Clades, Grades, and the Genus Problem: A Case for Revising Hominin Taxonomy.

Taxonomic practice in evolutionary biology increasingly prioritizes the recognition of monophyletic groups (clades), yet paleoanthropology still applies this principle inconsistently. Here, I outline these inconsistencies and broader challenges in hominin taxonomy and propose a potential solution. Australopithecus is widely considered non-monophyletic. In addition, the temporal depth of Homo is shallow relative to most primate genera, with recently diverged genera often taxonomically unstable due to complex morphological and genetic signatures and extensive gene flow among closely related taxa. The hominin fossil record exhibits pronounced morphological mosaicism and homoplasy, with supposedly key traits increasingly blurred both within and among genera. This obscures putative adaptive distinctions and contributes to persistent taxonomic disagreement. One potential solution is the subsumption of Australopithecus and Paranthropus within an expanded genus Homo. Such a revision would help address problems of non-monophyly, avoid conceptual difficulties associated with genera with shallow time depths, and facilitate more coherent evolutionary comparisons within a more stable taxonomy. Within this framework, Hominini is best defined as the Pan-Homo clade, with Hominina retained as the appropriate subtribal designation for the human clade. Although similar proposals have been advanced previously, accumulating fossil and genetic evidence makes this a timely moment to reconsider hominin taxonomy. This proposal has limitations, including the loss of taxonomic information relating to adaptive and ecological variation, particularly within Paranthropus and large-brained Homo. At a minimum, however, this article argues for greater consistency and transparency in distinguishing clades from grades in homininan taxonomy, and for more consistent criteria for the recognition of genera.