Ardipithecus ramidus

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Ardipithecus ramidus
Temporal range:
Ma
↓
A skull
A. ramidus at the Museo Nacional de Ciencias Naturales
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Genus: †Ardipithecus
Species:
†A. ramidus
Binomial name
†Ardipithecus ramidus
(White, Suwa & Asfaw, 1994)
Synonyms
  • Australopithecus ramidus

Ardipithecus ramidus is a species of

great apes. Its discovery, along with Miocene apes, has reworked academic understanding of the chimpanzee–human last common ancestor from appearing much like modern-day chimpanzees, orangutans and gorillas
to being a creature without a modern anatomical cognate.

The facial anatomy suggests that A. ramidus males were less aggressive than those of modern chimps, which is correlated to increased parental care and monogamy in primates. It has also been suggested that it was among the earliest of human ancestors to use some proto-language, possibly capable of vocalizing at the same level as a human infant. This is based on evidence of human-like skull architecture, cranial base angle and vocal tract dimensions, all of which in A. ramidus are paedomorphic when compared to chimpanzees and bonobos. This suggests the trend toward paedomorphic or juvenile-like form evident in human evolution, may have begun with A. ramidus. Given these unique features, it has been argued that in A. ramidus we may have the first evidence of human-like forms of social behaviour, vocally mediated sociality as well as increased levels of prosociality via the process of self-domestication—all of which seem to be associated with the same underlying changes in skull architecture. A. ramidus appears to have inhabited woodland and bushland corridors between savannas, and was a generalized omnivore.

Taxonomy

Map showing discovery locations of various australopithecines

The first remains were described in 1994 by American anthropologist

corrigendum recommending it be split off into a separate genus, Ardipithecus; the name stems from Afar ardi "ground" or "floor".[2] The 4.4-million-year-old female ARA-VP 6/500 ("Ardi") is the most complete specimen.[3]

Fossils from at least nine A. ramidus individuals at As Duma, Gona Western Margin, Afar, were unearthed from 1993 to 2003. The fossils were dated to between 4.32 and 4.51 million years ago.[4]

In 2001, 6.5- to 5.5-million-year-old fossils from the

A. kadabba.[6] A. kadabba is considered to have been the direct ancestor of A. ramidus, making Ardipithecus a chronospecies.[7]

The exact

Au. afarensis.[3]

African
hominin timeline (in mya)
View references
H. sapiensH. nalediH. rhodesiensisH. ergasterAu. sedibaP. robustusP. boiseiH. rudolfensisH. habilisAu. garhiP. aethiopicusLD 350-1K. platyopsAu. bahrelghazaliAu. deyiremedaAu. africanusAu. afarensisAu. anamensisAr. ramidusAr. kadabba


Before the discovery of Ardipithecus and other pre-Australopithecus hominins, it was assumed that the

morphologies), and their ancestors were comparatively poorly adapted to suspensory behavior or knuckle walking, and did not have such a specialized diet. Also, the origins of bipedality were thought to have occurred due to a switch from a forest to a savanna environment, but the presence of bipedal pre-Australopithecus hominins in woodlands has called this into question,[12] though they inhabited wooded corridors near or between savannas. It is also possible that Ardipithecus and pre-Australopithecus were random offshoots of the hominin line.[13]

Description

Reconstruction of Ardipithecus skeleton

Assuming subsistence was primarily sourced from climbing in trees, A. ramidus may not have exceeded 35–60 kg (77–132 lb). "Ardi," a larger female specimen, was estimated to have stood 117–124 cm (3 ft 10 in – 4 ft 1 in) and weighed 51 kg (112 lb) based on comparisons with large-bodied female apes.[14] Unlike the later Australopithecus but much like chimps and humans, males and females were about the same size.[3]

