The genus Paranthropus represents an iconic taxon in the hominin fossil record. Its early discovery, together with its morphological distinctiveness, have defined this genus as a unique hominin lineage parallel to our own. Paranthropus boisei, who lived in East Africa between 2.3 and 1.4 million years ago, stands out amongst ‘robust australopithecines’ for its hyper expression of the megadont features diagnostic of the genus, while its vast dental and mandibular hypodigm makes it the best represented species in the early hominin fossil record. Yet, while megadont traits allow the clear identification of fossils as Paranthropus rather than Homo or Australopithecus, their expression is far from homogenous. This has resulted in a paradox of a seemingly morphologically ‘well-defined’ species that, nevertheless, encompasses a large intraspecific variability that does not always respect this definition. This paradox has been at the centre of conflicting interpretations regarding the evolutionary history and taxonomy of P. boisei. This work represents a critical examination of the degree of variation in the size and shape of mandibles assigned to P. boisei combining anatomical descriptions, high-resolution imaging, landmark-based 3D geometric morphometrics, and machine learning methodologies. Comparisons of ca. 60% of all original P. boisei fossil mandibles to those of living hominoids reveal the first sex estimation of individual fossils with an associated statistical likelihood, and a unique expression of sexual dimorphism in the extinct group. Further analyses reveal that the high morphological variability observed is structured independently of sexual dimorphism. In contrast, this thesis presents a novel interpretation to this variability and recognizes two distinct craniomandibular ecomorphotypes within the species’ hypodigm, typified by the two most emblematic cranial specimens ascribed to P. boisei (i.e. OH 5 and KNM-ER 406). Moreover, this work discusses the potential evolutionary scenarios leading to the morphological and ecological differences underlying this partitioning with significant implications for our current understanding of the taxonomical affinities, ecological interpretations and evolutionary history of P. boisei. Finally, this thesis presents the first description of two P. boisei mandibles from the site of Koobi Fora, Kenya, and discusses these new fossil hominin specimens in the context of the ecomorphotypes identified.