Ainash Childebayeva

1 Introduction Today, more than 81 million people globally live at altitudes ≥ 2,500 m (Tremblay and Ainslie 2021), which corresponds to less than 73% of the oxygen present at sea level, dropping exponentially downwards with increasing elevation. This reduced atmospheric oxygen content, known as high-altitude hypoxia, presents a pronounced physiological challenge to human health, well-being, and reproduction.

Abstract The Caucasus and surrounding areas, with their rich metal resources, became a crucible of the Bronze Age1 and the birthplace of the earliest steppe pastoralist societies2. Yet, despite this region having a large influence on the subsequent development of Europe and Asia, questions remain regarding its hunter-gatherer past and its formation of expansionist mobile steppe societies3,4,5.

Abstract The ancient city of Chichén Itzá in Yucatán, Mexico, was one of the largest and most influential Maya settlements during the Late and Terminal Classic periods (ad 600–1000) and it remains one of the most intensively studied archaeological sites in Mesoamerica1,2,3,4. However, many questions about the social and cultural use of its ceremonial spaces, as well as its population’s genetic ties to other Mesoamerican groups, remain unanswered2.