Short answers, pulled from the story.
Social equality requires that all individuals within society possess equal rights, liberties, and status. It demands the absence of legally enforced social class or caste boundaries and discrimination based on inalienable parts of an individual's identity.
The Abolitionist Movement fought to end slavery between the 1700s and 1800s across Britain and the U.S. Campaigns during this period emphasized the equal dignity of all humans.
Thomas Jefferson wrote the famous phrase about men being created equal in the Declaration of Independence which appeared in 1776. This document became a foundational principle for later equality movements despite limited application at the time.
Economic development and industrialization correlate strongly with increased social equality across nations as developing countries become developed. Further economic growth in developed countries continues this trend toward greater equality through education, healthcare, and democratic institutions.