According to a collaborative study published in Nature Ecology & Evolution by the University of Bristol and Nanjing University, researchers analyzed 51 fossil species and 29 living species, revealing that some early reptiles, the ancestors of modern birds, did not lay eggs but gave birth to live young.
The Evolutionary Origin of Shelled Eggs
Fossil evidence indicates that eggs, particularly hard-shelled ones, appeared around 195 million years ago, long before chickens existed. According to Professor Michael Benton of the University of Bristol, early tetrapods evolved into amniotes around 320 million years ago, developing waterproof skin and amniotic eggs. This biological milestone allowed ancient animals to reproduce away from water bodies, establishing a foundational reproductive system that preceded modern avian species.
The study challenged the assumption that all ancient reptiles and early bird precursors relied exclusively on external egg-laying. By categorizing specimens into oviparous (egg-laying) and viviparous (live-bearing) groups, the research showed that some early reptile ancestors actually retained embryos for extended periods to give live birth before egg-laying became a dominant evolutionary strategy for embryonic protection.
Domestication and the Modern Chicken Egg
While biological eggs existed far earlier in the fossil record, the specific chicken egg as known today required the prior existence of the chicken itself. Evolutionary biologists trace modern chickens to the red junglefowl, shaped through thousands of years of hybridization and natural selection.
Research into the biochemical mechanisms of poultry reproduction identified ovocleidin-17 (OC-17), a specialized protein produced in the ovaries of hens that is essential for constructing eggshells. Because this protein is synthesized directly by modern chickens, it suggests that chicken-specific eggs came after the development of chickens. However, some scientists believe that genetics might have caused jungle fowl to reproduce mutations through natural selection, leading to the first chicken emerging from an egg laid by a non-chicken ancestor. Consequently, while the broader category of amniotic and bird eggs existed for hundreds of millions of years, the first true chicken egg required a chicken-like progenitor to produce it.
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