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Ardipithecus radius
Ceph Wrote:Ardipithecus radius

[Image: ramidus_illustration_KC_sq.jpg]
Image Credit: Karen Carr Studio

Temporal range: Late Miocene - Early Pliocene, 5.6–4.4Ma

Scientific classification
Tribe:(debated[1] Hominini)
Species:Ardipithecus kadabba

Ardipithecus ramidus was first reported in 1994; in 2009, scientists announced a partial skeleton, nicknamed ‘Ardi’. The foot bones in this skeleton indicate a divergent large toe combined with a rigid foot – it's still unclear what this means concerning bipedal behavior. The pelvis, reconstructed from a crushed specimen, is said to show adaptations that combine tree-climbing and bipedal activity. The discoverers argue that the ‘Ardi’ skeleton reflects a human-African ape common ancestor that was not chimpanzee-like. A good sample of canine teeth of this species indicates very little difference in size between males and females in this species.

Ardi’s fossils were found alongside faunal remains indicating she lived in a wooded environment. This contradicts the open savanna theory for the origin of bipedalism, which states that humans learned to walk upright as climates became drier and environments became more open and grassy.

Nickname:  Ardi
Where Lived:  Eastern Africa (Middle Awash and Gona, Ethiopia)
When Lived:  About 4.4 million years ago
[Image: ramidus_timeline_0.png?1268659147]

Year of Discovery:  1994
History of Discovery: 
A team led by American paleoanthropologist Tim White discovered the first Ardipithecus ramidus fossils in the Middle Awash area of Ethiopia between 1992 and 1994. Since that time, White’s team have uncovered over 100 fossil specimens of Ar. ramidus . White and his colleagues gave their discovery the name Ardipithecus ramidus (‘ramid’ means ‘root’ in the Afar language of Ethiopia and refers to the closeness of this new species to the roots of humanity, while ‘Ardi’ means ‘ground’ or ‘floor’). At the time of this discovery, the genus Australopithecus was scientifically well established, so White devised the genus name Ardipithecus to distinguish this new genus from Australopithecus. In 2009, scientists formally announced and published the findings of a partial skeleton (ARA-VP-6/500), nicknamed "Ardi", first found in 1994.

Females: average 3 ft 11 inches (120 centimeters)
Females: average 110 lbs (50 kg)
Height & Weight Supplemental Information: 
The only body size estimate scientists have made so far is based on the partial female skeleton ARA-VP-6/500 ("Ardi"). She is estimated to have stood 3 ft 11 in (120 cm) tall, weighed approximately 110 lbs (50 kg). Based on the size of the upper canine teeth in males is not much larger than the canines of females, scientists don’t expect Ar. ramidus to have shown much body size sexual dimorphism - so a male individual would have been similar to a female in size.  It’s possible that Ar. ramidus males did not compete against each other for dominance, and therefore did not need to grow bigger in size.

We don’t know everything about our early ancestors—but we keep learning more! Paleoanthropologists are constantly in the field, excavating new areas with groundbreaking technology, and continually filling in some of the gaps about our understanding of human evolution.

[Image: ardipithecusramidus3.jpg]

Below are some of the still unanswered questions about Ardipithecus ramidus that may be answered with future discoveries:

Does the pelvis of Ar. ramidus support the hypothesis that this early human species was bipedal? The pelvis was reconstructed from crushed fossils and, according to some scientists, is only suggestive of bipedalism.
What is the average size of male Ar. ramidus individuals? If more fossils support the original finding of relatively low sexual dimorphism, how does this relate to male and female size differences in other early humans at the base of our family tree -- and what does it mean?
First paper:

White, T.D., Suwa, G., Asfaw, B., 1994. Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371, 306-312. 

[big]Ardipithecus ramidus[/big]

A. ramidus was named in September 1994. The first fossil found was dated to 4.4 million years ago on the basis of its stratigraphic position between two volcanic strata: the basal Gaala Tuff Complex (GATC) and the Daam Aatu Basaltic Tuff (DABT). The name Ardipithecus ramidus stems mostly from the Afar language, in which Ardi means "ground/floor" (borrowed from the Semitic root in either Amharic or Arabic) and ramid means "root". The pithecus portion of the name is from the Greek word for "ape".

