Ardipithecus ramidus

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Ardipithecus ramidus
Temporal range: Zanclean 4.5–4.32  Ma
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Ardipithecus ramidus.jpg
A. ramidus at the Museo Nacional de Ciencias Naturales
Scientific classification OOjs UI icon edit-ltr.svg
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 australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). A. ramidus, unlike modern hominids, has adaptations for both walking on two legs (bipedality) and life in the trees (arboreality). However, it would not have been as efficient at bipedality as humans, nor at arboreality as non-human 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 Map of the fossil sites of the earliest hominids (35.8-3.3M BP).svg
Map showing discovery locations of various australopithecines

The first remains were described in 1994 by American anthropologist Tim D. White, Japanese paleoanthropologist Gen Suwa, and Ethiopian paleontologist Berhane Asfaw. The holotype specimen, ARA-VP-6/1, comprised an associated set of 10 teeth; and there were 16 other paratypes identified, preserving also skull and arm fragments. These were unearthed in the 4.4-million-year-old (Ma) deposits of the Afar region in Aramis, Ethiopia from 1992 to 1993, making them the oldest hominin remains at the time, surpassing Australopithecus afarensis . They initially classified it as Australopithecus ramidus, the species name deriving from the Afar language ramid "root". [1] In 1995, they made a 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 Middle Awash were classified as a subspecies of A. ramidus by Ethiopian paleoanthropologist Yohannes Haile-Selassie. [5] In 2004, Haile-Selassie, Suwa and White split it off into its own species, A. kadabba . [6] A. kadabba is considered to have been the direct ancestor of A. ramidus, making Ardipithecus a chronospecies. [7]

The exact affinities of Ardipithecus have been debated. White, in 1994, considered A. ramidus to have been more closely related to humans than chimpanzees, though noting it to be the most ape-like fossil hominin to date. [1] In 2001, French paleontologist Brigitte Senut and colleagues aligned it more closely to chimpanzees, [8] but this has been refuted. [5] In 2009, White and colleagues reaffirmed the position of Ardipithecus as more closely related to modern humans based on dental similarity, a short base of the skull, and adaptations to bipedality. [9] In 2011, primatologist Esteban Sarmiento said that there is not enough evidence to assign Ardipithecus to Hominini (comprising both humans and chimps), [10] but its closer affinities to humans have been reaffirmed in following years. [11] White and colleagues consider it to have been closely related to or the ancestor of the temporally close Australopithecus anamensis , which was the ancestor to 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. kadabbaArdipithecus ramidus

Before the discovery of Ardipithecus and other pre-Australopithecus hominins, it was assumed that the chimpanzee–human last common ancestor and preceding apes appeared much like modern-day chimpanzees, orangutans and gorillas, which would have meant these three changed very little over millions of years. Their discovery led to the postulation that modern great apes, much like humans, evolved several specialized adaptations to their environment (have highly derived 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 Ardipithecus Gesamt1.jpg
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 arboreal lifestyle), though it was likely not as specialized for grasping as it is in modern great apes. [9] [17] Its tibial and tarsal lengths indicate a leaping ability similar to bonobos. [10] It lacks any characters suggestive of specialized suspension, vertical climbing, or 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 sacrum to the hip joint) is shorter than in any known ape. It is inferred to have had a long lumbar vertebral series, and lordosis (human curvature of the spine), which are adaptations for bipedality. However, the legs were not completely aligned with the torso (were anterolaterally displaced), and Ardipithecus may have relied more on its quadriceps than hamstrings which is more effective for climbing than walking. [7] [19] However, it lacked foot arches and had to adopt a flat-footed stance. These would have made it less efficient at walking and running than Australopithecus and Homo. It may not have employed a bipedal gait for very long time intervals. [3] It may have predominantly used palm walking on the ground, [20] Nonetheless, A. ramidus still had specialized adaptations for bipedality, such as a robust fibularis longus muscle used in pushing the foot off the ground while walking (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) Modell eines Schadels des Pan troglodytes (Schimpanse, weiblich).jpg
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 monogamy. [7] [9] Because of this, it is assumed that A. ramidus lived in a society similar to bonobos 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 Maciej Henneberg argued that such shortening of the skull—which may have caused a descension of the larynx—as well as lordosis—allowing better movement of the larynx—increased vocal ability, significantly pushing back the origin of language to well before the evolution of Homo. They argued that self domestication was aided by the development of vocalization, living in a pro-social society, as a means of non-violently dealing with conflict. They conceded that chimps and A. ramidus likely had the same vocal capabilities, but said that A. ramidus made use of more complex vocalizations, and vocalized at the same level as a human infant due to 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 Nut cracking Ardipithecus.png
Hypothetical restoration of a female Ardipithecus using a hammer and anvil to crack open a nut

