Ardipithecus ramidus Presentation

Ardipithecus ramidus  

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Outlline

●Geology, geochronology, and environment●Morphology●Diet and dentition●Social behavior

Ardipithecus ramidusOrder: PrimatesSuborder: AnthropoideaSuperfamily: HominoideaEtymology: Ramid= Root, Ardi=groundHolotype: ARA-VP-6/1  Left I1,C,P3, P4  Right I1,C (broken),P3,M2  Lower Right P3,P4Paratypes: Post Cranial Elements

Geology, Geography, and Environment

Geography●Aramis - Aramis and Adgantoli drainages, west of Awash River (Aramis VP locality 6 [paratypic specimens, 10°28,74'N, 40°26,26E, roughly 625m in elevation])

●Lower Aramis Member strata between Daam Aatu Basaltic Tuff and Gaala Vitric TuffoSilt, sand, and clayo3-6m thick layer of strata

●Northernmost section of Rift Valley

http://www.scitechexplained.com/2010/04/ardi-ardipithecus-ramidus-the-story-of-the-earliest-bipedal-hominin-species-ever-found/

Geology●Exact delineations of major tectonic plates not known

●Area stressed by movement of Saudi Arabia away from Africao Triple Junction - 3 plates pulling away from one another, with Afar Triangle in the center Nubian, Somalian, and Arabian Plates Nubian and Somali Plates were previously collectively known as the African Plate, but significant rifting has separated it into two, with the Somali Plate drifting away from from Africa East African Rift - between Nubian and Somalian Plates

Red Sea Rift - between Nubian and Arabian Plates

Aden Ridge - between Somali and Arabian Plates

●Rifting provides new contexts and easier access to in situ fossils by uncovering previously inaccessible strata and uplifting sedimentary layers

http://www.sciencemag.org/content/326/5949/65/F1.expansion.html

General outline, Afar Triangle

http://records.viu.ca/~earles/afar-dyke-jul06.htm

http://geology.com/articles/east-africa-rift.shtml

Environment

●Pliocene climate - cooler and drier than Miocene

●Ar/Ar dating allows for dating of volcanic activity in region

●Predominance of arboreal adaptations and specifications over terrestrial ones support predominant living in woodland (not to be confused with forested) habitatoUnlike forested habitats, woodlands may have grasses at ground level, and upper tree canopies do not necessarily cover the entire landscape, whereas forests tend to be much thicker, with the upper levels covering the vast majority of the ground levels (closed canopy), discouraging grass growth at ground floor

Environment

●Isotopic analysis indicates woodland environment with small patches of denser forestoContradicts savannah theory, oEnvironment generally favors omnivory and arboreality

oMinimal vegetation component according to dental analysis, inconsistent with grassland feeding

Environment

●Carbon/oxygen isotope analysis of sediment in locality implies predominantly woodland environment, along with conciliatory evidence provided by woodland avian and mammalian fossils found in the same contextoColobine monkeys (leaf eating and largely arboreal) found within same locality, along with Tragelophine bovids, such as the spiral-horned antelope, which preferred woodland environments

oMajority of mammals practiced frugivory, omnivory, or herbivory, consistent with woodland environments

oFew hippopotamids, giraffids, etc. (grazing species), especially in comparison to areas further south, further reducing likelihood of swampland or savannah habitat

Environment

●Majority of mammal biomass were likely browsers and frugivores, both of which are largely unsustainable in environments without an abundance of trees

●Fossilized pollen, seeds, wood, etc. indicate palm and fig trees, not in true forest 

●Specimens found in wide range of environments, such as Gona, do not necessarily mean that Ar. ramidus took full advantage of each different ecological niche opportunity

Morphology

Cranial DescriptionARA-VP-1/125 & 1/500•Temporal and Occipital region of the skull.•The morphology is similar to that of a chimpanzee. –The occipital  condyle is  small–The foramen magnum is intersected by a bicarotid chord connecting the carotid foramina–The temporomandibular joint is very flat. Weak interior protection.–The hypoglossal canal is anterior to the internal auditory meatus than in great apes.  This is because of a shortening of the basioccipital part of the cranial base relative to modern African apes.

