Their rationale was that newly discovered remains in fact had smaller cheek teeth and thinner tooth enamel than members of the genus Australopithecus. Perhaps the most tantalizing part of the correction, though, was a brief mention of a partial skeleton that had been found near the type specimen. We have had to wait almost 15 years for that skeleton to be further described.
Part of the reason for this is that it was incredibly fragmentary and the team had to spend thousands of research hours preparing and conserving it. Between the initial announcement of Ar.
Ardipithecus kadabba also harks from Ethiopian fossil-bearing deposits but is considerably older than Ar. The remains mainly consist of teeth and are relatively meager compared to what we have for Au. Although it also has thin dental enamel, Ar. It has also been suggested that Ar. So what about the Ar.
There are many parts of the skull, including most of the teeth. Below the neck, there is a rather crushed and distorted pelvis, most of the forearm and hands, and most of the lower leg and foot. There is an incomplete femur but sadly, no humerus or scapula, and little in the way of ribs or vertebrae White et al.
There is enough of the skull preserved to allow a reconstruction. The result has provided some interesting results Suwa et al. The cranial capacity is between and cubic centimeters, which is about what we see in modern day chimpanzees. However the face is described as having a mixture of features.
The way the face projects outwards is rather chimpanzee-like in the middle part i. The base of the skull is rather short at the back, and perhaps most critically, the position of the foramen magnum is argued to be anteriorly placed, as in later hominins.
As discussed earlier, this last feature is seen as important in indicating bipedal locomotion. The postcranial skeleton is fascinating. The arm and hand bones indicate a highly arboreal animal with specialist adaptations to careful climbing in the trees. The authors argue that there are no knuckle-walking features in the wrist and finger bones, meaning that this specialized form of terrestrial quadrupedal locomotion was unlikely in Ardipithecus and its direct ancestors.
Proconsul was a lot lighter, and more work will be needed to try and assess whether such a large animal would have been able to comfortably move quadrupedally along tree branches. The foot also has several characteristics clearly related to arborealism. Most critically it has an opposable hallux or big toe , which would make it the only hominin with such a primitive feature Lovejoy et al.
All other known hominins have lost the ability to grasp with their hallux, indicating a strong shift away from arboreal grasping behaviors. Where Ardipithecus is really surprising is in the pelvis. The original is highly distorted, but there is some anatomy preserved, and in combination with an elaborate three-dimensional reconstruction, it appears that the pelvis shares some features with later hominins Lovejoy et al.
Most importantly, the iliac blades appear a little shorter than they do in apes, and there is a structure present called the anterior inferior iliac spine or AIIS. This is a feature on the anterior or front part of the pelvis which indicates a strong attachment for both the iliofemoral ligament, which helps with balance during upright walking, and a muscle that helps fully extend the knee called rectus femoris.
We therefore have a creature with reduced canines, packed full of climbing-related features that also was capable of some degree of bipedal locomotion. The paleoenvironmental reconstruction of where Ardipithecus lived also points to woodland habitat that is consistent with a predominantly arboreal species WoldeGabriel et al.
Based on the anatomical findings summarized above, the overall conclusion was that Ardipithecus is an undisputed hominin, albeit one very close to the LCA of chimpanzees and modern humans White et al. Some researchers have even begun to question its hominin status Harrison ; Sarmiento , but we have to wait until more information and full descriptions become available. Just a few years after the arrival of Ar. These remains are extremely fragmentary and come from quite a wide geographical area.
They are dated to approximately 6 Ma, which at the time made them the oldest putative hominin on record. Orrorin is represented by a handful of teeth and several postcranial remains, including a partial femur and humerus.
Its discoverers, Martin Pickford and Brigit Senut, argued that it was a hominin based on its thick dental enamel and the morphology of the femur Senut et al. They suggested that Orrorin was capable of bipedal locomotion based on a feature, called the obturator externus groove, on the upper part of the femur.
They have also argued that the inferior or lower part of the femoral neck was disproportionally thick, which has been suggested by some to be a feature that reflects increased downward loading at the hip joint due to upright locomotion.
