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The Ardipithecus ramidus Skull and Its Implications for Hominid Origins

Gen Suwa et al.

The key feature that distinguishes Homo sapiens from other primates is our unusually large brain, which allows us to communicate, make tools, plan, and modify our environment. Understanding how and when our cognitive ability evolved has been a special focus in anthropology and, more recently, genetics. Fossil hominid skulls provide direct evidence of skull evolution and information about diet, appearance, and behavior. Skulls feature prominently in the characterization of species, in taxonomy, and in phylogenetic analyses of both extinct and living primates.

Unfortunately, hominid skulls are relatively rare in the fossil record. A number of partial skulls and crania (skulls without a lower jaw) of early Homo and its predecessor, Australopithecus (which lived ~1 to 4 million years ago), have been recovered, but relatively few are complete enough for extensive comparisons. One surprisingly complete but distorted cranium from 6 to 7 million years ago was discovered in central Africa (Chad). This fossil, Sahelanthropus tchadensis (a.k.a. “Toumaï”), is thought by many to represent the earliest known hominid, although some have argued that it is a female ape.

The Ardipithecus ramidus skull is of particular interest because it predates known Australopithecus and thereby illuminates the early evolution of the hominid skull, brain, and face. The Ar. ramidus skull was badly crushed, and many of its bones were scattered over a wide area. Because the bones were so fragile and damaged, we imaged them with micro–computed tomography, making more than 5000 slices. We assembled the fragments into more than 60 key virtual pieces of the braincase, face, and teeth, enough to allow us to digitally reconstruct a largely complete cranium.

The fossil skulls of Australopithecus indicate that its brain was ~400 to 550 cm3 in size, slightly larger than the brains of modern apes of similar body size and about a third of those of typical Homo sapiens. Its specialized craniofacial architecture facilitated the production of strong chewing forces along the entire row of teeth located behind its canines. These postcanine teeth were enlarged and had thick enamel, consistent with a hard/tough and abrasive diet. Some species exhibited extreme manifestations of this specialized chewing apparatus and are known as “robust” Australopithecus.

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(Right) Oblique and side views of a female chimpanzee (right) and the Ar. ramidus female reconstruction (left; the oblique view includes a separate mandible). (Left) Comparison of brain and tooth sizes (arrows) of chimps (Pan, blue), Ar. ramidus (red), and Australopithecus (green). Means are plotted except for individual Ar. ramidus and Au. afarensis cranial capacities. Canine unworn heights (bottom) are based on small samples, Ar. ramidus (females, n = 1; males, n = 3), Au. afarensis (n = 2), Pan (females, n = 19; males n = 11).

Ar. ramidus had a small brain (300 to 350 cm3), similar to that of bonobos and female chimpanzees and smaller than that of Australopithecus. The Ar. ramidus face is also small and lacks the large cheeks of “heavy chewing” Australopithecus. It has a projecting muzzle as in Sahelanthropus, which gives it a decidedly ape-like gestalt. Yet the Ar. ramidus skull is not particularly chimpanzee-like. For example, the ridge above the eye socket is unlike that of a chimpanzee, and its lower face does not project forward as much as a chimpanzee’s face. Chimps primarily eat ripe fruits and have large incisors set in a projecting lower face. Ar. ramidus instead was probably more omnivorous and fed both in trees and on the ground. Additionally, in chimpanzees, forward placement of the entire lower face is exaggerated, perhaps linked with their large tusklike canines (especially in males) and elevated levels of aggression. This is not seen in Ar. ramidus, implying that it was less socially aggressive.

Like Ar. ramidus, S. tchadensis had a brain that was less than 400 cm3 in size. It also resembled Ar. ramidus in having small nonsharpened canines. Details of the bottom of the skull show that both Ar. ramidus and Sahelanthropus had a short cranial base, a feature also shared with Australopithecus. Furthermore, we infer that the rear of the Ar. ramidus skull was downturned like that suggested for Sahelanthropus. These similarities confirm that Sahelanthropus was indeed a hominid, not an extinct ape.

These and an additional feature of the skull hint that, despite its small size, the brain of Ar. ramidus may have already begun to develop some aspects of later hominid-like form and function. The steep orientation of the bone on which the brain stem rests suggests that the base of the Ar. ramidus brain might have been more flexed than in apes. In Australopithecus, a flexed cranial base occurs together with expansion of the posterior parietal cortex, a part of the modern human brain involved in aspects of visual and spatial perception.

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