Ardipithecus ramidus and the evolution of the human cranial base

Abstract

The early Pliocene African hominoid Ardipithecus ramidus was diagnosed as a having a unique phylogenetic relationship with the Australopithecus + Homo clade based on nonhoning canine teeth, a foreshortened cranial base, and postcranial characters related to facultative bipedality. However, pedal and pelvic traits indicating substantial arboreality have raised arguments that this taxon may instead be an example of parallel evolution of human-like traits among apes around the time of the chimpanzee-human split. Here we investigated the basicranial morphology of Ar. ramidus for additional clues to its phylogenetic position with reference to African apes, humans, and Australopithecus. Besides a relatively anterior foramen magnum, humans differ from apes in the lateral shift of the carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened, trapezoidal basioccipital element. These traits reflect a relative broadening of the central basicranium, a derived condition associated with changes in tympanic shape and the extent of its contact with the petrous. Ar. ramidus shares with Australopithecus each of these human-like modifications. We used the preserved morphology of ARA-VP 1/500 to estimate the missing basicranial length, drawing on consistent proportional relationships in apes and humans. Ar. ramidus is confirmed to have a relatively short basicranium, as in Australopithecus and Homo. Reorganization of the central cranial base is among the earliest morphological markers of the Ardipithecus + Australopithecus + Homo clade.A s the confluence of the neural, locomotor, and masticatory systems, the cranial base has been the site of profound structural change in human evolution. The modern human basicranium differs from that of our closest living relatives, the great apes, in numerous aspects of shape and morphological detail (1-4). In humans, the foramen magnum and occipital condyles are more anteriorly located, the midline basicranial axis is relatively short anteroposteriorly and strongly "flexed" internally, and the bilateral structures marking vascular and neural pathways through the central part of the base are more widely separated. This organization alters the relationships between the petrous and tympanic parts of the temporal bone. These phylogenetically derived features are already seen in the earliest known skulls of Australopithecus, ca. 3.0-3.4 Ma (5, 6).The cranium of Ardipithecus ramidus, an early Pliocene (4.4 Ma) hominoid from Ethiopia, was shown to have a relatively anterior foramen magnum on a short basicranium, corroborating evidence of nonhoning canine teeth and terrestrial bipedality for phylogenetic attribution of this taxon. These sets of derived characters are shared uniquely with the Australopithecus + Homo clade (7-10). At the same time, pelvic and pedal characters indicate that Ar. ramidus also retained considerable arboreal capabilities (11)(12)(13)(14). Despite the evidence for a unique phylogenetic relationship with the Australopithecus + Homo clade, it has been a...

Ontogenetic variation in the mandibular ramus of great apes and humans

“…In humans, these stages correspond roughly to chronological ages 0–6.5 years (Category 1), 6.5–12.5 years (Category 2), 12.5–18/20.5 years (Category 3), 18/20.5–<25 years (Category 4), and >25 years (Category 5) (Ubelaker, ; White, ; AlQahtani et al, ). Known eruption times for the permanent molars in the African apes ( P. troglodytes and G. gorilla ) are approximately 3–3.5 years for M1, 6.5–7 years for M2, and 10.25–11 years for M3 (summary data presented in Smith et al, ); eruption of the molars is generally considered to be relatively later in Pongo (M1 = 4.6 years) (Kelley and Schwartz, ). These age categories are unlikely to be exactly homologous across species; however, the sequence of molar eruption is similar among hominids (Godfrey et al, ) and these eruption patterns are important markers of weaning (M1) and the onset of sexual maturation (M3) in all of these taxa (Smith, ).…”

Tropical forests and the genus Homo

“…Similarly, the lower limb proportions and the foot and hand morphology of Ardipithecus ramidus ( c . 4.4 Ma) have been associated with tree climbing, while its pelvic morphology has been linked to ground‐based bipedal walking …”

Genetic and developmental basis for parallel evolution and its significance for hominoid evolution

“…40 Derived cranial-dental characters, such as a shortened cranial base and reduction of the canines with loss of the functional honing complex, are shared with Sahelanthropus and Australopithecus. 90,91 The pelvis is broad and short, freeing the lumbar column from articulation with the flared iliac blades, facilitating lordosis. The pelvis also displays evidence of the distinctive human and australopithecine separate ossification center for the anterior inferior iliac spine and a muscular conformation favoring hip stabilization during the single support phase of bipedal gait.…”

“…96 However, the characters that link these fossils to Australopithecus and Homo are extensive. 40,90 Such a hypothesis requires an explanation of why evolutionary experimentation with bipedalism and canine feminization was so prominent at the Miocene-Pliocene transition compared to the 20 Ma history of diversity in quadrupedal locomotor modes across hominoids. Alternatively, the affinities between Ardipithecus and archaic hominoids could represent reversals from an African ape-like LCA.…”