Ardipithecus ramidus

The following is an overview of series of articles in Science about Ar. ramidus published in October, 2009. At some point, the Ar. ramidus was anointed as our potential ancestor. However, there are many who question legitimacy of it. Ar. ramidus discovery stirred scientific community by shattering established textbook views and opinions and offering fresh and exciting perspectives on human origin and evolution.

Site: Ethiopia, Middle Awash

Ardipithecus ramidus was found in sediments flanking the Awash River, Ethiopia at Aramis locality. The Awash River valley is located in the Afar region (or Afar Triangle). The Awash River is a major river of Ethiopia and is some 1200 km long. Afar lies at the intersection of the Red Sea, Gulf of Aden, and Main Ethiopian Rifts. Rift or rift valley is a valley with steep sides formed when two parallel cracks (faults) develop in the earth's surface and the land between them sinks.

Sediments result from deposition of sands, rocks and other solid particles that were carried by water, wind or other natural processes to places where they gradually accumulated and were left undisturbed for arbitrary lengths of time, sometimes for millions of years. The A. ramidus-bearing sediments consist of silt and clay deposited between two volcanic markers, each dated to 4.4 million years ago, which implies that the fossils themselves were the same age and were deposited within time interval between 100 to 10,000 years.

Several of the most important hominid fossils have been found north and west of the Awash River, including Hadar (Australopithecus afarensis "Lucy" skeleton dated 3.2-million years ago), Gona (oldest stone tools dated at 2.6 million years ago), and the Middle Awash, Aramis ("Ardi" site).

The excavation of fossils has begun in 1981. Since then, research team of more than 70 scientists has collected 2,000 geological samples, thousands of artifacts (including stone tools), and tens of thousands of plant and animal fossils.

On the basis of the geology, paleobotanical, and isotopic data, researchers inferred that Pliocene vegetation at this location included palms, trees and shrubs (e.g. hackberry) as well as grasses. Combined evidence indicates that A. ramidus lived in habitats ranging from woodland to forest patches rather than in the open savanna that once was considered to be the predominant econiche of the earliest hominids.

Closed woodland has less continuous canopies than forest but its ground level is still sparse and lacks grasses. Woodland has stands of trees from 8 to 20 m high, which canopies cover about 40% of the ground and do not overlap extensively. Woodland's ground layer contains sun-loving grasses, herbs, and underbrush consisting of small trees and shrubs. Shrub woodland has a canopy height less than 8 m. As proportions of bushes, shrubs and grasses increase, woodland becomes bushland/thicket or wooded grassland. All these habitats can grade into each other continuously.

Overviews of faunal assemblages re-constructed by scientists can be found here (opens in new window).

Fossil: introduction

Ardipithecus was represented by 110 specimens from a minimum of 36 individuals. Fossils were scattered widely across the excavation area.

When describing results of reconstruction analysis, evolutionists frequently use terms derived and primitive. Knowing a meaning of these terms is very important for understanding conclusions made by the scientists. Primitive features are attributes, which all members of the group possess. For example, all apes and hominids are tailless. Absence of the tail is a primitive trait of our ancestors. In contrast, humans have large brain, which clearly distinguishes them from all other hominoids. Therefore, large brain is derived characteristic of Homo sapiens.

The Ardipithecus ramidus fossils from 4.4 million years ago are not old enough to represent the chimpanzee/human last ancestor, who was estimated to live at least 6-8 million years ago. Largest and most complete skeleton ARA-VP-6/500 apparently belonged to a large bodied female (lovingly named Ardi), who stood about 120 cm tall and weighed about 51 kg.

The cause of death was impossible to determine. The only pathology was a partially healed osteolytic lesion on the 5^th ray of the left foot attributed to a local infection.

Fossil: small cranium, weak prognathism, orthogrady

Skull of Ar. ramidus harbored brain (had endocranial volume) that in size was similar to chimpanzee and gorilla's brain (300 to 350 cm³). In comparison, Australopithecus afarensis brain ranged from 380 to 430 cm³. Average human brain is about 1450 cm³.

Overall face structure was inferred from most complete specimen ARA-VP-6/500. The face was described as having weaker prognathism than in genus Pan (chimpanzee and bonobos) and being somewhat short. However, because modern ape data show that the facial feature is highly variable within sexes and species, the authors did not consider the reconstructed face to be characteristic to Ar. ramidus as species.

