Scientists reckon they've cracked the most baffling of evolutionary conundrums - whether the chicken or the egg came first.
Everyone from scholars to schoolchildren have been stumped by the chicken-and-egg question, but now researchers have discovered early ancestors of modern birds and reptiles may have given birth to live young.
In evolutionary terms, eggs were being laid by the chicken's distant dinosaur ancestors millions of years before the first chickens evolved.
But a research project has now found the chicken's earliest reptilian ancestors, dating back millions of years before the dinosaurs evolved, may not have laid eggs as previously thought.
University of Bristol's School of Earth Sciences led the study, which saw 51 fossil species and 29 living species categorised as oviparous - laying hard or soft-shelled eggs - or viviparous - giving birth to live young - were examined.
Experts there and at Nanjing University say that although the hard-shelled egg has often been seen as one of the greatest innovations in evolution, this research implies extended embryo retention - when the young are retained by the mother for a varying amount of time - actually gave this particular group of animals the ultimate protection.
Professor Michael Benton, from the University of Bristol, said: "Before the amniotes, the first tetrapods to evolve limbs from fishy fins were broadly amphibious in habits.
"They had to live in or near water to feed and breed, as in modern amphibians such as frogs and salamanders.
"When the amniotes came on the scene 320million years ago, they were able to break away from the water by evolving waterproof skin and other ways to control water loss.
"But the amniotic egg was the key.
"It was said to be a 'private pond' in which the developing reptile was protected from drying out in the warm climates and enabled the Amniota to move away from the waterside and dominate terrestrial ecosystems."
He added: "Our work, and that of many others in recent years, has consigned the classic 'reptile egg' model of the textbooks to the wastebasket.'
Project leader Professor Baoyu Jiang added: '"This standard view has been challenged.
"Biologists had noticed many lizards and snakes display flexible reproductive strategy across oviparity and viviparity.
"Sometimes, closely related species show both behaviours, and it turns out that live-bearing lizards can flip back to laying eggs much more easily than had been assumed."
Conducted by researchers from Nanjing University and the University of Bristol, the study is published in Nature Ecology & Evolution.
