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That it took hundreds of thousands of years to get from hand ax to ax with handle suggests that as of 50,000 B.C., during the Middle Paleolithic, the social brain was not humming very vibrantly. There were only 2 million or 3 million "neurons"--a.k.a., people--scattered across the whole planet, and lacking fiber optics or even postal service, they weren't exactly in constant contact.
Still, even back then, the social brain, through positive feedback, was maturing. With each advance in subsistence technology, survival grew more secure, hastening population growth; and as population grew, the advances came more quickly. By the Mesolithic Age, around 10,000 B.C., with the neuronal population up to around 4 million, the rate of advance had moved from one major innovation per 20,000 years to a sizzling one per 200--including such gifts to posterity as combs and beer.
It was around this time that, as the economist Michael Kremer has noted, Mother Nature happened to conduct an experiment that underscored the value of large social brains. Melting polar ice caps severed Tasmania from Australia and the New World from the Old World. Thereafter, just as you would expect, the larger the landmass and hence the population, the faster subsistence technology progressed. The people of the vast Old World invented farming before the people of the smaller (and, at first, thinly populated) New World. And the Aborigines of yet smaller Australia never farmed. As for tiny Tasmania, modern explorers, on contacting the Tasmanians, found them lacking such Australian essentials as fire, bone needles and boomerangs.
Farming, which took shape in the Old World around 8,000 B.C. and in the New World a few millenniums later, is a much misunderstood meme. Anthropologists sometimes call it an "energy technology," since food does, after all, energize us; but farming may have originally mattered more as a kind of information-processing technology. By radically increasing the human population that a given acre could support, farming sped up the synergistic exchange of cultural information, lubricating innovation; it packed lots of neurons together, raising both the size and the efficiency of social brains.
The results were epoch making. In both the New World and the Old World, within a mere 5,000 years of the inception of farming, there were dazzling technological advances, including monumental temples, big dams and, above all, a whole new information technology: writing.
Early writing didn't spur invention the way writing does now. There were no technical journals to convey news of inventions, no patents to file. No, the main service of writing, like that of farming, was to permit bigger, faster social brains; to allow neurons to be packed more densely still, further boosting intellectual synergy. After all, it was via writing that royal bureaucracies kept large cities functioning. And writing also meant clear, precise legal codes, which kept urban life peaceful, even though people now lived cheek by jowl with lots of other people who were neither friends nor family.