Like most hominids, but unlike all previously recognized hominins, it had a grasping hallux or big toe adapted for locomotion in the trees. It is not confirmed how much other features of its skeleton reflect adaptation to bipedalism on the ground as well. Like later hominins, Ardipithecus had reduced canine teeth.

In 1992–1993 a research team headed by Tim White discovered the first A. ramidus fossils—seventeen fragments including skull, mandible, teeth and arm bones—from the Afar Depression in the Middle Awash river valley of Ethiopia. More fragments were recovered in 1994, amounting to 45% of the total skeleton. This fossil was originally described as a species of Australopithecus, but White and his colleagues later published a note in the same journal renaming the fossil under a new genus, Ardipithecus. Between 1999 and 2003, a multidisciplinary team led by Sileshi Semaw discovered bones and teeth of nine A. ramidus individuals at As Duma in the Gona Western Margin of Ethiopia's Afar Region. The fossils were dated to between 4.35 and 4.45 million years old.

Ardipithecus ramidus had a small brain, measuring between 300 and 350 cm3. This is slightly smaller than a modern bonobo or female common chimpanzee brain, but much smaller than the brain of australopithecines like Lucy (~400 to 550 cm3) and roughly 20% the size of the modern Homo sapiens brain. Like common chimpanzees, A. ramidus was much more prognathic than modern humans.

The teeth of A. ramidus lacked the specialization of other apes, and suggest that it was a generalized omnivore and frugivore (fruit eater) with a diet that did not depend heavily on foliage, fibrous plant material (roots, tubers, etc.), or hard and or abrasive food. The size of the upper canine tooth in A. ramidus males was not distinctly different from that of females. Their upper canines were less sharp than those of modern common chimpanzees in part because of this decreased upper canine size, as larger upper canines can be honed through wear against teeth in the lower mouth. The features of the upper canine in A. ramidus contrast with the sexual dimorphism observed in common chimpanzees, where males have significantly larger and sharper upper canine teeth than females.

The less pronounced nature of the upper canine teeth in A. ramidus has been used to infer aspects of the social behavior of the species and more ancestral hominids. In particular, it has been used to suggest that the last common ancestor of hominids and African apes was characterized by relatively little aggression between males and between groups. This is markedly different from social patterns in common chimpanzees, among which intermale and intergroup aggression are typically high. Researchers in a 2009 study said that this condition "compromises the living chimpanzee as a behavioral model for the ancestral hominid condition."

A. ramidus existed more recently than the most recent common ancestor of humans and chimpanzees (CLCA or Pan-Homo LCA) and thus is not fully representative of that common ancestor. Nevertheless, it is in some ways unlike chimpanzees, suggesting that the common ancestor differs from the modern chimpanzee. After the chimpanzee and human lineages diverged, both underwent substantial evolutionary change. Chimp feet are specialized for grasping trees; A. ramidus feet are better suited for walking. The canine teeth of A. ramidus are smaller, and equal in size between males and females, which suggests reduced male-to-male conflict, increased pair-bonding, and increased parental investment. "Thus, fundamental reproductive and social behavioral changes probably occurred in hominids long before they had enlarged brains and began to use stone tools," the research team concluded.


[Image: Ardipithecis_Ramidus_skeleton_1994-1996.jpeg]

On October 1, 2009, paleontologists formally announced the discovery of the relatively complete A. ramidus fossil skeleton first unearthed in 1994. The fossil is the remains of a small-brained 50-kilogram (110 lb) female, nicknamed "Ardi", and includes most of the skull and teeth, as well as the pelvis, hands, and feet. It was discovered in Ethiopia's harsh Afar desert at a site called Aramis in the Middle Awash region. Radiometric dating of the layers of volcanic ash encasing the deposits suggest that Ardi lived about 4.4 million years ago. This date, however, has been questioned by others. Fleagle and Kappelman suggest that the region in which Ardi was found is difficult to date radiometrically, and they argue that Ardi should be dated at 3.9 million years.

The fossil is regarded by its describers as shedding light on a stage of human evolution about which little was known, more than a million years before Lucy (Australopithecus afarensis), the iconic early human ancestor candidate who lived 3.2 million years ago, and was discovered in 1974 just 74 km (46 mi) away from Ardi's discovery site. However, because the "Ardi" skeleton is no more than 200,000 years older than the earliest fossils of Australopithecus, and may in fact be younger than they are, some researchers doubt that it can represent a direct ancestor of Australopithecus.

