FOUR LEAPS. “Taking a long historical long view, we see steady total growth rates punctuated by rare transitions when new faster growth modes appeared with little warning. We know of perhaps four such “singularities”: animal brains (∼600 MYA), humans (∼2 MYA), farming (∼10 kYA), and industry (∼0.2 kYA).”
http://intelligence.org/files/AIFoomDebate.pdf