Ardipithecus ramidus is a hominin species that has been dated to between 4.5 and 4.2 million years ago using paleomagnetic and radioisotopic dating methods. (Paleomagnetism uses the known periodic reversals in the Earth’s magnetic field throughout Earth’s history; the radioisotopic method utilizes the known rate of decay of one radioisotope into another). Importantly, Ar. ramidus represents the oldest species that possesses features unequivocally linked to the hominin lineage. Consequently, Ar. ramidus is considered by some researchers to be the best evidence discovered so far for the earliest hominin ancestor. Fossils of this species, found in Ethiopia at the Middle Awash and Gona sites, possess derived features (features different from those found in the ancestor) in the skull and teeth. The postcranial skeleton of Ar. ramidus, however, suggests this species had not evolved obligate bipedality (“obligate” meaning the skeletal anatomy limits locomotion to one means, in this case bipedality; obligate is the opposite of functional bipedality and is possessed by chimpanzees, for example, who can walk upright for short distances or climb in trees). This combination of traits is important because scientists have long considered obligate bipedality to be a defining characteristic of the hominin lineage. The traits possessed by Ar. ramidus, however, demonstrate that hominin-like skulls and teeth evolved before obligate bipedality and suggest the earliest hominins were not obligate bipeds.
The skull of Ar. ramidus is represented by most of the cranial vault (the part of the skull that covers the brain), parts of the cranial base (the part of the skull below the brain, in which the brain sits), and most of the right half of the face. The features of the skull link this species to other hominin species but also suggest primitive similarities shared with living apes. For example, the skull is small relative to later hominins (the cranial capacity is 300 to 350 cubic centimeters, about the size seen in female chimpanzees and bonobos [pygmy chimpanzees]). Like apes and other early hominins, Ar. ramidus exhibits substantial prognathism (forward projection) of the face. Unlike apes, however, Ar. ramidus does not exhibit prognathism below the nasal aperture. The posterior (towards the back) portion of the cranial base is shorter than in chimpanzees, a trait also seen in other hominins. It should be noted that these two traits (i.e., lack of prognathism in the lower part of the face and a short cranial base) are also found in Sahelanthropus tchadensis: an earlier, purportedly hominin species.
The dentition of Ar. ramidus is well represented, including all types of teeth (incisors, canines, premolars, and molars) and associated upper and lower teeth. In general, the absolute and relative sizes of the teeth (excluding the canines) are roughly similar to those of chimpanzees, but the incisors are smaller and the second and third molars are larger than in chimpanzees. The canine teeth of Ar. ramidus show derived hominin features. Most notably they are substantially reduced relative to that seen in living apes and less sexually dimorphic (the male and female canines do not show significant size difference). In addition, Ar. ramidus lacks the canine honing complex whereby the upper canine is sharpened on the anterior surface of the lower premolar, a feature found in all living apes and no hominin species. Ar. ramidus retains a diastema (gap) between the incisors and canines, which is seen in apes and variably in Au. afarensis; however, this diastema is much smaller than that seen in apes and more like that found in Au. afarensis.
The Ar. ramidus forelimb is known from several specimens including one nearly complete forelimb fossil. Fossils of the Ar. ramidus humerus (the bone in the upper arm) closely resemble humeri (plural of humerus) of other hominin species, including an elliptically-shaped humeral head (the part of the humerus that connects to the shoulder blade to form the shoulder joint) and a shallow groove for the long head of the biceps brachii muscle. In these ways, the humerus of Ar. ramidus also differs from those of living apes. Fragmentary radii (the plural of radius, the bone on the outside of the lower arm) and ulnae (the plural of ulna, the bone on the inside of the lower arm) are also represented and possess features not exhibited by living African apes. In addition, the metacarpals (hand bones) are different from those of the living apes in that they lack prominent ridges and grooves where the hand bones connect to the bones of the wrist. The Ar. ramidus hand phalanges (finger bones), by contrast, are longer than those of other hominins, being intermediate in length between chimpanzees and gorillas—i.e., the Ar. ramidus hand phalanges are shorter than those of chimpanzees, but longer than those of gorillas.