A recent independent statistical analysis of measurements taken from the Orrorin femur has also confirmed it to be very hominin-like and similar in shape to Australopithecus Richmond and Jungers Other researchers have noted that a thick inferior femoral neck and obturator externus groove are features only weakly related to bipedal locomotion Lovejoy et al.
There is also the issue of the Orrorin upper limb remains. There is one highly curved finger bone and a partial humerus with a strong attachment for a muscle used in climbing Senut et al. Overall, Orrorin could well be a hominin based on its femoral morphology, but if so, it was also a strong climber that was comfortable in the trees.
There is also the problem of whether the femur and dental remains actually come from the same species or not. They were found a very long distance from each other, and their association must be treated with some caution. Not long after Orrorin made the news, another possible species of hominin was announced.
They were originally dated to between 6 and 7 Ma based on faunal remains found at the site Vignaud et al. The best-known specimen is a relatively complete cranium called TM The researchers argued that because it appeared to have a relatively small canine, in combination with a narrow and less prognathic protruding face, it must have been a very early hominin.
However, the skull is heavily distorted and cracked, which has obscured some important diagnostic characters. The result has two very particular features of note, the position and the angulation of the foramen magnum. In the original, this structure is hard to position, but in the reconstruction its position and angulation are more hominin-like, indicating an affinity for bipedal locomotion.
A few more specimens of S. Weaving all these various threads of evidence together into something cohesive can be an overwhelming task. Some of the specimens discussed above have only been recently announced, and most of them are still being worked on by the teams that discovered and described them, making it difficult for other researchers to independently assess them.
As a result, the evolutionary relationships between these different species are still in a state of flux. Various opinions have thus been expressed, and it really boils down to how one views variation within and between named fossil species. After the announcement of S. This could still be the case, but others have suggested that we are overestimating the level of species diversity in early hominin fossils and that Ardipithecus , Sahelanthropus , and Orrorin could very likely all belong to the same genus White In terms of a broader evolutionary context, again, it is still early days.
The team that discovered Ardipithecus has suggested that the evidence from Ethiopia and northern Kenya strongly point to a Ar. This is possible, but where Sahelanthropus and Orrorin might fit into the sequence remains to be seen. There is also the issue of Au. We find specimens as old as 4. So what can we definitively say about early hominins?
We have possibly as many as four species and three genera between 7 and 4. Twenty years ago, none of these species had been discovered or named, so we are doing well. The dating of these species coincides very well with the chimpanzee—modern human divergence dates predicted by molecular genetic work. All of them seem to exhibit adaptations to increased levels of bipedalism, but at least two genera Ardipithecus and Orrorin would have been very competent climbers as well.
We also see a reduction in canine size that might be associated with behavioral shifts in male competition and aggressive threat displays. Finally, and this we can be sure of, the hard work that goes into finding these specimens in often very remote places promises many more delights and surprises in the years to come. Therefore, it is important to note that this change was advantageous for the humans but not advantageous for the other great apes.
A retracing of the evolutionary traits backwards in an evolutionary timeline from the modern Homo sapiens to the Homo heidelbergensis, H. Nature Publishing Group. In addition, several studies have indicated that the extinct hominin Ardipithecus, which was extremely similar to the common chimpanzee ancestor, possessed the ability to walk on two feet, while spending time in the trees.
Indeed, the first hominin, or the first common ancestor was partially bipedal, i. The following paragraphs will review several prominent theories of bipedalism. The different models of bipedalism will be examined in accordance with the factors of natural selection. Then a comprehensive approach based on an evolutionary timeline of other great apes in the Hominini tribe and even in the Homininae subfamily will be explained considering the aforementioned perspective of multiple answers.
The savanna-based theory was one of the earliest models to explain the origins of bipedalism and gathered support from several anthropologists Dart Dart RA. It mainly suggested that the early hominids were forced to adapt to an open savanna after they left the trees by walking erect on two feet Shreeve Shreeve, J. Sunset on the Savanna. Discover Magazine.