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Some researchers infer from the form of her pelvis and limbs and the presence of her abductable hallux, that "Ardi" was a facultative biped: bipedal when moving on the ground, but quadrupedal when moving about in tree branches. A. ramidus had a more primitive walking ability than later hominids, and could not walk or run for long distances. The teeth suggest omnivory, and are more generalised than those of modern apes.


The toe and pelvic structure of A. ramidus suggest to some researchers that the organism walked upright.

According to Scott Simpson, the Gona Project's physical anthropologist, the fossil evidence from the Middle Awash indicates that both A. kadabba and A. ramidus lived in "a mosaic of woodland and grasslands with lakes, swamps and springs nearby," but further research is needed to determine which habitat Ardipithecus at Gona preferred.

Alternative views and further studies

Due to several shared-characters with chimpanzees, its closeness to ape divergence period, and due to its fossil incompletenes, the exact position of Ardipithecus in the fossil record is a subject of controversy. Independent researcher such as Esteban E. Sarmiento of the Human Evolution Foundation in New Jersey, had systematically compared in 2010 the identifying characters of apes and human ancestral fossils in relation to Ardipithecus, and concluded that the comparison data is not sufficient to support an exclusive human lineage. Sarmiento noted that Ardipithecus does not share any characters exclusive to humans and some of its characters (those in the wrist and basicranium) suggest it diverged from the common human/African ape stock prior to the human, chimpanzee and gorilla divergence. His comparative (narrow allometry) study in 2011 on the molar and body segment lengths (which included living primates of similar body size) noted that some dimensions including short upper limbs, and metacarpals are reminiscent of humans, but other dimensions such as long toes and relative molar surface area are great ape-like. Sarmiento concluded that such length measures can change back and forth during evolution and are not very good indicators or relatedness. The Ardipithecus length measures, however, are good indicators of function and together with dental isotope data and the fauna and flora from the fossil site indicate Ardipithecus was mainly a terrestrial quadruped collecting a large portion of its food on the ground. Its arboreal behaviors would have been limited and suspension from branches solely from the upper limbs rare.

However, some later studies still argue for its classification in the human lineage. Comparative study in 2013 on carbon and oxygen stable isotopes within modern and fossil tooth enamel revealed that Ardipithecus fed both arboreally (on trees) and on the ground in a more open habitat, unlike chimpanzees and extinct ape Sivapithecus, thereby differentiating them from apes. In 2014 it was reported that the hand bones of Ardipithecus, Australopithecus sediba and A. afarensis consist of distinct human-lineage feature (which is the presence of third metacarpal styloid process, that is absent in apes). Unique brain organisations (such as lateral shift of the carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened, trapezoidal basioccipital element) in Ardipithecus are also found only Australopithecus and Homo clade. Comparison of the tooth root morphology with those of Sahelanthropus tchadensis also indicated strong resemblance,[24] implying its correct inclusion in human lineage.

Ardipithecus ramidus and the Paleobiology of Early Hominids

[Image: ardipithecusramidus.jpg]

Hominid fossils predating the emergence of Australopithecus have been sparse and fragmentary. The evolution of our lineage after the last common ancestor we shared with chimpanzees has therefore remained unclear. Ardipithecus ramidus, recovered in ecologically and temporally resolved contexts in Ethiopia’s Afar Rift, now illuminates earlier hominid paleobiology and aspects of extant African ape evolution. More than 110 specimens recovered from 4.4-million-year-old sediments include a partial skeleton with much of the skull, hands, feet, limbs, and pelvis. This hominid combined arboreal palmigrade clambering and careful climbing with a form of terrestrial bipedality more primitive than that of Australopithecus. Ar. ramidus had a reduced canine/premolar complex and a little-derived cranial morphology and consumed a predominantly C3 plant–based diet (plants using the C3 photosynthetic pathway). Its ecological habitat appears to have been largely woodland-focused. Ar. ramidus lacks any characters typical of suspension, vertical climbing, or knuckle-walking. Ar. ramidus indicates that despite the genetic similarities of living humans and chimpanzees, the ancestor we last shared probably differed substantially from any extant African ape. Hominids and extant African apes have each become highly specialized through very different evolutionary pathways. This evidence also illuminates the origins of orthogrady, bipedality, ecology, diet, and social behavior in earliest Hominidae and helps to define the basal hominid adaptation, thereby accentuating the derived nature of Australopithecus.

Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus

Social behaviour of fossil hominoid species is notoriously difficult to predict owing to difficulties in estimating body size dimorphism from fragmentary remains and, in hominins, low canine size dimorphism. Recent studies have shown that the second-to-fourth digit ratio (2D : 4D), a putative biomarker for prenatal androgen effects (PAEs), covaries with intra-sexual competition and social systems across haplorrhines; non-pair-bonded polygynous taxa have significantly lower 2D : 4D ratios (high PAE) than pair-bonded monogamous species. Here, we use proximal phalanx ratios of extant and fossil specimens to reconstruct the social systems of extinct hominoids. Pierolapithecus catalaunicus, Hispanopithecus laietanus and Ardipithecus ramidus have ratios consistent with polygynous extant species, whereas the ratio of Australopithecus afarensis is consistent with monogamous extant species. The early anatomically modern human Qafzeh 9 and Neanderthals have lower digit ratios than most contemporary human populations, indicating increased androgenization and possibly higher incidence of polygyny. Although speculative owing to small sample sizes, these results suggest that digit ratios represent a supplementary approach for elucidating the social systems of fossil hominids.

At long last, meet Ardipithecus ramidus

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Two restorations of “Ardi”, a 45% complete skeleton of Ardipithecus ramidus published in this week’s issue of Science. Restorations (including the full skeletal restoration below) by artist Jay Matternes.

The stories of “Ida” and “Ardi” could hardly be more different.

Ida was a lemur-like primate that lived 47 million years ago in an area that is now Messel, Germany. Ardi was much closer to us; she was one of the earliest hominins and lived 4.4 million years ago in what would become known as Ethiopia.

When the bones of Ida were discovered they were held in a private collection for years before being sold for an undisclosed sum to paleontologist Jorn Hurum. The first bones of Ardi were found in the field in 1992 by paleoanthropologist Gen Suwa. Enough fossils were found for an initial publication two years later, but the search continued for years afterward. That search has yielded over 100 bones from Ardi’s kind, representing about 36 individuals. Ardi is the most complete individual, with about 45% of the skeleton intact.

From almost the time of her acquisition by Hurum, Ida was groomed to be a star. The media company Atlantic Productions began to work with Hurum and immediately started the production of a book, several documentaries, and a bombastic public unveiling of a discovery they said would “CHANGE EVERYTHING.” The scientists describing Ida were forced to rush their study to meet the deadline set by the company The bones of Ardi and her kin, by contrast, were scrutinized with great detail over 15 years. So secret was the study of her bones that some frustrated paleoanthropologists called it the “Manhattan Project of anthropology.”

Ida was introduced to the public as the “ancestor of us all”, but the science behind this claim was flimsy. She was more closely related to lemurs than to us. Ardi, however, sits relatively close to the base of the human (i.e. hominin) family tree. Whether our species can trace its ancestral heritage back through hers will be something that will be debated in the months and years to come, but she is nevertheless one of our closest extinct relatives.

Ardi, of course, is short for Ardipithecus ramidus, one of the earliest hominins found to date. Her skeleton (see image below), as well as bits and pieces of other skeletons of the same species, were described this week in a special edition of the journal Science. While a close relative of Australopithecus afarensis (made famous by “Lucy”), Ardipithecus ramidus is about half a million years older than the earliest Australopithecus afarensis and is a bit closer to the last common ancestor between living chimpanzees and humans.* As such the remains of Ardi and her kind give us a closer look at how some of the earliest humans evolved.

*[I apologize if these technical names are cumbersome, but they are important. There are only a few hominin genera but many species, and when talking about Ardipithecus it is important to distinguish between the species given attention in this post, Ardipithecus ramidus, and its older sister species Ardipithecus kadabba.]