Analysis of skull position, namely, of foramen magnum (an aperture in the base of the skull, through which the medulla oblongata enters and exits the skull vault) relative to other structures of the cranium to elucidate degree of Ardi's orthogrady was inconclusive. However, various skull metrics indicate that Ardi's skull differs from that of extant apes and leans toward hominid conditions. Some indirect computations place Ardi "squarely within the hominid range".

Most important conclusion made by scientists after analysis of Ar. ramidus skull is that it shared many characteristics with Sahelanthropus and Australopithecus, which are also unique to hominid clade. In lights of this, many skull features of extant apes, such as extreme prognathism in chimpanzee, reduction of facial size in bonobos, and overdeveloped lower face in gorillas appear to be derived and were developed independently from hominids.

Fossil: dentition of non-aggressive omnivore

In apes and monkeys, the male's upper canine tooth usually has a prominent, dagger-like crown that is continuously sharpened (honed) by wear against a specialized lower premolar tooth. Together, these teeth form so-called C/P₃ complex, also referred as sectorial canine complex (SCC). This complex was developed in the course of evolution not as a dietary adaptation but rather as as a formidable weapon in various social conflicts. Size of the canines also reflects status of the individual in the social hierarchy. Males demonstrate their canines by gaping widely at each other thus establishing their positions in the group. All fossil apes had the honing C/P₃ complex.

In contrast, Ar. ramidus has dramatically "feminized" canines: they are shorter, diamond-shaped and there is no evidence of honing. The hominid canines from about 6 million years ago are similar in size to those of Ar. ramidus. Australopithecus afarensis had more robust canines but honing was absent. This suggests that male canine size and prominence were reduced by ~6 to 4.4 million years ago from an ancestral ape with a honing C/P₃ complex.

Subdued shape of canines in Ar. ramidus implies that selection, probably, sexual, played an important role in elimination of agonistic behavior observed in monkeys and extant apes. Thus, fundamental reproductive and social behavioral changes probably took place in groups of hominids long before their brain started to enlarge.

Even in modern humans, the canine is metrically the most dimorphic tooth: human male canines are about 4 to 9% larger than that in females. Analysis of Ar. ramidus fossils indicated that Ar. ramidus was probably only marginally more dimorphic than modern humans.

Analysis of Ardi's incisors and postcanine teeth suggests that she was not as intensive a frugivore as are Pan (chimpanzee) and Pongo (orangutan). Molar structure, enamel thickness, and wear evidence indicate that Ar. ramidus diet was much less abrasive than that of Australopithecus. Isotopic analysis of enamel indicates that Ardi predominantly consumed (~85 to 90%) C₃ plant sources (shade-loving plants) in woodland habitat, which she most likely supplemented with more protein rich items like insects, insect larvae, eggs, etc.

Fossil: forelimbs of "careful climber"

Human hands, short and wide and highly mobile at the wrists, are adapted to a variety of grasps, which manifests in ability to perform complex dexterous manipulations with tiniest of objects. Hands of knuckle-walkers / agile climbers differ from human hands by several features, among which significant elongation of all metacarpals except of thumb, weak thumb-to-palm opposition, and stiffening of the wrist are most important. Ardi's hands are very different from those of African apes. They lacked virtually all of the above mentioned specializations that protect great ape hands from injury while they climb and feed in trees. Most of all, midcarpal joint of Ardi's hand is even more flexible than our own and allowed for greater dorsiflexion (the movement which decreases the angle between the palm and the arm). Ardi's thumb was larger and more robust than in extant apes, and together with short metacarpals, made her hand very similar to human. Consequently, while moving along the tree branches, Ardi could support her entire weight on her palms, and after she laid her palms flat on the tree branch, she was able to move her body well forward without releasing the supporting hand from the substrate (this type of locomotion, called palmigrady, is used by some Old World monkeys). Ardi's hands resemble those in many Miocene apes, including Proconsul. Because of this, her wrist's structure is considered to be primitive (ancestral).

Ardipithecus discoverers suggest that Ardi was a "careful" climber incapable of powerful vertical climbing and suspensory locomotion as seen in chimpanzee and orangutan.

Fossil: pelvis at dawn of upright walking

The pelvic girdle is formed by the sacrum (fused vertebrae), coccyx ("tail"), and the two coxae (hip bones). Each coxa is attached to the sacrum and to each other at the pubic symphysis. A coxa is formed by the fusion of three bones, the ilium, ischium, and pubis, which meet in the acetabulum (hip socket).

