Damien Esquerre has been fascinated by lizards since he was a child.

Key points: Egg-laying lizards appear to have gone from eggs to birthing live young and back again

Egg-laying lizards appear to have gone from eggs to birthing live young and back again It's a transition that's believed to be extremely rare

It's a transition that's believed to be extremely rare South America's Andes mountains are 'sky islands' and hotbeds for biodiversity

Now the Chilean-born researcher has made a groundbreaking — but controversial — discovery about the scaly critters, one that challenges a fundamental rule of evolution.

He found egg-laying lizards propelled to the cold Andean mountaintops in South America had evolved to give birth to live young.

But in a remarkable backflip, a handful of the species appears to have returned to an egg-laying state when moved back down the mountain to warmer areas.

The rapid rise of the Andes mountains meant lizard species were isolated and biodiversity flourished on 'sky islands'. ( Supplied: Damien Esquerre )

"Eggs do not incubate very well at cold temperatures, and of course the mountaintops in the Andes are pretty cold. Laying eggs wouldn't work in that kind of environment, so they had to evolve giving birth to live young," Mr Esquerre said.

"We have strong evidence that suggests that they re-evolved laying eggs when they re-colonised warmer areas."

"No mammal has ever re-evolved egg laying, but these lizards seem to."

Damien Esquerre, a researcher at the Australian National University, studies the evolution of lizards in South America and pythons in Australia. ( Supplied: Lannon Harley, ANU )

This switch from eggs to live young and back again marks an evolutionary transition believed to be extremely rare, Mr Esquerre said in recent paper for peer-reviewed journal Evolution.

He said Dollo's Law of irreversibility states that once a species loses a trait through evolution, it's extremely unlikely to regain it.

"For example, snakes lost their limbs, so re-evolving limbs is very unlikely or impossible … Laying eggs is one of the classic examples of Dollo's Law," he said.

Louis Dollo first proposed the principle in 1983.

Since then, it's been widely accepted — Australian National University calls it a "golden rule" of biology — though there are numerous exceptions.

Some studies have noted that certain sand boas and vipers, for example, have re-evolved egg laying.

Some lizards in Chile have developed brown, subdued hues to blend into rocky terrain. ( Supplied: Damien Esquerre )

But his research comes with a caveat: more study is needed.

"We think it's significant and slightly controversial. We think a lot of scientists are going to — with reason — be very sceptical," he said.

"We can never be 100 per cent certain about this.

"This opens a window to start looking into this with more detail. It would be a very significant evolutionary event if true, and we have some evidence that it actually happens."

If two populations are separated, as they were with the formation of the Andes mountains, over time they're going to become two entirely different species. ( Supplied: Damien Esquerre )

'Sky islands' fuelling biodiversity

The group of lizards featured in Mr Esquerre's study are considered one of the most diverse in the world, comprising 260 species. Around half give birth to live young, while the other half lay eggs.

Between three and eight of those species could have reverted back to egg laying after evolving to birth live young, he said.

The lizards are highly adaptable — they can be found at the southern tip of South America, close to the Antarctic, and also in the Atacama desert, one of the direst places on earth.

The origin of the group of lizards studied had roughly the same age as the Andes mountains — 20 million years. ( Supplied: Damien Esquerre )

Some lizards have brown, subdued hues to blend next to rocky terrains, while those that live in the rainforest sport bright green and blue tones.

"We tried to date the origin of this group and we found that it has roughly the same age as the Andes — 20 million years — which is actually really, really young in terms of geology and evolution," Mr Esquerre said.

Lizards trapped in these cold climates adapted and evolved to birthing live young. ( Supplied: Damien Esquerre )

To get a grasp on the timeline, he pointed out that dinosaurs went extinct 65 million years ago.

"The Andes are formed by the collision of two tectonic plates, and when they collide they are smushing each other upwards, so they form mountain chains. As they were being uplifted, the number of species in this group increased. How do we explain that?

The evolution of lizard pregnancies can partly be discovered through tracing DNA and creating family trees. ( Supplied: Damien Esquerre )

"The best way to explain it is thinking of mountains as islands. As mountaintops rose, different populations of these lizards were getting trapped — just like if a group of islands gets divided, the populations of animals will be divided and isolated from each other.

"New species form when you have isolation plus time … if two populations are separated, given enough time they're going to become two entirely different species."

To reconstruct evolution at this scale, Mr Esquerre and his team worked on making a family tree of the species by sequencing and comparing DNA, to see which lizards shared similarities and which were more distantly related.

Natural selection played a large part in the findings, he said.

"The ones who lay eggs quickly will start to die off and be selected out, whereas the ones who that retain their eggs for longer and incubate it in their uterus for longer will have higher chances of survival," he said.

"Just by slowly selecting that egg retention strategy, you'll eventually end up with females that don't even lay eggs — they just incubate their eggs inside and hatch inside.

"Given more and more and more time and natural selection, you'd end up with a completely live-bearing species."

Some lizard species hold their eggs in their uterus until they hatch — a midway point between laying eggs and giving birth to live young. ( Supplied: Damien Esquerre )

Scientific scepticism

But Dr Oliver Griffith, an evolutionary biologist at the University of Melbourne who specialises in pregnancy in lizards, urged caution about the findings.

"We really need those next experiments before we can make any strong claims," he said.

"I think their model doesn't consider the difficulty of going from being a live bearing animal to an egg laying one."

Phymaturus darwini, from the Andes of the Metropolitan Region of Chile. ( Supplied: Damien Esquerre )

Dr Griffith said while scientists had a good understanding about how animals changed from egg-laying to live bearing, "we don't have any evidence at all of how it might go in the opposite direction".

But he liked the study's examination of the impacts of climate on these wildly varied lizards.

The discovery is controversial — scientists say more experiments are necessary. ( Supplied: Damien Esquerre )

"I think they've done a really good job of looking at how climate might drive the evolution of pregnancy in this group of lizards," he said.

He added that Dollo's Law was not meant to be a hard and fast rule, but "basically it's that you can't go back in time in evolution". While it may be possible to return to a certain state, "you can't really go back in the same way that you got there in the first place," by retracing footsteps, he said.

"So if you find a way to climb up a building, then you're not going to climb down the building the same way that you got up there — you might slide down a pole, rather than find individual bricks."

He said the study primarily focused on studying the historical relationships of lineages of species, but there needed to be more work on the physiological side of things — the internal machinery of laying eggs —something Mr Esquerre also noted in his paper.

"The crux is, can we get some of these other types of evidence to show this actually has occurred?" Dr Griffith said.