Which structure would be most affective to use to describe the timeline associated with the development of the polio vaccine

Read this excerpt from leah missbach day's foreword to wheels of change. bicycles have long played a role in my life. as a young woman, i rode one year-round before i had a car. but it was later in adulthood that the bicycle became more than a source of transportation for me. the bicycle actually began to truly shape the way i saw the world. what is the author's purpose for including this in the foreword?