MARRIAGE AND THE MISSING LINK
Our immediate human ancestor, Homo Ergaster, emerged 1.7 million years ago. The males of Ergaster's ancestors, Australopithicus and Habilis, were twice as large as the females - meaning they were polygamous. In species where males have a lot of competition for access to mates, the bigger the male is the greater his selective advantage. The more sex he has with as many females as he can, the more his genes get replicated.
This changes with Ergaster. The males became 50% bigger than their ancestors, while the females became 70% bigger. This reduction of dimorphism signals that the greatest revolution in human society was occurring: pair-bonding. One man mating with one woman, the nuclear family as the foundation of a human community.
"Pair-bonding" is what paleontologists call it. We call it marriage. Marriage - monogamous pair-bonding for mutual support and to raise children (human infants are far more helpless than those of other primates - this again starts with Ergaster) - is what made us human. Marriage is the missing link.

