I was reading The Skeptical Adaptationist (Randy Nesse's blog) and came across the discussion about the most important invention of the past 2000 years. He suggested printing. I would say the contraceptive pill, because of its effect on human behaviour.
Feminism has been around at least since the French Revolution, but it only began to take off when the pill gave women control over their own fertility. I remember a discussion at breakfast in my Oxford college in 1961. The other students, all male, were excited at the thought that women would no longer have an excuse not to yield to them. I argued that this would mean the end of marriage as we knew it and perhaps the collapse of parenthood. At the time all my women friends were scrambling to get engaged before their last undergraduate year ended. Every week someone else started showing off her ring.
And what has happened? Where the pill is freely available hardly any students think of marriage. People live together for years without marrying. The age of first pregnancy has moved from the mid twenties into the early thirties. A large proportion of young women say they never want babies, and some are freezing their eggs in anticipation of perhaps choosing motherhood when they are past menopause. The birth rate in most of Europe has fallen below replacement rate. In fact this one factor alone has slowed down the world's population growth and may reverse it just in time to prevent calamity.