American primatologist Craig Stanford postulated that A. ramidus behaved similarly to chimps, which frequent both the trees and the ground, have a polygynous society, hunt cooperatively, and are the most 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 omnivore and fruit eater which predominantly consumed 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 spiral-horned antelope and colobine monkeys (namely Kuseracolobus and Pliopapio ). There are a few specimens of primitive white and black rhino species, and elephants, giraffes and hippo specimens are less abundant. These animals indicate that Aramis ranged from wooded grasslands to forests, but A. ramidus likely preferred the closed habitats, [27] specifically riverine areas as such water sources may have supported more canopy coverage. [28] Aramis as a whole generally had less than 25% canopy cover. [13] There were exceedingly high rates of scavenging, indicating a highly competitive environment somewhat like Ngorongoro Crater. Predators of the area were the hyenas Ikelohyaena abronia and Crocuta dietrichi , the bear Agriotherium , the cats Dinofelis and Megantereon , the dog Eucyon , and crocodiles. [29] Bayberry, hackberry and 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]

Carbon isotope analyses of the herbivore teeth from the Gona Western Margin associated with A. ramidus indicate that these herbivores fed mainly on C4 plants and grasses rather than forest plants. The area seems to have featured bushland and grasslands. [31]

See also

Related Research Articles

<i>Ardipithecus</i> Extinct genus of hominins

Ardipithecus is a genus of an extinct hominine that lived during the Late Miocene and Early Pliocene epochs in the Afar Depression, Ethiopia. Originally described as one of the earliest ancestors of humans after they diverged from the chimpanzees, the relation of this genus to human ancestors and whether it is a hominin is now a matter of debate. Two fossil species are described in the literature: A. ramidus, which lived about 4.4 million years ago during the early Pliocene, and A. kadabba, dated to approximately 5.6 million years ago. Initial behavioral analysis indicated that Ardipithecus could be very similar to chimpanzees, however more recent analysis based on canine size and lack of canine sexual dimorphism indicates that Ardipithecus was characterised by reduced aggression, and that they more closely resemble bonobos.

<span class="mw-page-title-main">Bipedalism</span> Terrestrial locomotion using two limbs

Bipedalism is a form of terrestrial locomotion where a tetrapod moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped, meaning 'two feet'. Types of bipedal movement include walking or running and hopping.

<span class="mw-page-title-main">Homininae</span> Subfamily of mammals

Homininae, also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini ―and 2) the tribe Gorillini (gorillas). Alternatively, the genus Pan is sometimes considered to belong to its own third tribe, Panini. Homininae comprises all hominids that arose after orangutans split from the line of great apes. The Homininae cladogram has three main branches, which lead to gorillas and to humans and chimpanzees. There are two living species of Panina and two living species of gorillas, but only one extant human species. Traces of extinct Homo species, including Homo floresiensis have been found with dates as recent as 40,000 years ago. Organisms in this subfamily are described as hominine or hominines.

<i>Australopithecus</i> Genus of hominin ancestral to modern humans

Australopithecus is a genus of early hominins that existed in Africa during the Pliocene and Early Pleistocene. The genera Homo, Paranthropus, and Kenyanthropus evolved from some Australopithecus species. Australopithecus is a member of the subtribe Australopithecina, which sometimes also includes Ardipithecus, though the term "australopithecine" is sometimes used to refer only to members of Australopithecus. Species include A. garhi, A. africanus, A. sediba, A. afarensis, A. anamensis, A. bahrelghazali and A. deyiremeda. Debate exists as to whether some Australopithecus species should be reclassified into new genera, or if Paranthropus and Kenyanthropus are synonymous with Australopithecus, in part because of the taxonomic inconsistency.

