Next time you ask whether the chicken or egg came first, think of the lizards in South America's Andes Mountains who laid eggs, evolved to birth live young, then evolved to lay eggs again.
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Australian National University researcher Damien Esquerré and colleagues found species of the Liolaemus lizards had basically broken Dollo's Law.
"[Dollo's Law says] once you lose a structure like limbs in a snake, or eyes in cave animals, or eggs in lizards, it's very, very unlikely to regain it," Mr Esquerré said.
He said it's been reported in two species of snakes, one in Africa and one in the Amazon.
Lizards have been known to evolve from laying eggs to birthing live young, but never back again, Mr Esquerré said.
"It is a very, very rare occurrence," Mr Esquerré.
He had originally set out to explain why the law-breaking, egg-laying Liolaemus lizards had some of the most diverse species in the world and how they'd adapted to life in the extremes on the mountain.
Mr Esquerré and his team mapped the DNA of over 200 species of the Liolaemus lizards, finding that several egg-laying and live-birthing lizards shared common DNA.
He said if you took a family tree of the lizards with a common ancestor giving birth to live young, but then discovered a later species laid eggs, you could say that species had evolved back to laying eggs.
"We have found that egg laying actually disappeared early in their history," Mr Esquerré said.
"But egg laying re-evolved later in other species."
So how did this happen? Mr Esquerré said Liolaemus lizards are as old as the Andes themselves and as the mountains they lived on uplifted, they evolved to give birth to live young.
The extreme conditions on the mountains made it too difficult for eggs to incubate.
The mountains were also so tall that it separated the lizards, creating what Mr Esquerré described as "sky islands".
"Given enough time isolated populations become different species," he said.
But it was when the lizards began to repopulate the lowlands below the mountains again that they changed, with some evolving back to laying eggs thanks to the more favourable conditions for incubating eggs.
"Live bearing and egg laying are two extremes of the same continuum," Mr Esquerré said.
He said some current species of the lizard female keep the eggs inside them for longer, where they eventually hatch inside - an early evolutionary stage of live bearing.
Mr Esquerré said it was thanks to the Andes the genus of Liolaemus lizards had some of the most diverse range of species in the world.
He said his research opened the window into looking how other mountain ranges, including the Snowy Mountains, promoted biodiversity.