Contrary to the tirades of creationists, paleontologists have identified many evolutionary transitions in the fossil record, and the early history of hominins presents us with some of the most compelling evidence for evolutionary change. The nearly 45% complete skeleton of Ardipithecus ramidus, along with numerous other skeleton fragments from the same species, will enhance our understanding of what our earliest human relatives were like. Depending on your expectations, the skeleton is either very surprising or is consistent with ideas that have been kicked around for some time now.

One of the BIG questions in paleoanthropology, the kind that grabs the attention of the public as well as researchers, is “What was the last common ancestor of chimpanzees and humans like?” The traditional view is that the last common ancestor was a chimpanzee-like knuckle-walker, thus meaning that living chimpanzees are evolutionarily conservative (or have not changed much) while humans have changed radically. A key feature that makes us different from our closest ape relatives is our habitually bipedal posture. How, when, and why did early humans rise up off their knuckles and stand tall?

I say “stand tall” because there has long been an element of mythic storytelling in hypotheses involving the origins of our kind. We want to know the moment our ancestors went from “dumb monkeys” to “noble apes.” (See the book Narratives of Human Evolution for more on this.) As such the origin of bipedalism has often been attributed to the beginnings of other “defining” human characters. Our ancestors stood up to free their hands to gesticulate and carry tools, to allow mothers to carry babies more comfortably, to let them see predators and potential prey in the tall grass of the savanna, &c., &c. &c. The emphasis has long been on why our earliest ancestors stood up, but Ardipithecus ramidus suggests that they were already standing upright when they came down from the trees.

[Image: i-28d65815880a8f5463bb238c3ce953e1-ar-ra...-20053.jpg]

Since the discovery of “Lucy” paleoanthropologists have recognized that the early australopithecines, a diverse group of hominins including both our ancestors and other now-extinct lineages, had their hands in the trees and their feet on the ground. They had long arms with curved fingers suited to climbing in trees yet from the hips down they were adapted to walking bipedally. When the remains of even older hominins were found, like the ~6 million year old Orrorin tugenensis from Kenya and the controversial ~7 million year old hominid Sahelanthropus tchadensis from Chad, they too appeared to posses some traits related to bipedalism. The problem was that these older fossils were very incomplete; Orrorin is represented by a few carnivore-gnawed scraps and Sahelanthropus by a skull and a few teeth. More complete skeletons would be needed to see to what extent early hominins possessed bipedal and arboreal traits. Ardipithecus ramidus does so nicely.

Much like Australopithecus afarensis, Ardipithecus ramidus had upper-body traits that exhibit adaptation to life in the trees while it had relatively broad hips more consistent with bipedal locomotion. The arms of Ardipithecus ramidus were very long (it could put its hands on its knees standing up; take a moment and try to do the same) and it had hands tipped in curved fingers well-adapted to grasping branches (see image to the upper left). It does not appear to have moved through the canopy by swinging from limb to limb, like a gibbon, but instead moved through the trees on all fours, grasping the branches below it rather than hanging from those above.

[Image: i-4a147bced9ac5f8a3227b21be7f4e446-ar-ra...-20051.jpg]

The hips of Ardipithecus ramidus, however, suggest that it probably spent a good deal of time walking upright. (See image to the left. Grey reconstruction is Ardipithecus ramidus. Yellow is Australopithecus afarensis.) In knuckle-walking apes like chimpanzees the blades of the pelvis are flat and come up over the back. In Ardipithecus ramidus the blades of the pelvis form are somewhat more bowl-shaped, a shape that helps hold the viscera of the abdomen in place in hominins that were constantly walking upright. The arrangement in Ardipithecus ramidus did not provide as much support as in our own genus, Homo, or even later australopithecines, but it offered more support than the same bones in chimpanzees. (It should be noted, though, that this interpretation is already controversial.)

The femur, or thigh bone, may also provide a crucial clue to the bipedal habits of Ardipithecus ramidus. Based upon the skeletal restoration published in the Science papers the femur this hominin was oriented to meet the knee-joint at an angle. It might not look like it when you view your own legs, but if you could see your own skeleton when you stood up straight you would see the same orientation, only to a greater extent. This, along with the development of muscles between the top of the femur and the hip, are essential to keeping balance while walking upright.