<i>Australopithecus afarensis</i> Extinct hominid from the Pliocene of East Africa

Australopithecus afarensis is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not take place until the 1970s. From 1972 to 1977, the International Afar Research Expedition—led by anthropologists Maurice Taieb, Donald Johanson and Yves Coppens—unearthed several hundreds of hominin specimens in Hadar, Ethiopia, the most significant being the exceedingly well-preserved skeleton AL 288-1 ("Lucy") and the site AL 333. Beginning in 1974, Mary Leakey led an expedition into Laetoli, Tanzania, and notably recovered fossil trackways. In 1978, the species was first described, but this was followed by arguments for splitting the wealth of specimens into different species given the wide range of variation which had been attributed to sexual dimorphism. A. afarensis probably descended from A. anamensis and is hypothesised to have given rise to Homo, though the latter is debated.

Paleoanthropology or paleo-anthropology is a branch of paleontology and anthropology which seeks to understand the early development of anatomically modern humans, a process known as hominization, through the reconstruction of evolutionary kinship lines within the family Hominidae, working from biological evidence and cultural evidence.

<i>Australopithecus anamensis</i> Extinct hominin from Pliocene east Africa

Australopithecus anamensis is a hominin species that lived approximately between 4.2 and 3.8 million years ago and is the oldest known Australopithecus species, living during the Plio-Pleistocene era.

<span class="mw-page-title-main">Afar Triangle</span> Geological depression caused by the Afar Triple Junction

The Afar Triangle is a geological depression caused by the Afar Triple Junction, which is part of the Great Rift Valley in East Africa. The region has disclosed fossil specimens of the very earliest hominins; that is, the earliest of the human clade, and it is thought by some paleontologists to be the cradle of the evolution of humans. The Depression overlaps the borders of Eritrea, Djibouti and the entire Afar Region of Ethiopia; and it contains the lowest point in Africa, Lake Assal, Djibouti, at 155 m (509 ft) below sea level.

<i>Australopithecus garhi</i> Extinct hominid from the Afar Region of Ethiopia 2.6–2.5 million years ago

Australopithecus garhi is a species of australopithecine from the Bouri Formation in the Afar Region of Ethiopia 2.6–2.5 million years ago (mya) during the Early Pleistocene. The first remains were described in 1999 based on several skeletal elements uncovered in the three years preceding. A. garhi was originally considered to have been a direct ancestor to Homo and the human line, but is now thought to have been an offshoot. Like other australopithecines, A. garhi had a brain volume of 450 cc (27 cu in); a jaw which jutted out (prognathism); relatively large molars and premolars; adaptations for both walking on two legs (bipedalism) and grasping while climbing (arboreality); and it is possible that, though unclear if, males were larger than females. One individual, presumed female based on size, may have been 140 cm tall.

<span class="mw-page-title-main">Hominini</span> Tribe of mammals

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<span class="mw-page-title-main">Tim D. White</span> American paleoanthropologist

Tim D. White is an American paleoanthropologist and Professor of Integrative Biology at the University of California, Berkeley. He is best known for leading the team which discovered Ardi, the type specimen of Ardipithecus ramidus, a 4.4 million-year-old likely human ancestor. Prior to that discovery, his early career was notable for his work on Lucy as Australopithecus afarensis with discoverer Donald Johanson.

<span class="mw-page-title-main">Middle Awash</span> UNESCO World Heritage Site in Ethiopia

The Middle Awash is a paleoanthropological research area in the northwest corner of Gabi Rasu in the Afar Region along the Awash River in Ethiopia's Afar Depression. It is a unique natural laboratory for the study of human origins and evolution and a number of fossils of the earliest hominins, particularly of the Australopithecines, as well as some of the oldest known Olduwan stone artifacts, have been found at the site—all of late Miocene, the Pliocene, and the very early Pleistocene times, that is, about 5.6 million years ago (mya) to 2.5 mya. It is broadly thought that the divergence of the lines of the earliest humans (hominins) and of chimpanzees (hominids) was completed near the beginning of that time range, or sometime between seven and five mya. However, the larger community of scientists provide several estimates for periods of divergence that imply a greater range for this event, see CHLCA: human-chimpanzee split.

<span class="mw-page-title-main">Third trochanter</span>

In human anatomy, the third trochanter is a bony projection occasionally present on the proximal femur near the superior border of the gluteal tuberosity. When present, it is oblong, rounded, or conical in shape and sometimes continuous with the gluteal ridge. It generally occurs bilaterally without significant side to side dimorphism. A structure of minor importance in humans, the incidence of the third trochanter varies from 17 to 72% between ethnic groups and it is frequently reported as more common in females than in males. Structures analogous to the third trochanter are present in other mammals, including some primates. It is called the third trochanter in reference to the greater and lesser trochanters that are always present on the femur.