According to this theory, the evolution of bipedal locomotion would have been helpful in a savanna because the posture would allow hominins to watch over tall grasses, hunt effectively, or be aware of predators. Unfortunately, the paleoclimatological evidence has argued against the savanna-based theory, and the fossil record shows that the early bipedal hominines were still adapted to climbing trees as well. Several researches have indicated that bipedalism evolved in trees.
The second model is the postural feeding hypothesis, which has been proposed by Kevin Hunt at Indiana University. This stated that bipedal movements might have evolved into regular habits because they were convenient for obtaining food and keeping balance Hunt Hunt KD. The evolution of human bipedality: ecology and functional morphology. J Hum Evol. It has been observed that chimps were only bipedal when they ate.
Chimpanzees would reach up for fruit hanging from the trees, and orangutans used their hands to stabilize themselves, while navigating thinner branches Stanford Stanford CB.
Arboreal bipedalism in wild chimpanzees: Implications for the evolution of hominid posture and locomotion. Australopithecus afarensis had hand and shoulder features that demonstrated hanging habits, whereas their hip and hind limb clearly indicated bipedalism. Because a bipedal posture was utilized for grabbing from an overhead branch and harvesting food, Hunt argues that bipedalism evolved more as a feeding posture than as a walking posture.
Retrace the steps back to the common ancestor showed clues to address these perplexing issues and theories. The earliest hominins, Sahelanthropus and Ardipithecus have been suggested to have been bipedal and partly arboreal Nelson Nelson SV. Chimpanzee fauna isotopes provide new interpretations of fossil ape and hominin ecologies.
Interestingly, extinct hominins that were close to the common chimpanzee ancestor were partially bipedal. The evolutionary momentum gradually pushed the common ancestor, which was limitedly bipedal and arboreal, to become chimps that were mostly arboreal with limited bipedal motion quadrupedal mostly on ground in one branch and hominins that were mostly terrestrial with full bipedal locomotion in the other evolutionary branch.
Evolution, therefore, did not have a single direction from the common ancestor toward Homo sapiens. Ancestors of both chimps and humans that apparently possessed ambiguous traits of humans and chimps evolved in two ways: one toward chimpanzees, which included great chimpanzees and bonobos, and the other toward Homo sapiens Patterson et al.
Genetic evidence for complex speciation of humans and chimpanzees. Importantly, hominins slowly evolved to walk like modern humans over a continuous scale. Therefore, the important question was not why the earliest hominins were partially bipedal but rather why hominins became more bipedal over time and replaced their less-bipedal ancestors. This specific evolutionary trait of bipedalism was not necessary for the chimps and their extinct ancestors that lived on the trees.
Despite the alleged lack of evidence, the fact remained that full bipedalism had not been documented in other great apes.
Chimps, for example, were agile climbers and nested on the trees to rest around noon and sleep at night. During the day, gorillas climbed the trees, swung from the branches, and chased one another. Most arboreal great apes, such as orangutans, spent nearly all of their time on the trees. Bipedalism full bipedalism observed in Homo erectus and modern humans was not a beneficial trait when moving from one tree to another in an arboreal life.
In addition, a change in environment moving away from trees cannot be a cause of the partial bipedalism in early hominins, as suggested by fossil evidence. Nevertheless, when hominins started to settle on the ground, the savannah-based theory could provide an explanation for why hominins evolved to walk like modern humans, replacing their less-bipedal ancestors Dart Dart RA.
Furthermore, the savannah-based theory incorporated several models of bipedalism, such as the sentinel response, threat display, and endurance running, all of which provided general evidence for how bipedalism aided the survival of hominins in the savannah.
Another model was the postural feeding hypothesis, which was supported by the evidence from several studies. However, there were logical problems associated with it. There were possibly two aspects of bipedalism: i how were the earliest hominins partially bipedal in the first place? Hunt's theory could not explain the second aspect of why partially bipedal hominins evolved to walk like modern humans.