Features  •The wrist is not as stiff

because of the midcarpal joint. •Lacks the expansion of the metacarpal head that is typical of knuckle walking.•MC5-hamate allows more flexibility than any other form extant hominoid from a neutral position.

Science, Vol. 326, October 2, 2009

Postcranial Description•ARA-VP-7/2–3 bones from the left arm–Larger than A. afarensis  from Hadar., but smaller than other Ar. ramidus.–Humeral head is 30% smaller than the smallest  A. afarensis. •Characteristics that are shared with other hominids–Elliptical humeral head–Blunt proximally extended ulnar olecranon process–Anteriorly facing ulnar brachialis insertion, and anteriorly oriented trochlear notch.•Characteristics that are shared with apes–Strong angulation of the distal radial articular surface, because of a large styloid process–Strong lateral trochlear ridge on the distal humerus, this is also seen in some specimens of A. afarensis•Differences from African apes–Proximal humerus does not have the deep bicipital groove seen in most African apes.

Features•Primitive features absent in specialized apes.•Short metacarpals•Extended joint surface on the 5th digit•Thumb is more robust than apes•Capitate head allows strong palm flexion•Hands lacked ape specialization that protected them while climbing trees.•Metacarpals are short, no prominent ridges associated with knuckle walking.

Science, Vol. 326, October 2, 2009

Central Joint Complex•Palmer portion of the lateral side of the capitate has been extended to create a capitate MC2, which the MC2 to act as a buttress against rotation.•Lateral portion of the capitate’s dorsodistal surface has been withdrawn proximally.•A screw affect is created by dorsal portions of the joint surfaces that are angled opposite of their palmar portions.–This screw affect creates a way to dissipate energy during loading•The CJCs in apes have increased rigidity and permit energy dissipation while in suspension and vertical climbing, and possibly during knuckle walking.

CJC of Ar. ramidus

Science, Vol. 326, October 2, 2009

●Simple planar joint surface, similar to Old World Monkeys.

●All four CJCs meet at together to form a single dosopalmer axis

●It can only resist rotation by its subcondral surface.

Locomotion 

●We know that our last common ancestor had limbs similar in proportion to those of a monkey that of an ape.oBracholindices(BI) R/Hx100oRatio of bones similar to quadrupeds, T/Fx100

●The lower back is thought to have had 6 lumbar vertebrate,instead of 3 or 4 like African apes. 

●The metacarpal bends to a degree that would allow it to bend backwards.  This would allow it to move through branches on its palms.

●Relatively long phalanges to Pan, abbreviated to Homo●Metacarpals 2-5 are shorter than those of extant ape.●Ardipithecus is the only hominid with a metacarpal head reminiscent metacarpal phalangeal joint found in Miocene hominoids.

The Foot●Adapted to climbing straight up.

●Os peroneum  is key to the evolution of the foot.oIt is attached to the fibularis longus.

●Apes lack Os peroneum●Our feet evolved in a different direction from apes.

●Talar axis angle is similar to apes, other hominids have lower angles.

●Flexor hallucis has a longer groove which createsmore angle and increased trapezoidal size.

●Creates more knee rotation, knee and hip extensionduring upright gait.

●Midfoot structure is similar to those of Old World Monkeysthan Apes.

Diet and Dentition(Diet)

Enamel thickness:Thin enamel (as seen in chimpanzees) suggests a diet of "soft" foods, while thick enamel (like that of Homo) would imply a diet of more "abrasive" foods.Enamel thickness in Ardipithecus ramidus is in the middle, which points to omnivory.

There is debate about whether Ar. ramidus dentition would suggest frugivory.

