Anatomically modern humans first appeared 200 or 300 thousand years ago. For most of the time since, ten thousand generations or so, they lived as yet another unobtrusive species of the genus Homo, a middle of the food chain animal that was sparse in numbers, had a limited range in East Africa, and used the same old technologies for millenium after millenium.
About 70,000 years ago that changed rather abruptly. A host of cultural innovations occurred, including the sewing needle, watercraft of some sort, and perhaps most importantly, art. Quite abruptly our ancestors became top predators, spread over the world, and swept away the rest of the Human species and, eventually, half the genera of large mammals. What changed? How did it happen? It won't surprise even non-skeptics to learn that nobody really knows. There are some plausible hypotheses. Prof Harari thinks that language is the key. Human language is uniquely expressive. It's especially useful for gossip, one of the most crucial human communication functions in the kind of small bands people lived in for most of our existence. Best of all, he thinks, is its ability to express and allow us to comprehend hypotheticals.
Of course we lack any direct evidence that such language emerged 70,000 years ago, and we lack evidence that the other human species lacked it. The most important piece of indirect evidence is appearance of art. Art is inherently hypothetical, and some early art, for example a sculpture of a lion headed man, is prettly clearly hypothetical.
Today we know that language is essentially hard-wired into the human brain - full-featured language complete with hypotheticals. Was this the result of a series of abrupt mutations? We don't know, but at least potentially we can trace the genes for language eventually, and compare them with, say, the Neandertal genome.