Answer:

The first chicken eggs

But wait—weren’t there some scientists who claimed that, in fact, the chicken came first?

This claim came from some researchers studying how chicken eggshells form. Eggshell is mostly made from calcium carbonate (CaCO₃). Hens get their supply of calcium for eggshell production from dietary sources (calcium-rich seafood shells, such as oyster or prawn shells, are a popular snack for backyard hens for this reason).

To form a shell, the calcium needs to be deposited in the form of CaCO₃ crystals, and hens rely on specific proteins that enable this process. One such protein, called ovocleidin-17 (or OC-17 for short), is only found in the ovary of a chicken, leading to the suggestion that the chicken must have come before the chicken egg, since without OC-17, there can be no chicken egg formation. (Interestingly, it seems that this protein is responsible for speeding up the rate of eggshell formation, enabling hens to build an egg from scratch and lay it within a 24-hour timeframe.)

Three chickens in a compost heapDomestic chickens are extremely efficient egg layers, capable of producing a fresh egg roughly every 24 hours. Image adapted from: Australian Academy of Science

So, can we lay this age-old conundrum to rest? Or are scientists and philosophers still scrambling to find an answer?

At the end of the day, the question is something of a false dichotomy. Eggs certainly came before chickens, but chicken eggs did not—you can’t have one without the other. However, if we absolutely had to pick a side, based on the evolutionary evidence, we’re on Team Egg.

This article was written by Emma Berthold, Content Producer, Australian Academy of Science and has been reviewed by the following experts: Professor Rick Shine AM FAA Professor of Evolutionary Biology, School of Biological Sciences, University of Sydney; Associate Professor Trevor Worthy Vertebrate palaeontologist and Vice-Chancellor's Postdoctoral Research Fellow, College of Science and Engineering, Flinders University; Dr Walter Boles Senior Fellow, Ornithology Section, Australian Museum