This talk will be about change and how we experience it. Humans go through several transitions during the course of their life. The transition from one life history phase to the next, e.g. from infancy to childhood or from reproductive to post-reproductive life, represents a physiological challenge as well as a sociocultural one. We have been evaluating the interaction among biocultural variables underlying key life history transitions and identifying the implicated trade-offs. With a different analytical focus, we are also interested in exploring transitions at a population level, like the ones indigenous communities are experiencing all over the world. For the last 20 years, we have been working with Qom and other indigenous communities in northern Argentina. Biological and ethnographic data are combined to understand the theoretical and applied components of these life and population transitions. I will present an overview of what we have learned so far and share some of the contributions we have made to evolutionary anthropology, human biology, biocultural studies, and global public health.