![]() Second, the description of Pliobates cataloniae, a small-bodied (4–5 kg) hominoid from the Miocene of Spain (11.6 Ma), argues for a gibbon-sized common ancestor of all crown hominoids, rather than an extant great ape-sized ancestor with hylobatids evolving as a dwarfed lineage 29. Chimpanzee-like postcranial morphology and body mass are not necessarily linked, although this is often implied by many models that suggest a chimpanzee-like LCA. First, this hypothesis coincides with the assumption of an overall chimpanzee-like morphology for the chimpanzee–human and hominid LCA, a topic of much debate 6, 7, 30, 31, 32, 33, with some researchers suggesting that current fossil evidence and analyses point to a generalized monkey-like ancestor 34. However, a few recent findings argue caution with acceptance of a chimpanzee-sized series of LCAs stretching back to before the divergence of hylobatids from other hominoids around 19.5 Ma. Another primary reason for this omission may be a consensus among many researchers that-reinforced by the wealth of molecular work showing humans and chimpanzees are sister taxa 27-the mass of the chimpanzee–human LCA resembled common chimpanzees 28 (~45 kg), and a chimpanzee-sized ancestor represents the LCA of African hominids (i.e., hominines) 10, hominids 10, and hominoids 29. ramidus, possibly even substantially so” 7-a range extending down from the fossil’s predicted body mass (~50 kg) 7 and encompassing almost all primates from chimpanzees to diminutive monkeys. ramidus, which argues that the chimpanzee-human and African hominid LCAs were likely to be “equal to or smaller than Ar. Issues with body mass variation in Miocene hominoids are evident in the description of Ar. 26) further complicate the usefulness of these data. 24, 25), and questions about how these species relate to one another and to crown hominoids (reviewed in ref. In addition, body sizes in the more well-sampled Miocene hominoid (all living and extant apes and humans) taxa (e.g., Proconsul) appear to be extremely variable (e.g., refs. One important reason for this omission is the paucity of African fossil hominids during the period when the chimpanzee and human lineages are believed to have diverged, perhaps 4–6 Ma (million years ago) 17 or earlier at 6–8 Ma 18, 19, 20, with the notable exceptions of putative basal hominins Orrorin tugenensis (~6 Ma) 21, Sahelanthropus tchadensis (6–7 Ma) 22, Ardipithecus kadabba (5.5–6.4 Ma) 23, and the later Ardipithecus ramidus (4.4 Ma) 7. Note that here we define body size as body mass 16. 14) implicitly assumed a smaller-body mass in order to maintain balance and stability on deformable branches of different diameters 15. Various classic models 9, 11, 12, 13 proposed a body mass increase as a proximate factor in the evolution of suspensory adaptations and the transition from an arboreal to terrestrial hominid (great apes plus humans and our fossil ancestors) as larger sizes dictated a switch between locomotor modes, while models based around an arboreal quadruped ancestor (e.g., ref. 10)-depend on inferences about body mass at and prior to the root of our lineage. This omission is startling because numerous arguments over one of the most contested topics in hominin evolution-what were the selective regimes that led to the origins of bipedalism 6, 7, 8, 9 (but see ref. ![]() ![]() 1, 2, 3, 4, 5), the body mass of the last common ancestor (LCA) of chimpanzees and humans remains unexplored in any rigorous fashion. Though the timing, causes, and biological implications of the increase in body mass that took place during human evolution continue to inspire a wealth of research (e.g., refs. Thus, understanding the evolution of this trait is a necessary step in reconstructing the paleobiology of extinct fossil species. Locomotion, behavior, diet, social organization, energy requirements, and a host of other vital biological and ecological characteristics are directly or indirectly tied to body mass. Body mass impacts almost every aspect of an animal’s biology and ecology.
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