The human ilium is broad and low, while the chimp's ilium is narrow and high.

The Ar. ramidus pelvis morphology is mosaic: the ilium is broad as in all post-Miocene hominoids (especially Symphalangus, Gorilla, and Australopithecus) and Homo; at the same time, ischium is dramatically ape-like - substantially longer than any known Australopithecus example.

The exceptionally derived ilium implies an early adaptation to habitual terrestrial bipedality while retention of a capacity for arboreal locomotion.

Fossil: prehensile yet propulsive foot

The primary difference between the chimp and human foot is the contrast between prehensile function of the former and the propulsive anatomy of the latter. Like the hand, the foot has three anatomical regions: (1) 7 bones of the tarsus form the ankle and the heel (from distal to proximal: 3 cuneiforms, cuboid, navicular, talus, calcaneus); (2) 5 metatarsals form the middle portion of the foot; (3) phalanges have the same number and arrangement as in the fingers.

Opposable hallux (big toe), long phalanges and greatly foreshortened cuboid, navicular, and lateral cuneiforms (bones located next to the metatarsals) of the chimp allow conformity to and efficient grasping of the substrate. The human foot has greatly reduced phalanges, with all metatarsals elongated and parallel, which increase the lever arm of the tarsus for striding. At the same time, a longitudinal arch provides a shock absorbing and weight distribution system. The orientation of the ankle joint allows efficient balance and straighter path for foot falls.

Ardi's foot is unlike those of African apes as well as humans. It retains largely primitive morphology including os peroneum, the bone, which is imbedded within a tendon that helps to abduction of big toe while the foot is grasping the substrate. This bone is present in Old World monkeys and gibbons but not in African apes. The Ar. ramidus is the only known hominid with opposable big toe. The toe served for grasping btanches on the trees and balancing on the ground.

Anteroposterior lengths of cuboid and navicular in Ar. ramidus are longer than in extant apes but not as long as in humans. Nevertheless, they should have been able to enhance Ardi's bipedal capabilities to some degree.

These and other pedal characters that were described in Ardi's allowed her to take advantage of both worlds as arboreal palmigrade quadruped and terrestrial primitive biped. This unique combination had to be instrumental in her survival abilities in pre-historic woodlands of modern Ethiopia.

Fossil: unique morphology of the vertebral column

The human vertebral column consists of 33 vertebrae divided into five regions: 7 cervical vertebrae (neck), 12 thoracic vertebrae (support rib cage), 5 robust lumbar vertebrae (lower back), 5 fused sacral vertebrae (sacrum, axial skeleton in the pelvis), 4 caudal vertebrae (coccys, "tail"). Viewed from the side, human vertebral column has a series of four curvatures (S-shaped). The dorsal (back) outline of the cervical and lumbar regions are concave (the curvature of the lumbar region is called lordosis) while the thoracic and sacral regions are convex (the curvature of the thoracic region is called kyphosis).

Chimpanzees generally have one more thoracic, one less lumbar vertebra, and one less caudal vertebra than do humans, which is reflected in stiff vertebrate column and acute angle between the lumbar and sacral regions.

Some species of arboreal quadrupedal monkeys, especially those which perform spectacular leaping feats, have flexible, elongated backs (with extra vertebra).

It now appears that Ar. ramidus had six lumbar vertebrae and a four-segment sanctum - formula that is unlike of any extant ape. In combination with situational anterior pelvic tilt, Ardi could achieve substantial lordosis during upright walking that would stabilize her gait considerably. The pelvic and vertebrate data imply that spinal morphology of extant apes is derived and developed as a result of adaptation to vertical climbing and suspension.

Inferences

Ardi's discovery falsifies chimpanzee-centric theory of human origin, which viewed Australopithecus as transitional between an ape-like ancestor and early Homo. Anatomy of Ar. ramidus demonstrates that the anatomy of extant African apes is not primitive but instead exhibits highly derived features that evolved as a result of adaptations to their specialized lifestyles (forest dwelling, diet, reproductive strategy, and social organization). Ar. ramidus preserves many primitive (ancestral) characteristics of the last common ancestor with two notable exceptions - ability to walk upright and the absence of the large projecting canine teeth that signify social organization with competition and aggression between males. Consistent with a reduced inter- and intra-group agonism, the differences between body sizes in males and females of Ardipithecus ramidus reflect only moderate sexual dimorphism.

