In a groundbreaking study published in the Journal Nature Ecology and Evolution, researchers from the University of Bristol and Nanjing University claim to have unraveled the age-old conundrum of what came first: the chicken or the egg. Their findings challenge the prevailing belief that hard-shelled eggs were the key to the success of amniotes, a group of animals whose embryos develop inside an amnion, a protective membrane or sack.
The team of scientists examined 51 fossil species and 29 living species classified as oviparous (laying hard or soft-shelled eggs) or viviparous (giving birth to live young). Surprisingly, they discovered that the early ancestors of modern birds and reptiles might have actually given birth to live young instead of laying eggs. These findings indicate that extended embryo retention within the mother’s body provided the ultimate protection for the developing embryo, rather than relying on the evolution of a hard-shelled 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),” explained the study.
The researchers’ findings revealed that all branches of Amniota, including mammals, exhibit signs of retaining embryos within their bodies for extended periods of time. This prolonged embryo retention served as a mechanism to delay birth until more favorable environmental conditions were present.
Professor Michael Benton from the University of Bristol expressed the implications of their work, stating, “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 favorable.”
The research not only sheds light on the evolutionary history of amniotes but also challenges the notion that the hard-shelled egg was the key innovation that allowed these animals to thrive. Instead, it suggests that the extended retention of embryos within the mother’s body provided superior protection, ultimately leading to their success.
Project Leader Professor Baoyu Jiang added, “Sometimes, closely related species show both behaviors, and it turns out that live-bearing lizards can flip back to laying eggs much more easily than had been assumed.”
This groundbreaking study not only revolutionizes our understanding of the reproductive strategies of ancient animals but also highlights the intricate and complex processes that shaped the evolution of life on Earth. With this new knowledge in hand, scientists can now explore even more fascinating avenues of research, further unraveling the mysteries of our evolutionary past.