[Image: i-1243a21b04b6fef839d27551700753c2-ar-ra...-20055.jpg]

This does not mean that Ardipithecus ramidus walked upright all the time or walked like we do, however. It probably only walked upright while on the ground, preferring to move on all fours by grasping while in the trees, and the muscles important to stabilization while walking were not as developed as in later, habitually bipedal hominins. Ardipithecus ramidus also had a very ape-like foot (see image to the left) with a divergent big toe. In our foot our big toe is in line with the rest and assists with the big push-off at the end of a step. Ardipithecus ramidus lacked this adaptation for walking on the ground. Between its foot morphology and lesser development of the hip muscles for balance, Ardipithecus ramidus may have even slightly swayed side to side while walking.

Yet there was an important difference between the foot of Ardipithecus ramidus and that of living chimpanzees and gorillas. The living African apes have flexible feet useful for grasping, almost like a second pair of hands. In monkeys and other primates, however, a particular bone (the os peroneum) embedded within a foot tendon helps keep the foot rigid, especially when jumping from one surface to another. According to the authors of the study, our close ape relatives have lost this “rigid foot” adaptation while our species has retained it. It appears that Ardipithecus ramidus had this “rigid foot” mechanism too, and this relatively simple trait might have had a major effect on its ability to walk on land. Being able to keep a rigid foot while walking on the ground might have been a subtle feature that nevertheless had great importance as early hominins began to walk on the ground more often.

[Image: i-2605258adc5c777e2bd7f445b03f49a7-ar-ra...-20057.jpg]

In general, though, it appears that the adaptations that allowed Ardipithecus ramidus to walk upright on the ground were exaptations. Adaptations that its ancestors possessed for life in the trees were advantageous to walking on the ground and were modified to fit a more terrestrial lifestyle. Pre-existing adaptations to life in the trees were put to new use on the ground and began to be shaped by this new mode of life.

But why would Ardipithecus ramidus have walked on the ground so often? What pressures caused the selection of bipedal traits? These questions are difficult to answer, but the scientists behind the descriptive papers hypothesize that Ardipithecus ramidus was a more generalized feeder than living chimpanzees. It did not go as high up into the canopy in search of fruit, and instead ate a wider variety of food more often, something that drew it to the ground. This hypothesis might be able to be tested by looking at what istopes are preserved in the teeth of Ardipithecus ramidus, but even then we can’t necessarily be sure we’re asking the right questions. I have no doubt these questions will be debated for some time to come.

The hypothesis that Ardipithecus ramidus was arboreally-adapted but did not knuckle-walk, however, is consistent with recent studies that suggest that knuckle-walking was not the ancestral mode of locomotion in the last common ancestor of humans and chimpanzees. In fact, slight differences in the way that gorillas and chimpanzees knuckle-walk may even mean that this “typical” ape mode of locomotion evolved more than once. And while most of the focus has been on Ardipithecus ramidus, the constellation of traits we see in its skeleton may suggest that living chimpanzees are more evolutionarily specialized than we previously thought. What we need to find are early fossil chimpanzees; their side of the family tree is practically blank. Being able to compare early humans with early chimpanzees would tell us much about the evolution of both groups.

There is plenty more that could be said (and certainly will be said) about Ardipithecus ramidus, but the question many people want answered is “How does it fit into our family tree?” The answer is not as straightforward as you might think, and whatever we say now will have to be tested against new discoveries. It will be important to compare even earlier fossil humans and chimpanzees to Ardipithecus ramidus to test what we hypothesize today.

Ardipithecus ramidus is more similar to other early hominins, like australopithecines, than chimpanzees. This makes it one of our early human relatives, but a question remains; was it ancestral to us or does it simply represent the form our true hominin ancestors exhibited at one point in time? Tim White, one of the lead architects of the Ardipithecus ramidus analysis has long favored a straight-line-progression of early hominins, and he slots Ardipithecus ramidus into this sequence. (See his recent essay in The Paleobiological Revolution.)

If White’s view is correct, Ardipithecus ramidus would be a chronospecies, or a particular stage in a direct, gradual line of descent that did not have any side branches or splits. This would mean that paleoanthropologists are looking at different forms of a single species at different points in time. As such Ardipithecus ramidus would not have evolved as a result of a speciation event but would instead represent a particular “phase” in a narrow line of hominin evolution. Ardipithecus kadabba transitions to Ardipithecus ramidus which shades into Australopithecus anamensis which turns into Australopithecus afarensis, after which the human evolutionary tree splits between the earliest members of the genus Homo and other australopithecines.