Diet and Dentition(Diet)

Low bunodont cusps suggest that the Ar. ramidus diet was not particularly abrasive but may have included some hard foods. It is consistent with a partially terrestrial, partially arboreal pattern of feeding. From left to right:

Ar. ramidus

Au. anamensis

Au. afarensis

Diet and Dentition(Diet)

  According to one Middle Awash research team, "Ar. ramidus was more omnivorous than chimpanzees based on the size, shape, and enamel distribution of its teeth. It probably supplemented woodland plants such as fruits, nuts, and tubers with the occasional insects, small mammals, or bird eggs" (Ann Gibbons).

Diet and Dentition(Diet) 

Carbon-isotope studies of teeth from five individuals show that Ar. ramidus ate mostly woodland, rather than grassland, plants. This new evidence overwhelmingly refutes the once favored but now moribund hypothesis that upright-walking hominins arose in open grasslands (Ann Gibbons).

Favoring a woodland environment as opposed to grasslands is a somewhat disputed issue. However, it does seem as if a large majority agrees that based on evidence of diet, "Ar. ramidus was a denizen of woodland with small patches of forest" (Science).

Diet and Dentition(Dentition)

●Ar. ramidus dentition is closely related in size to that of chimpanzees. However, "the Ar. ramidus P3 is relatively smaller than that of Pan... and is comparable to those of Au. anamensis and Au. afarensis" (Science, Vol 326).

●There is no evidence of honing. The upper and lower canines do exhibit wear slopes, as is also seen in Au. afarensis and Au. anamensis. 

● "The Ar. dentition suggests omnivory. It exhibits none of the specializations seen among modern apes; neither the large incisors of Pongo or Pan, nor the specialized molar morphology of Pongo, Pan, or Gorilla" (Science).

Diet and Dentition(Dentition)

●Molars overlap with Pan in some measures of enamel thickness but differ in overall thickness and structure (Science).

●Molar morphology between Pan and Ar. ramidus differ greatly. Chimpanzee molars have a broad basin with thin enamel. This is most likely an adaptation to crushing soft foods (ripe fruits) while retaining some shearing capacities (Science).

In contrast to Pan dentition... 

Diet and Dentition(Dentition)

The lower canines of Ar. ramidus retain more apelike morphology than do the uppers. 

The Ar. ramidus canines are morphologically derived in the direction of later hominids.

Diet and Dentition(Dentition)

Ar. ramidus teeth show trend of reduced canine/premolar size. 

There's debate about whether sexual dimorphism is present or not. 

Diet and Dentition (Dentition)

The canine tooth changed and started evolving into a more "diamond-shaped" structure.

What could be the implications of reduced canines in Ar. ramidus?

Social Behavior

1)      Paleobiological implications of the Ardipithecus ramidus dentitiona)      "The Ar. ramidus dental morphology and wear pattern are consistent with a partially terrestrial, omnivorous/frugivorous niche. Analyses show that the ARA-VP-6/500 skeleton is female and that Ar. ramidus was nearly monomorphic in canine size and shape. The canine/tower third premolar complex indicates a reduction of canine size and honing capacity early in hominid evolution, possibly driven by selection targeted on the male upper canine."

1)  Ardipithecus ramidus and the paleobiology of early hominidsa)      "Its ecological habitat appears to have been largely woodland-focused."b)      "Ar. ramidus had a reduced canine/ premolar complex and a little-derived cranial morphology and consumed a predominantly [C.sub.3] plant--based diet (plants using the [C.sub.3] photosynthetic pathway)."

