Scientists have attempted to solve the age-old question of whether the chicken or the egg came first. A recent study published in the Journal Nature Ecology and Evolution proposes that the ancestors of birds and reptiles gave birth to live young.
The researchers from Nanjing and Bristol University challenged the notion that hard-shelled eggs were crucial for the success of amniotes, which are animals that have embryos developing inside an amnion within the egg. They found that the amniotic egg, which is different from the eggs of current amphibians, consists of fetal membranes and an external shell that can be either strongly or weakly mineralized.
The study, led by the University of Bristol’s School of Earth Sciences, analyzed 51 fossil species and 29 living species. They categorized these species as either oviparous (laying hard or soft-shelled eggs) or viviparous (giving birth to live young). The findings revealed that all branches of Amniota, including mammals, showed signs of retaining embryos within their bodies for extended periods.
Professor Michael Benton from the University of Bristol stated that their work, along with other recent studies, dismisses the traditional reptile egg model found in textbooks. He explained that the earliest amniotes evolved extended embryo retention instead of hard-shelled eggs to protect the developing embryo until more favorable conditions were present for birth.
Additionally, project leader Professor Baoyu Jiang noted that closely related species sometimes exhibit both behaviors of live birth and egg laying. The study discovered that live-bearing lizards can easily transition back to laying eggs, contrary to previous assumptions.
In simple terms, the study suggests that the ancestors of birds and reptiles gave birth to live young, and the development of hard-shelled eggs came later in evolution. This finding challenges the conventional belief that the egg came before the chicken.
