles. But when they started doing simple genetic tests around a decade ago, the data didn鈥檛 fit this simple story.

One explanation for the genetic results was that live-bearing evolved at least twice. Another was that egg-laying re-evolved in one population, but this was dismissed by many as unlikely.

Making some eggs

鈥淭here is not really any consensus,鈥� says of the University of Glasgow, UK. So 听her team collected 76 lizards from around Europe and carried out more detailed genetic studies, looking at over 200,000 sites in the genome. They used this data to build a detailed family tree of common lizards.

This tree shows that the egg-laying lizards in the southern Alps are a remnant of the ancestral population. Live-bearing lizards evolved once from this ancestral population and split into several different groups. In one of these, evolution went backwards and egg-laying re-evolved, giving rise to the Spanish population.

鈥淚 consider this strong evidence of regaining egg-laying,鈥� says of Southeastern Louisiana University. In 2015 she found . But what we don鈥檛 know, she says, is why or how eggs re-evolved in common lizards.

It must have happened relatively recently, as these lizards only became live-bearing around 2 million years ago. Perhaps the genetic programme involved in egg-laying lay dormant for that time, then was simply re-activated. Elmer鈥檚 team is now working out what genetic changes occurred.

However, in the snakes known as , . It is unlikely that the original genetic program survived this long, so the sand boas may have re-evolved eggs from scratch.

Making a U-turn

It is becoming clear that it is very common for evolution for change direction. For instance, on one island on the Galapagos ground finches evolved thicker beaks during a period of dry years when small seeds were rare, then re-evolved thinner beaks when the climate became wetter.

But it is a different matter for evolution to go into reverse long enough for a trait as complex as egg-laying to reappear. 鈥淚 think it鈥檚 quite uncommon,鈥� says Elmer. 鈥淚 don鈥檛 know of many other cases.鈥�

But it undoubtedly does happen, contrary to a claim by 19th-century biologist Louis Dollo. His assertion that such reversions are impossible, although only ever a hypothesis, is wrongly called Dollo鈥檚 law.

Other examples include metamorphosis re-evolving in salamanders that had lived their entire lives as juveniles, toes reappearing in snake-like lizards that had lost them, wings re-evolving in several stick insects and . More examples may emerge from genetic studies.