The idea that early hominins shaded into one another in a straight line of descent requires substantial evidence to corroborate. Based upon what we see elsewhere in the fossil record, and even among living species, it is difficult to believe that there were no speciation events that resulted in ancestor species and descendant species living side-by-side for a time. We expect splits, and it may be that what appears to be a straight chain of evolution is only that way because we are still dealing with an incomplete record in terms of time and biogeography. It is also difficult to be sure of direct ancestor-descendant relationships when dealing with extinct organisms, and this is true of hominins just as well as other extinct vertebrates. (White’s complaints about “cladists” aside.) We want to know whether this or that early hominin was ancestral to us, but in many cases we cannot know for sure. I have no doubt that there are plenty of other early hominins to be discovered, and as they are found it I would expect a “bushier” picture of human evolution to emerge.

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Three hypotheses of early hominin evolution. Top – That there was a straight line of hominin evolution, each “species” being the phase or grade of one true natural species. Middle – A single line of hominin evolution with a speciation event in the same area in which the earliest Australopithecus split from Ardipithecus ramidus. Bottom – Ardpithecus ramidus as one of the last members of a more archaic lineage which existed after a speciation event elsewhere in Africa that gave rise to the first Australopithecus. Published in this week’s issue of Science.

While the authors of the present study seem to prefer Ardipithecus ramidus as a “chronospecies” between Ardipithecus kadabba and Australipithecus anamensis (a hominin known from fragmentary remains thought to be intermediate between Ardipithecus and Australopithecus afarensis), they do present two alternative hypothesis (though not in much detail). There may have been some sort of speciation event between the last Ardipithecus and the first Australopithecus, either within the same geographical area or outside it, and either of which could falsify the idea that there was just one lineage of hominins evolving in a gradual fashion in one place. It may even be that Ardipithecus ramidus represents a remnant of an earlier evolutionary split among the earliest hominins. Future discoveries will be needed to test these ideas, but at the moment I must admit that the “chronospecies” pattern preferred by White is a little difficult for me to swallow. It would appear to be consistent with the evidence at present, but I very much doubt that all the relevant evidence is in yet.

Arguments over the tempo and mode of evolution aside, the assorted collection of Ardipithecus ramidus fossils are very impressive and will no doubt be important to our understanding of early human evolution for many years to come. While some have said that the new fossils are more important than “Lucy”, I think this is a mistake. Ardipithecus ramidus is so impressive because of the mosaic of features it exhibits when compared to other early hominins and living apes (including us). It is best understood as part of an transitional series in which the evolution of particular features can be tracked. If anything, the fossils of Ardipithecus ramidus make the bones of other early hominins more important; by comparing them to each other that we can achieve a better understanding of our own evolutionary heritage.

[If the discussions on blogs are not enough to keep you busy, log onto Facebook between 12 PM and 2 PM (eastern time) tomorrow to talk about Ardipithecus ramidus with science writers Ann Gibbons and Elizabeth Culotta. They have written some excellent and easily-accessible summaries about this hominin. You can join in by visiting the ScienceNOW page on Facebook. To get the papers summarized in this blog post, visit the special Ardipithecus ramidus webpage hosted by Science.]
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  • Claudiu Constantin Nicolaescu
Human ancestors were 'grounded,' new analysis shows

by New York University

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An evolutionary tree depicting the relationships among living apes, Ardi, and modern humans. Each branch on the tree represents a species and their intersections represent their common ancestors. The dots represent hypothetical evolutionary changes associated with the evolution of ground-living adaptations in the common ancestor of African apes and humans as well as the evolution of bipedalism, which is supported by the analysis. This shows that human bipedalism evolved from an ancestral form similar to the living African apes. Credit: Thomas Prang

