Humanity could be the first SI (that we know of). Humanity has developed superior heuristics (scientific method), memory capabilities (writing, information storage), information processing speed (language, Internet), and data gathering (any instrument of science).
The scientific community can be thought of as one massive parallel computer to work on a vast number of issues simultaneously. Guided by the rigorous (most of the times, that is) application of the scientific method, the scientific community constantly diagnosis its current knowledge base. The scientific community constantly shifts resources to problems that are more complicated and/or hold greater promise than others.
The AI and SI movements can be viewed as attempts to create more efficient and robust nodes within the neural net of the scientific community. Take that costly human physicist who works only 15 hours a day and suffers from rigid heuristics after a few decades and replace him/her with a tireless research operating at superior speeds and malleable heuristics.