The single species hypothesis: truly dead and pushing up bushes, or still twitching and ripe for resuscitation?

Frank Livingstone proclaims himself to be the last living proponent of the single species hypothesis. In sharp contrast, a species-rich, bushy phylogeny is favored by most human paleontologists. Is Livingstone's proclamation merely contrarian posturing, or does closer inspection warrant reconsideration of just how speciose the hominin lineage is? The high-speciation perspective draws on evidence of speciosity in the Cercopithecoidea and punctuated equilibria theory for support. If blue monkeys and redtail monkeys are indistinguishable skeletally, this reasoning goes, or if red colobus and black and white colobus are likewise indistinguishable, should we not expect that there are more species of hominin than is apparent from skeletal evidence alone? A contrarian perspective notes that not all monkey taxa are speciose. Importantly, two broadly distributed, partly terrestrial monkeys have not speciated at all: vervets and baboons. Nor are monkeys the first choice as a hominin speciation model. If expectations of species numbers are based on the Hominoidea, a taxon more closely related to hominins, more similar in body size, and found in more hominin-like habitats than monkeys, a single-species perspective is more appealing. No great ape genus has even two sympatric species. Moreover, despite a separation of 1.6 Ma, West African chimpanzees have not speciated from Pt. troglodytes nor Pt. schweinfurthii. It is notable that no two contemporaneous species of hominin were separated by significantly more than this interval. A biological--as opposed to an ecological or geographical--species definition would place all hominins in a single, phenotypically diverse species. Since divergence from the chimpanzee, "species" distinctness in hominins may have been maintained by temporary allopatry and centripetal niche separation. The hominin lineage may have evolved as a single, phenotypically diverse, reticulately evolving species.