Ardi was a woodland dweller. Long established textbook opinions that bipedality evolved in response to loss of forests and expansion of grasslands and savannahs as a result of climate change are now shaken by apparent signs of bipedality found in Ardi's skeleton.

The projecting canines play very important role in reproductive success in the apes and other primates therefore their disappearance had to be accompanied with profound changes in behavior, which would not merely compensate for loss of fitness but instead turn it to advantage. The scientists, who unearthed Ardi suggest, that the major shift in the social organization was associated with a major life-history strategy changes, which involved three additional behavioral adaptations:

• regular food carrying
• pair-bonding
• reproductive crypsis (in which females did not advertize ovulation, unlike the case in chimpanzee).

Researchers offer sexual selection as a driving force of canine teeth reduction phenomenon. Obvious female preference for a dominant male becomes less advantageous when offspring requires long and intensive parenting. Under such conditions mothering female needs help in providing food and shelter for herself as well as for her offspring, whose survivorship becomes paramount for species success. Less aggressive smaller-canined males may become progressively more attractive to dependent females, who could encourage such males to target them in preference to other females in exchange for food and protection. Repeated copulations made display of heightened receptivity during ovulations unnecessary and, at some point, even detrimental to pair forming process and bonding, hence, reproductive crypsis in hominid females.

Possible mechanism of sexual selection might be gleaned from sexual behavior and secondary sexual characteristics of male gelada baboons. A handful of males grow rapidly at puberty, develop distinctive golden manes and become almost twice the size of the females. These males usually succeed in acquiring their own harems. After a relatively brief but intense reproductive life and constant conflicts with rivals, they are displaced by younger stronger males and never breed again. However, when many such males are present in the social group, an adolescent male may adopt a distinctly different phenotype and behavior - he remains female-like in size and appearance, does not develop the mane and acts as inconspicuous as possible. Such males hang around the big males' harems, mating with females when harem-owner is distracted. Because small and sneaky male never has to fight for females, he is likely to have longer, if less intense sexual life, and in the long run he even may achieve better reproductive success than the aggressive male.

The discoverers of Ar. ramidus conclude:

Elimination of the SCC occurred long before the eventual dentognathic hypertrophy of Australopithecus, and long before the likely horizon at which sufficient reliance on tool use would have encouraged abandonment of food and/or safety in the arboreal substrate. It is far more likely that our unique reproductive behavior and anatomy emerged in concert with habituation to bipedality and the elimination of the SCC. It is also now equally clear that Pan's specialized reproductive constellation has been driven by an entirely different locomotor and dietary history.

One of the most important conclusions that we can thank Ardi and her discoverers for is that the last common ancestor, which according to genetic studies lived 5 to 7 million years ago, did not look like chimpanzee.

References

Articles

• Suwa G et al. The Ardipithecus ramidus skull and its implications for hominid origins. Science. 2009 Oct 2;326(5949):68e1-7. PMID: 19810194
• Suwa G et al. Paleobiological implications of the Ardipithecus ramidus dentition. Science. 2009 Oct 2;326(5949):94-9. PMID: 19810195
• Lovejoy CO et al. The great divides: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African apes. Science. 2009 Oct 2;326(5949):100-6. PMID: 19810199
• Lovejoy CO. Reexamining human origins in light of Ardipithecus ramidus. Science. 2009 Oct 2;326(5949):74e1-8. PMID: 19810200
• White TD et al. Ardipithecus ramidus and the paleobiology of early hominids. Science. 2009 Oct 2;326(5949):75-86. PMID: 19810190
• Lovejoy CO et al. Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus. Science. 2009 Oct 2;326(5949):72-72e8. PMID: 19959791
• Lovejoy CO et al. The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science. 2009 Oct 2;326(5949):71-71e6. PMID: 19959790
• Suwa G et al. The Ardipithecus ramidus Skull and Its Implications for Hominid Origins. Science. 2009 Oct 2;326(5949):68-68e7. PMID: 19959787
• Greenfield LO. Canine "honing" in Australopithecus afarensis. Am J Phys Anthropol. 1990 Jun;82(2):135-43. PMID: 2113767
• Bateson P, Mameli M. The innate and the acquired: useful clusters or a residual distinction from folk biology? Dev Psychobiol. 2007 Dec;49(8):818-31. PMID: 18023000

Other sources

• Is there fossil evidence of "missing links" between humans and apes? by Dr Colin Groves
• Chimpanzees become Human?

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