He stated the advantage of obtaining food from branches, or balancing in an arboreal task Hunt Hunt KD. However, hominins evolved to walk like modern humans on the ground, not on the trees. The evolutionary change that was driven by balancing and reaching on the trees should not have affected the hominins that were abandoning the arboreal life.
Hominins continuously evolved to possess terrestrial adaptations and eventually lived on the ground. However, Hunt's theory could explain the origin of bipedalism, i. Among the nonhuman primates, monkeys that habitually walk on the ground, such as baboons, Japanese macaques, rhesus macaques, and patas monkeys, do not always exhibit clear hindlimb dominance in terms of foot force.
In these animals, the hindlimb force is sometimes equal to or less than the forelimb force. On the other hand, arboreal primates that mainly utilize the arboreal environment, such as chimpanzees, orangutans, spider monkeys, and capuchins, typically show clear hindlimb dominance in terms of foot force Kimura T.
Hafner, New York facsimile of edition. Hildebrand M. Cartmill M. Zoological Journal of the Linnean Society, — Prost J. Tomita M. Rollinson J. Symposia of the Zoological Society of London, — Figure 1Quadrupedal and bipedal walking.
From left to right, a quadrupedal dog representing the common mammal, the quadrupedal and bipedal walking of a chimpanzee, and a bipedal human, respectively. Black circles indicate the center of gravity of the body, which is higher in the bipedal than in the quadrupedal position. The quadrupedal footfall order of nonhuman primates is mainly a diagonal-sequence in which the fore- and hindfoot of the same side touch the substrate at about the same place at successive times. Common mammals mainly adopt the lateral-sequence, where the fore- and hindfoot of the same side touch at quite separate places.
Many common mammals touch the substrate mainly with their toes and interdigital pads digitigrade , but many arboreal nonhuman primates such as chimpanzees support their body with their whole sole plantigrade , similar to humans.
In nonhuman primates, the distal humerus is positioned in front of the shoulder at forefoot touchdown during quadrupedal walking. On the other hand, the excursion of the humerus is not so large in common mammals. Figure 2Functional differentiation of the limbs in quadrupedal nonhuman primates.
Nonhuman primates show larger acceleration and body supporting force components in the hindlimbs than in the forelimbs during quadrupedal locomotion. By contrast, common mammals accelerate and support the body mainly using the forelimbs.
The steering activity of both groups is situated at the front. Quadrupedal and bipedal walking. Functional differentiation of the limbs in quadrupedal nonhuman primates. Primate quadrupedal locomotion is unique in comparison with that of common mammals, showing a more protracted arm and forelimb at the forefoot touchdown of a step Larson S.
American Journal of Physical Anthropology, 87— Larson S. Journal of Zoology, — Nonhuman primates frequently sit with an orthograde trunk and use the forefoot as a manipulating hand during foraging Jolly C.
Man, New Series. Kawai M. Morbeck M. Hunt K. American Journal of Physical Anthropology, 83— Remis M. Keith A. British Medical Journal, —, —, —, —, —, — Extant nonhuman primates have larger limbs than common mammals. The limb bones, especially the hindlimb bones, of primates are relatively longer Alexander R.
In: Ehara A. Elsevier, New York, pp. Polk J. Figure 3Scatter diagram of osteological data for mammal limb long bones. The scatter diagram is based on principal component PC analysis from 25 linear measurements, the original data of which were divided by the cube root of body mass to eliminate differences in body mass.
PC1 is the primary component associated with the large size, evenly controlled differences in body mass, especially epiphyseal diameters.
Species on the positive side of PC2 have long metapodials and several forelimb bones with large diameters, whereas those on the negative side have long limb bones and several hindlimb bones with large diameters. Solid circles: 79 species of nonprimate quadrupedal mammals; open circles: 68 species of nonhuman primates, which form a clear group separate from the nonprimate mammals and are characterized by long bone lengths, except metapodials, and large bone diameters, especially of the hindlimbs; triangle: humans, which are in the nonhuman primate group.
