I occasionally find myself trying to teach the word ‘threshold’, usually by pointing at a nondescript strip of floor near the door.


Students find this a bit mystifying. The threshold is an imaginary space – mental as well as physical. In its more interesting sense, it refers to a point in a system at which a rapid change of state occurs. It is loosely linked to ideas of tolerance, and critical mass.

So we talk of pain thresholds (the point at which pain become unbearable), threshold knowledge (core competencies in education which, when attained, transform performance in a given discipline) and poverty thresholds (the point below which a household cannot support a minimum standard of living). And so on.

In this short animated film, Professor David Christian describes six evolutionary thresholds – points at which the development and proliferation of species on earth suddenly accelerated.


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