Social Behavior

a)    “Functionally important sex-related size dimorphism is not apparent.”b)     “The largest, presumably male, specimens are as morphologically derived as the smallest, showing that dimorphic canine morphology was virtually absent in these hominids by 4.4 Ma.”c)     "indicating that the canine was not an important component of adult sociobehavioral relationships"d)    “The Ar. ramidus canines are metrically and morphologically derived in the direction of later hominids, and we hypothesize that reduction and alteration of upper canine size and shape in this and earlier hominid species are related to changes in social behaviors (22, 31).”e)    “Most likely, Ardipithecus exhibited minimal skeletal body size dimorphism, similar to Pan, consistent with a male-bonded social system, most likely a primitive retention from the CLCA condition (31).”f)     “The CLCA probably also combined moderate canine dimorphism with minimal skull and body size dimorphism (22, 23), most likely associated with relatively weak male-male agonism in a male philopatric social system (22, 23, 31).”

Social Behavior

1)      “The Amazing Ardi” Kenneth W. Krausea)    “And her canines – along with those of her male counterparts – were smaller and less sharpened, or honed, possibly evidencing the adoption among early hominins of less aggressive and more cooperative social and reproductive strategies well prior to the development of enlarged brains and tool use.”Omnivorous (less competition)a)      "It is far more likely," he vies, "that our unique reproductive behavior and anatomy emerged in concert with habituation to bipedality and elimination of the [sectorial canine complex]." He summarizes: The ability of Ardi's kind to exploit both trees and land and to reduce male-to-male conflict merged with "three previously unseen behaviors"--regular food carrying, ovulatory crypsis, and pair bonding--to intensify parental investment among males."

Social Behavior

a)     “within a unique reproductive strategy that allowed early homini(n)s to thrive relative to their ape relatives and could have ultimately accommodated rapid development of the unusually energy-thirst(y) brain…in Homo.”b)    “the brain of Ar. ramidus may have already begun to develop some aspects of later hominid-like form and function.”

 c)      "And Cambridge University anthropologist William McGrew claims that in fact there was a sufficient size distinction between Ar. ramidus males and females to indicate heightened male aggression and promiscuity."

Social Behavior

"Breakthrough adaptations can transform life-history by deviating from typical reproductive strategy. Early hominids show feminized male canines (left) and primitive bipedality (right). These suggest that females preferred nonaggressive males who gained reproductive success by obtaining copulation in exchange for valuable foods (vested provisioning). Success would depend on copulatory frequency with mates whose fertility remained cryptic (e.g., absence of cycling in mammary size). The result would be reduced agonism in unrelated females, and cooperative expansion of day ranges among equally cooperative males, eventually leading to exploitation of new habitats."

Social Behavior

1)      Ardi and Human Sexual Evolutiona)      “This variation may have developed as a result of our emerging ability to use tools to adapt to otherwise inhospitable habitats--to create a protected environment, complete with warmth and stored food, that could allow babies to survive at any time of the year.”b)      “But Ardi’s canines are relatively small, so her discoverers speculate that pair-bonding may have been developing, with females exerting more control in the mating process than with other primates.”c)      “Ardi’s relatively small canines suggest that a similar pattern might have been emerging for this intermediate species, one in which males did not dominate females or compete aggressively for resources, but instead a situation closer to that of the bonobo, characterized by free-floating sexual liaisons and frequent eruptions of nonprocreative sex.”

Social Behavior

a)      “according to National Geographic, "The bonobo, an African ape closely related to humans, has an even bigger sexual appetite. Studies suggest 75 percent of bonobo sex is non-reproductive and that nearly all bonobos are bisexual." Thus the prevalence of homosexuality and bisexuality in our closest relatives is a matter of record.”b)      “the latter species has considerably larger canine teeth than does the bonobo--one of the prominent physical differences separating the two.”c)      Homosexuality as bonding mechanismi)        “females who had engaged in homosexual consorts during the mating season were likely to remain affinitively bonded (friends) throughout the year in contrast to male-female consort pairs which were not translated into year-round bonds.”ii)      “might be valuable as support networks for defending the young or dealing safely with outbreaks of aggression within the troop.”iii)    “perhaps a non-reproducing female could have served as a free-floating aunt to help her sisters raise their young”