The Ar. ramidus hindlimb is represented by a partial, but damaged, pelvis; two partial femora (thigh bones); and most of the foot. Unlike apes, the Ar. ramidus ilium (the upper, fan-shaped bone of the pelvis) is flared out to the side of the body. In this way, the pelvis of this species is like all other hominin pelves (plural of pelvis). The flaring of the ilium shifts the gluteal muscles more to the outside of the body, which, in turn allows weight to be borne on one foot during bipedal walking. Interestingly, in contrast to its hominin-like ilium, the Ar. ramidus ischium (lower bone of the pelvis) more closely resembles those of the African apes. In particular, the shape and size of the ischium in Ar. ramidus suggests the hamstring muscles were strongly developed. Remains of the foot demonstrate that, unlike all known hominin species, Ar. ramidus had an opposable big toe (a big toe that, like those of living apes, is mobile like a human thumb). In addition, like later, obligately bipedal hominins, the four other toes of Ar. ramidus are flat and rigid. It should be mentioned, however, that in this way the foot of Ar. ramidus is also similar to many extinct apes and living monkeys. The foot phalanges (toe bones) of this species are intermediate in length between chimpanzees and species in the genus Homo.
The extensive fossil record of Ar. ramidus permits reconstruction of the behavior of this species. For instance, the dental remains suggest the Ar. ramidus’ diet constituted tougher foods than those of later hominins but less tough than those of chimpanzees. The relatively small incisors and large molars may indicate Ar. ramidus relied less on ripe fruit than chimpanzees. Furthermore, dental wear indicates Ar. ramidus likely had a less abrasive diet than did the later Au. afarensis. The small size of the canines (relative to living apes) suggests male-male competition using canines in fighting or threat displays was less important in Ar. ramidus (and all later hominins) than in other primate species.
The locomotor behavior (behavior involved with traveling from place to place) of Ar. ramidus can also be reconstructed. In particular, scientists argue that the forelimb of Ar. ramidus lacks the specializations related to suspensory locomotion (moving around while suspended below branches) and knuckle-walking. In particular, these researchers argue Ar. ramidus lacked the stiffness of the wrist (which facilitates knuckle walking) that is exhibited by knuckle-walking apes. Instead, these researchers suggest that the Ar. ramidus wrist was mobile (like that in living monkeys). The shape and size of the ischium suggest that the hamstring muscles were well-developed, a condition seen in living primates that emphasize climbing in their locomotor behaviors. Based on these observations, researchers argue that Ar. ramidus emphasized climbing and spent most of its time walking on all four limbs above branches.
The evolutionary relationships between Ar. ramidus and other hominin species are of great interest to paleoanthropologists. An older hominin taxon, Ardipithecus kadabba, which is also found in the Middle Awash of Ethiopia, is argued by some researchers to be a direct ancestor of Ar. ramidus because both species share many features, such as thin dental enamel and larger canines. Furthermore, some researchers argue Ar. ramidus is on a single line of descent in eastern Africa, beginning with Ar. kadabba to Ar. ramidus to Australopithecus anamensis and ending with Au. afarensis. This hypothesis, however, would imply that a large amount of morphological change would have had to occur between Ar. ramidus and Au. anamensis in a very short period of time (approximately 200,000 years). Many scientists question whether this amount of morphological change could occur in a single lineage in such a short time.
Paleoanthropologists are also interested in Ar. ramidus because at 4.4 million years ago, it provides the first extensive fossil evidence that extends our understanding of the last common ancestor we shared with chimpanzees. Scientists argue that the morphology of Ar. ramidus demonstrates that great ape adaptations for forelimb suspension and knuckle-walking were not present in the last common ancestor of hominins. This argument also implies that living great apes evolved suspensory adaptations separately and that none is a good model of the anatomy and behavior of the last common ancestor of chimpanzees and humans.
The two sites from which Ar. ramidus fossils have been recovered, Middle Awash and Gona, differ slightly in their habitat reconstructions. Faunal evidence from the parts of the Middle Awash region where Ar. ramidus has been found, indicate that the ecological habitat of the species appears to have been closed woodland. At Gona, many large fossil mammals associated with Ar. ramidus are intense grazers, which would indicate a habitat with a significant grassy component, while other indicators from the same site suggest that the environment was a more mosaic habitat, consisting of closed woodlands and more open, grassy environments. Although the reconstruction of environmental conditions at Aramis in the Middle Awash, Ethiopia, suggests a more closed environment, both habitat reconstructions are consistent with the idea that bipedality initially evolved in a woodland environment rather than in a more open savanna grassland.