<i>Ardipithecus kadabba</i> Hominin fossil

Ardipithecus kadabba is the scientific classification given to fossil remains "known only from teeth and bits and pieces of skeletal bones", originally estimated to be 5.8 to 5.2 million years old, and later revised to 5.77 to 5.54 million years old. According to the first description, these fossils are close to the common ancestor of chimps and humans. Their development lines are estimated to have parted 6.5–5.5 million years ago. It has been described as a "probable chronospecies" of A. ramidus. Although originally considered a subspecies of A. ramidus, in 2004 anthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim D. White published an article elevating A. kadabba to species level on the basis of newly discovered teeth from Ethiopia. These teeth show "primitive morphology and wear pattern" which demonstrate that A. kadabba is a distinct species from A. ramidus.

Ardi (ARA-VP-6/500) is the designation of the fossilized skeletal remains of an Ardipithecus ramidus, thought to be an early human-like female anthropoid 4.4 million years old. It is the most complete early hominid specimen, with most of the skull, teeth, pelvis, hands and feet, more complete than the previously known Australopithecus afarensis specimen called "Lucy". In all, 125 different pieces of fossilized bone were found.

Claude Owen Lovejoy is an evolutionary anthropologist and anatomist at Kent State University Ohio. He is best known for his work on Australopithecine locomotion and the origins of bipedalism. "The origin of man", which he published in Science in January 1981, is cited as among his best-known articles. The 'C' of his name stands for Claude, but he never uses the name and is known only as Owen.

The chimpanzee–human last common ancestor (CHLCA) is the last common ancestor shared by the extant Homo (human) and Pan genera of Hominini. Estimates of the divergence date vary widely from thirteen to five million years ago.

Changes to the dental morphology and jaw are major elements of hominid evolution. These changes were driven by the types and processing of food eaten. The evolution of the jaw is thought to have facilitated encephalization, speech, and the formation of the uniquely human chin.

The savannah hypothesis is a hypothesis that human bipedalism evolved as a direct result of human ancestors' transition from an arboreal lifestyle to one on the savannas. According to the hypothesis, hominins left the woodlands that had previously been their natural habitat millions of years ago and adapted to their new habitat by walking upright.

Pliopapio is an extinct genus of Old World monkey known from the latest part of the Miocene to the early Pliocene Epochs from the Afar Region of Ethiopia. It was first described based on a very large series of fossils from the site of Aramis in the Middle Awash, which has been dated by 40Ar/39Ar to 4.4 million years old. It has since been found from similarly aged sediments at Gona, approximately 75 km to the North. Additional fossils from the Middle Awash extend its known time range back to at least 5.3 million years ago. There is only one known species, Pliopapio alemui.

References

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  8. ↑ Senut, B.; Pickford, M.; Gommery, D.; et al. (2001). "Premier hominidé du Miocène (formation de Lukeino, Kenya)" [First hominid from the Miocene (Lukeino Formation, Kenya)]. Comptes Rendus de l'Académie des Sciences (in French). 332 (2): 137–144. Bibcode:2001CRASE.332..137S. doi:10.1016/S1251-8050(01)01529-4. S2CID   14235881.
  9. 1 2 3 4 5 White, T. D.; Asfaw, B.; Beyene, Y.; Haile-Selassie, Y.; Lovejoy, C. O.; Suwa, G.; WoldeGabriel, G. (2009). "Ardipithecus ramidus and the Paleobiology of Early Hominids". Science . 326 (5949): 75–86. Bibcode:2009Sci...326...75W. doi:10.1126/science.1175802. PMID   19810190. S2CID   20189444.
  10. 1 2 3 Sarmiento, E. E.; Meldrum, D. J. (2011). "Behavioral and phylogenetic implications of a narrow allometric study of Ardipithecus ramidus". HOMO. 62 (2): 75–108. doi:10.1016/j.jchb.2011.01.003. PMID   21388620.
  11. ↑ Kimbel, W. H.; Suwa, G.; Asfaw, B.; Rak, Y.; White, T. D. (2014). "Ardipithecus ramidus and the evolution of the human cranial base". Proceedings of the National Academy of Sciences. 111 (3): 948–953. Bibcode:2014PNAS..111..948K. doi: 10.1073/pnas.1322639111 . PMC   3903226 . PMID   24395771.
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