Mary Leakey discovered a 1.8-million-year-old hominid skull at Olduvai Gorge, which was originally named Zinjanthropus boisei and later reclassified as Paranthropus boisei. This find, known as OH 5, was one of the first fossil ancestors to be accurately dated using the new technique of potassium-argon radiometric dating. The discovery of this robust hominid, alongside Homo habilis, definitively proved that multiple hominid species coexisted at the same place and time.

The discovery of Paranthropus boisei, a massive-jawed hominid that coexisted with members of our own genus, has been a powerful symbol of the branching, non-linear nature of human evolution. This find, which helped to disprove the 'single species' hypothesis, has inspired poets to think of human history not as a single story of progress but as a complex, interwoven tale of many different hominid lineages. The image of the robust, specialized hominid, living alongside our more generalized ancestors, is a potent metaphor for the contingency and diversity of the evolutionary process.