The question 'What came first: Chicken on the egg?' is one of the most baffling of evolutionary conundrums. Everyone from scholars to schoolchildren have been stumped by the question, but scientists seem to have found the answer. According to researchers from University of Bristol, the early ancestors of modern birds and reptiles may have given birth to live young rather than laying eggs, as per The Times. A study detailing the discovery has been published in the Journal Nature Ecology and Evolution.
Along with researchers from Nanjing University, scientists challenge the existing belief that hard-shelled eggs were the key to the success of amniotes - animals whose foetuses develop inside an amnion (membrane or sack) inside the egg.
"The amniotic egg is very different from the anamniotic egg of extant amphibians, which lacks an eggshell and extraembryonic membranes. The amniotic egg consists of a suite of fetal membranes, including the amnion, chorion and allantois, as well as an external shell that can be either strongly mineralized (as in rigid-shelled eggs) or weakly mineralized (as in parchment-shelled eggs)," the study said.
The research, led by University of Bristol's School of Earth Sciences, studied 51 fossil species and 29 living species categorised as oviparous, that lay hard or soft-shelled eggs, or viviparous, that give birth to live young, according to the outlet.
The study showed that all the branches of Amniota, including mammals, show signs of retaining embryos within their bodies for extended periods of time.
While the hard-shelled egg has often been seen as one of the greatest innovations in evolution, this research implies it was this extended embryo retention that gave this group of animals the ultimate protection.
Professor Michael Benton, from the University of Bristol, said: "Our work, and that of many others in recent years, has consigned the classic 'reptile egg' model of the textbooks to the wastebasket. The first amniotes had evolved extended embryo retention rather than a hard-shelled egg to protect the developing embryo for a lesser or greater amount of time inside the mother, so birth could be delayed until environments become favourable."
"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," Project Leader Professor Baoyu Jiang added.
