Final answer:
Social inequality first became a fundamental feature of social life during the Medieval era, with the feudal structure defining social classes based on land ownership. The Modern era, particularly the Industrial Revolution, further transformed social dynamics by introducing new means of wealth and social mobility.
Step-by-step explanation:
Social inequality as a fundamental feature of social life first emerged during the Medieval era. During this time, social classes were largely defined by the feudal structure, with land ownership determining hierarchy and wealth distribution. The Industrial Revolution in the Modern era later brought about significant shifts, providing new means of wealth and social mobility.