Differentiating goose eggshells through proteomics: A case study from Xitou, North China

Eggshells are significant cultural symbols linked to funeral customs and social development. Identifying their species is crucial in archaeological research. However, their small fragment size makes eggshells difficult to accurately identify using traditional methods. Recent mass spectrometry advances have significantly enhanced eggshell species identification in archaeological research, but their success remains limited. For example, Anser cygnoides and Anser anser eggshells cannot yet be reliably distinguished. This paleoproteomic study uses Zooarchaeology by Mass Spectrometry and Liquid Chromatography-Tandem Mass Spectrometry to identify by proteotyping the species of eggshells from the Xitou site in China. We established a series of peptide markers and amino acid variants to distinguish A. cygnoides from A. anser, and with this method attribute three Xitou eggshell specimens to the graylag goose. Carbon isotope analysis and shell thickness measurements were conducted to understand the domestication of the geese associated with these eggshells. The isotope results indicate that these geese primarily ate C₃ plants, and shell thickness results suggest they were possibly domesticated. These findings firstly provide direct molecular evidence of possibly domesticated graylag geese in China and offer new insights into poultry domestication and cultural practices. SIGNIFICANCE: Combing ZooMS and LC-MS/MS analysis, we have identified novel peptide markers to distinguish the eggshells of swan goose (Anser cygnoides) from those of graylag goose (Anser anser). Using a series of ZooMS markers and amino acid variations, the eggshells from Xitou site (north China) were identified to be derived from graylag goose. Further eggshells thickness and isotope analysis show that these geese were likely domesticated and primarily fed on C₃ plants, offering valuable insights into early geese farming and sustainable practices during the Western Zhou period. Furthermore, the discovery of eggs as burial items highlights the important role that geese played in ancient Chinese culture and rituals.