The role of character displacement in the molarization of hominin mandibular premolars

Abstract

Closely related species are likely to experience resource competition in areas where their ranges overlap. Fossil evidence suggests that hominins in East Africa c. 2-1.5 million years ago may have lived synchronically and sympatrically, and that competition may have contributed to the different tooth sizes observed in Homo and Paranthropus. To assess the likelihood that these taxa overlapped, we applied a character displacement model to the postcanine tooth size of fossil hominins and validated this model in populations of living primates. Mandibular fourth premolar (P4 ) crown size was measured from fossil taxa and from living primate species where dietary overlap is established. Dimensions of the P4 crown were fitted to a character matrix and described as the response variables of a generalized linear model that took taxon and location as input variables. The model recovered significant divergence in samples of closely related, living primates. When applied to fossil hominins the same model detected strong indications of character displacement between early Homo and Paranthropus (P = 0.002) on the basis of their P4 crown size. Our study is an example of how ecologically informed morphologies measured in appropriate extant referents can provide a comparative context for assessing community and ecological evolution in the fossil record.

Hominin taxic diversity: Fact or fantasy?

“…First, he did not recognize Telanthropus as a distinct taxon. Second, he was not convinced the two taxa were synchronic, to the extent that strict synchrony can ever be determined from the paleontological record (but see Schroer and Wood, 2015).…”

Deconstructing a Species-Complex: Geometric Morphometric and Molecular Analyses Define Species in the Western Rattlesnake (Crotalus viridis)

“…Body size drives the separation of sister-pairs over evolutionary time (as above), and diversifies species as they adaptively radiate [ 82 ]. However, rapid evolution is seldom observable and thus rarely detectable in fossil lineages [ 83 ], hence its recognition is always “after the fact.” We suggest that morphological displacement along a particular trajectory represents the residual of past competitive interactions and these are more easily detectable using GM approaches (as herein, [ 83 – 84 ]).…”

Diversity-dependent speciation and extinction in hominins

“…Ultimately, the climatic ‘Court Jester’ must set the stage upon which the ‘Red Queen’ of interspecific competition ‘dances’ 90 , and our results point to a need to further explore the relationship between climate and competition, and how this relationship drove macroevolution, in our own lineage. Finally, an important effect of the inferred longer species lifespans is notably extended periods of temporal overlap between sympatric species, such as P. boisei and early Homo in East Africa 19 as well as Paranthropus robustus and Australopithecus africanus in South Africa. Extended periods of sympatry provide the context for interspecific competition at smaller scales, with effects at equivalent scales, such as microevolutionary morphological evolution driven by competition-mediated niche separation 10 , 13 , 91 .…”

“…The consequences of competition can include three processes: speciation, extinction and morphological change through, for example, character displacement 13 , 18 . There is some indirect evidence that competition resulted in morphological evolution in our lineage: competition between Homo and Paranthropus in East Africa probably led to character displacement in the mandibular premolar morphology of these two groups 19 . However, much less work has been devoted to exploring the effects of competition on hominin speciation or extinction.…”