INTRODUCTION

As of 1995, over 140 million people worldwide lived at altitudes exceeding 2500 m (Niermeyer et al.,2001). The effects of hypobaric hypoxia – defined as a low environmental oxygen partial pressure – on cellular metabolic function, growth and development, physical activity, reproduction, and human health have made high altitude a unique setting in which to investigate human adaptation. This is especially true for traits that are directly or indirectly related to oxygen (O2) transport. Following Niermeyer et al. (2001), the term adaptation will be used to refer to a feature of structure, function, or behavior that is beneficial and enables survival in a specific environment. Such features may be genetic in origin, although features that arise via developmental and/or physiological processes may also be termed adaptations if they enable survival (see Chapter 2 of this volume). The term genetic adaptation will refer to a heritable feature that was produced by natural selection (or other force of evolution) altering allele frequencies over time. A developmental adaptation will refer to an irreversible feature that confers survival benefit and is acquired through lifelong exposure to an environmental stress or stressors. Developmental features that arise without clear adaptive benefit will be termed developmental responses. Finally, the term acclimatization will refer to a time-dependent physiological response to high altitude, which may or may not be adaptive.

Around the world, various regional populations show a wide diversity of time-depth exposure to altitude providing a “natural experiment” to test hypotheses of developmental and/or genetic adaptation (Moore, 1990).