African apes adapted to living on the ground, a finding that indicates human evolved from an ancestor not limited to tree or other elevated habitats. The analysis adds a new chapter to evolution, shedding additional light on what preceded human bipedalism.
African apes adapted to living on the ground, a finding that indicates human evolved from an ancestor not limited to tree or other elevated habitats. The analysis adds a new chapter to evolution, shedding additional light on what preceded human bipedalism.
"Our unique form of human locomotion evolved from an ancestor that moved in similar ways to the living African apes—chimpanzees, bonobos, and gorillas," explains Thomas Prang, a doctoral candidate in New York University's Department of Anthropology and the author of the study, which appears in the journal eLife. "In other words, the common ancestor we share with chimpanzees and bonobos was an African ape that probably had adaptations to living on the ground in some form and frequency."
The way that humans walk—striding bipedalism—is unique among all living mammals, an attribute resulting from myriad changes over time.
"The human body has been dramatically modified by evolutionary processes over the last several million years in ways that happened to make us better walkers and runners," notes Prang.
Much of this change is evident in the human foot, which has evolved to be a propulsive organ, with a big toe incapable of ape-like grasping and a spring-like, energy-saving arch that runs from front to back.
These traits raise a long-studied, but not definitively answered, question: From what kind of ancestor did the human foot evolve?
In the eLife work, Prang, a researcher in NYU's Center for the Study of Human Origins, focused on the fossil species Ardipithecus ramidus ('Ardi'), a 4.4 million-years-old human ancestor from Ethiopia—more than a million years older than the well-known 'Lucy' fossil. Ardi's bones were first publicly revealed in 2009 and have been the subject of debate since then.
In his research, Prang ascertained the relative length proportions of multiple bones in the primate foot skeleton to evaluate the relationship between species' movement (locomotion) and their skeletal characteristics (morphology). In addition, drawing upon the Ardi fossils, he used statistical methods to reconstruct or estimate what the common ancestor of humans and chimpanzees might have looked like.
Here, he found that the African apes show a clear signal of being adapted to ground-living. The results also reveal that the Ardi foot and the estimated morphology of the human-chimpanzee last common ancestor is most similar to these African ape species.
"Therefore, humans evolved from an ancestor that had adaptations to living on the ground, perhaps not unlike those found in African apes," Prang concludes. "These findings suggest that human bipedalism was derived from a form of locomotion similar to that of living African apes, which contrasts with the original interpretation of these fossils."
The original interpretation of the Ardi foot fossils, published in 2009, suggested that its foot was more monkey-like than chimpanzee- or gorilla-like. The implication of this interpretation is that many of the features shared by living great apes (chimpanzees, bonobos, gorillas, and orangutans) in their foot and elsewhere must have evolved independently in each lineage—in a different time and place.
"Humans are part of the natural world and our locomotor adaptation—bipedalism—cannot be understood outside of its natural evolutionary context," Prang observes. "Large-scale evolutionary changes do not seem to happen spontaneously. Instead, they are rooted in deeper histories revealed by the study of the fossil record.
"The study of the Ardi fossil shows that the evolution of our own ground-living adaptation—bipedalism—was preceded by a quadrupedal ground-living adaptation in the common ancestors that we share with the African apes."

Journal Reference:
Thomas Cody Prang. The African ape-like foot of Ardipithecus ramidus and its implications for the origin of bipedalism, eLife(2019). DOI: 10.7554/eLife.44433

The ancestral condition from which humans evolved is critical for understanding the adaptive origin of bipedal locomotion. The 4.4 million-year-old hominin partial skeleton attributed to Ardipithecus ramidus preserves a foot that purportedly shares morphometric affinities with monkeys, but this interpretation remains controversial. Here I show that the foot of Ar. ramidus is most similar to living chimpanzee and gorilla species among a large sample of anthropoid primates. The foot morphology of Ar. ramidus suggests that the evolutionary precursor of hominin bipedalism was African ape-like terrestrial quadrupedalism and climbing. The elongation of the midfoot and phalangeal reduction in Ar. ramidus relative to the African apes is consistent with hypotheses of increased propulsive capabilities associated with an early form of bipedalism. This study provides evidence that the modern human foot was derived from an ancestral form adapted to terrestrial plantigrade quadrupedalism.
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  • Claudiu Constantin Nicolaescu
Oh, yeah. Forgot to post that here, sorry.

Also, lol, "radius". Might wanna edit that to "ramidus"  Wink
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  • Taipan

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