Additional letters: BV, brown-throated three-toed sloth; CC, golden-backed squirrel; CN, prehensile-tailed porcupine; PF, kinkajou; TG, common tupai; and TV, common brushtail possum from nonprimate mammals, all of which are active in arboreal environments and situated either among the primates or near the border of the two groups.
In addition: EP, patas monkey; MF, Japanese macaque; PA, Anubis baboon; PH, hamadryas baboon from nonhuman primates, all of which use terrestrial surfaces daily and are located at the border of the two groups Kimura, Scatter diagram of osteological data for mammal limb long bones. In addition: EP, patas monkey; MF, Japanese macaque; PA, Anubis baboon; PH, hamadryas baboon from nonhuman primates, all of which use terrestrial surfaces daily and are located at the border of the two groups Kimura T.
Based on foot force patterns and bone robustness among nonhuman primates, arboreal species are more hindlimb dominant than terrestrial ones Kimura T. Folia Primatologica, 17— Primates who adapt to relatively arboreal environments show strong hindlimb dominance in their quadrupedal posture and locomotion, whereas relatively terrestrial nonhuman primates show weak hindlimb dominance. Arboreal species show large vertical and accelerating foot forces in the hindlimbs and robust hindlimb bones.
The hindlimb dominance of primates, which is useful for bipedal walking, is likely acquired in the arboreal environment. Many nonhuman primates include bipedal locomotion in their daily locomotor repertories, in spite of having a high center of gravity and the difficulties involved in maintaining balance while in the bipedal compared with the quadrupedal position Figure 1Quadrupedal and bipedal walking. The bipedalism of nonhuman primates, however, contains many characteristics that differ from that of humans.
The trunk of bipedal nonhuman primates never stands erect like that of humans, but rather inclines forward Figure 4Stick pictures depicting bipedal walking in a chimpanzee left and a human right on the sagittal plane.
One cycle of walking from a heel-strike to the next heel-strike is shown. The statures of both species are depicted as equal in size. Sticks connect the tragion ear point , shoulder joint, hip joint, knee joint, ankle joint, and tip of the third toe in the chimpanzee, and the vertex, shoulder joint, hip joint, knee joint, ankle joint, and dorsal surface of the big toe in the human. Points on the head and toe are different in the two species because of the relatively differently shaped body parts.
Human walking is characterized by an erect trunk with small angular movements, hyperextended hip joint, fully extended knee during the single stance-phase, and the rocking mechanism of the sole. Ishida H. Yamazaki N. Annales des Sciences Naturelles, Zoologie, 5: — Okada M. The host of advantages bipedalism brought meant that all future hominid species would carry this trait. Bipedalism allowed hominids to free their arms completely, enabling them to make and use tools efficiently, stretch for fruit in trees and use their hands for social display and communication.
They could also see further over the savannah grass — but this also could have been a disadvantage since predators could probably spot them more easily. Bipedal hominids could spend more time foraging and scavenging out in the open savannah because their bodies would be exposed to less sunlight standing upright. Walking on two limbs was also more energy efficient than walking on four — giving early hominids more energy to reproduce and therefore more chance of producing offspring bearing this unique trait.
But even with these advantages, these transitional hominids probably spent time in the trees as well. Similarly, further north in Africa, the Australopithecus species of Ethiopia and Tanzania between 3-million and 2-million years ago would have been able to climb trees better than modern humans, but were simultaneously adapting to more full-time upright walking. Australopithecus afarensis , which populated the Afar Depression in Ethiopia, would have lived in an environment typified by wetlands, woodland and forest.
But the bipedal footprints of Australopithecus afarensis in Laetoli, Tanzania, are found in an area where the environment was probably drier and sparsely wooded 3.
It probably could have walked and climbed effectively. The ball-and-socket joint is the most mobile type of joint, allowing us to swing our arms and legs in many different directions. The joint is made up of a ball-shaped bone which rotates in a cup-shaped cavity, as in our hips and shoulders. Hinge joints, such as the knee and elbow, act as a lever that enables our arms and legs to flex bend and straighten.
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