A. ramidus had a small brain, measuring 300–350 cc (18–21 cu in). This is slightly smaller than a modern bonobo or chimp brain, but much smaller than the brain of Australopithecus—about 400–550 cc (24–34 cu in)—and roughly 20% the size of the modern human brain. Like chimps, the A. ramidus face was much more pronounced (prognathic) than modern humans.[15] The size of the upper canine tooth in A. ramidus males was not distinctly different from that of females (only 12% larger), in contrast to the sexual dimorphism observed in chimps where males have significantly larger and sharper upper canines than females.[3][16]

A. ramidus feet are better suited for walking than chimps. However, like non-human great apes, but unlike all previously recognized human ancestors, it had a grasping big toe adapted for locomotion in the trees (an

knuckle walking; and it seems to have used a method of locomotion unlike any modern great ape, which combined arboreal palm walking clambering and a form of bipedality more primitive than Australopithecus. The discovery of such unspecialized locomotion led American anthropologist Owen Lovejoy and colleagues to postulate that the chimpanzee–human last common ancestor used a similar method of locomotion.[9][18]

The upper pelvis (distance from the

plantarflexion),[17] the big toe (though still capable of grasping) was used for pushing off, and the legs were aligned directly over the ankles instead of bowing out like in non-human great apes.[21]

Paleobiology

Chimp skull (note the large canines and elongated face)

The reduced canine size and reduced skull robustness in A. ramidus males (about the same size in males and females) is typically correlated with reduced male–male conflict, increased parental investment, and

ateline monkeys[16] due to a process of self domestication (becoming more and more docile which allows for a more gracile build). Because a similar process is thought to have occurred with the comparatively docile bonobos from more aggressive chimps, A. ramidus society may have seen an increase in maternal care and female mate selection compared to its ancestors.[22] Alternatively, it is possible that increased male size is a derived trait instead of basal (it evolved later rather than earlier), and is a specialized adaptation in modern great apes as a response to a different and more physically exerting lifestyle in males than females rather than being tied to interspecific conflict.[12]

Australian anthropologists Gary Clark and

selective pressure to become more social. This would have allowed their society to become more complex. They also noted that the base of the skull stopped growing with the brain by the end of juvenility, whereas in chimps it continues growing with the rest of the body into adulthood; and considered this evidence of a switch from a gross skeletal anatomy trajectory to a neurological development trajectory due to selective pressure for sociability. Nonetheless, their conclusions are highly speculative.[22][23]

Hypothetical restoration of a female Ardipithecus using a hammer and anvil to crack open a nut

American primatologist

technologically advanced non-human.[24] However, Clark and Henneberg concluded that Ardipithecus cannot be compared to chimps, having been too similar to humans.[22] According to French paleoprimatologist Jean-Renaud Boisserie, the hands of Ardipithecus would have been dextrous enough to handle basic tools, though it has not been associated with any tools.[25]

The teeth of A. ramidus indicate that it was likely a generalized

C3 plants in woodlands or gallery forests. The teeth lacked adaptations for abrasive foods.[9][10][16] Lacking the speed and agility of chimps and baboons, meat intake by Ardipithecus, if done, would have been sourced from only what could have been captured by limited pursuit, or from scavenging carcasses.[26]

Paleoecology

Half of the large mammal species associated with A. ramidus at Aramis are

palm trees appear to have been common at the time from Aramis to the Gulf of Aden; and botanical evidence suggests a cool, humid climate.[30] Conversely, annual water deficit (the difference between water loss by evapotranspiration and water gain by precipitation) at Aramis was calculated to have been about 1,500 mm (59 in), which is seen in some of the hottest, driest parts of East Africa.[13]

C4 plants and grasses rather than forest plants. The area seems to have featured bushland and grasslands.[31]

See also

  • Australopithecus – Genus of hominin ancestral to modern humans
  • Ardi – Designation of the fossilized skeletal remains of an Ardipithecus ramidus
  • Graecopithecus – Extinct genus of hominids
  • Orrorin – Postulated early hominin discovered in Kenya
  • Paranthropus – Contested extinct genus of hominins
  • Sahelanthropus – Extinct hominid from Miocene Africa

References

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External links

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