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Males were responsible for provisioning the females, whereas females protected their offspring Lovejoy Lovejoy CO. Hominid Origins-The Role of Bipedalism. Females would mate exclusively with the provisioning male, and other males would no longer need to fight with each other over the females.
Therefore, the males' jagged, blade-like canine teeth diminished over time. Several studies have demonstrated that chimpanzees could carry twice as many nuts in a bipedal position than when walking on all fours Carvalho et al.
Anthropological evidence also supported this theory. The downsizing of male canine teeth, decrease in antagonistic behavior and body size dimorphism corroborated Lovejoy's theory. As mentioned previously, there could be multiple answers to bipedalism, and there were two aspects of bipedal evolution: 1 that ancient hominins were already partially bipedal, and 2 that hominins evolved full bipedalism.
Although the postural feeding theory provided an explanation for the first aspect, the savanna-based theory could provide an answer to why hominins became increasingly bipedal over time. Lovejoy's provisioning model lied between these two theories. Hunt's theory, which suggested that bipedalism involved reaching for food and balancing on the branches, would logically fall before using the hands for provision.
The early hominins spent time on the trees, but the species eventually evolved to walk like modern humans on the ground.
The evolutionary momentum that was driven by balancing and reaching on the trees should have affected the early hominins. The provisioning model demonstrated how hominins became more bipedal over the time not only by food gathering but also by provisioning infants monogamy. Nonetheless, the provisioning model did not have sufficient evidence explaining why hominins would have begun to walk like modern humans and lost all adaptations to arboreal life. To this gap was where the savannah-based theory contributed its explanations.
When largely bipedal hominins started to settle on the ground, the savannah based-theory would be the explanation for their full bipedalism. The savannah based-theory included various other models that already assumed that hominins started to live a terrestrial life, such as sentinel behavior, threat, running endurance and thermoregulatory models. The general order of the theories was the following: postural-feeding, provisioning, and savannah-based theories.
However, there were no straight lines between these theories, and it was possible that the three forces worked together at one point. First, there were arboreal hominids that possessed ambiguous traits of bipedalism. These were gradually replaced by two lines of species: one consisting of Pan species and the other comprising hominins.
Hominin-like species and modern chimp-like species gradually evolved to undergo specific adaptations to live on the ground and on the trees, and the new hominins presented a survival advantage over their common ancestors. The ambiguous traits were eliminated through the choice and selection. However, when the split between the two species became clear, the hominins and chimpanzees would not have competed for resources. The stated biological relationship between the chimps and humans was similar to the remarkable relationship between the okapi and the giraffe.
Similar to the unique adaptation of bipedal locomotion that was only observed in Homo species, the giraffe's long neck was also an evolutionary product exclusive to this species Badlangana et al. Observations on the giraffe central nervous system related to the corticospinal tract, motor cortex and spinal cord: What difference does a long neck make?
The okapi and the giraffe are currently the only living members of the Giraffidae family. Although the short-necked okapi's outer appearance resembles a zebra, the okapi is the closest surviving species to the giraffe. Apparently, Darwinian natural selection has led the ancestral giraffes with long necks to reproduce and pass on their genes because they had a competitive advantage that enabled them to reach higher branches. Consequently, the giraffe ancestors fed on acacia leaves and spread through the savannah where tall trees grew.
In contrast, the long-neck adaptations became futile for the okapi species, which, ultimately, inhabited canopy forests and fed on the buds, grasses, ferns, fruits, or fungi Hart and Hart Hart JA, Hart TB. Ranging and feeding behaviour of okapi Okapia johnstoni in the Ituri Forest of Zaire: food limitation in a rain-forest herbivore.
Symposium of the Zoological Society of London. Abrir menu Brasil. Brazilian Archives of Biology and Technology. Abrir menu. Bipedalism: New Perspective As in other species, several characteristics of the ape-like hominin ancestors were advantageous for their survival.
The savanna-based theory 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. The postural feeding hypotheses The second model is the postural feeding hypothesis, which has been proposed by Kevin Hunt at Indiana University.
Review: Introducing a new perspective Retrace the steps back to the common ancestor showed clues to address these perplexing issues and theories. The threat model The original proponents of this model theorized that bipedalism originated as a natural defense strategy for early hominids. The thermoregulatory model Peter Wheeler proposed the thermoregulatory model, a model that stated that bipedalism would increase the amount of body surface area, which helped dissipate heat and reduces heat gain Wheeler Overcoming the disadvantage The clear advantage of bipedalism was the possibility for ancient hominin species to use their hands.
Provisioning model The last of prominent bipedalism theories, the provisioning model, was proposed by Owen Lovejoy, who suggested a modified version of Darwin's explanation.
Biological Analogy of Bipedal evolution First, there were arboreal hominids that possessed ambiguous traits of bipedalism. Ayala FJ. Dart RA. Darwin C. Eisner T, Grant R. David Strait deftly deals with this genus among others in the next chapter of this special issue.
Complementing the molecular estimates for the human—chimpanzee divergence dates, the last 20 years have heralded major breakthroughs in our understanding of early hominin morphology, behavior, and evolutionary relationships. This is due to a number of major paleontological discoveries dated between four and seven million years ago see Table 2. We will discuss the possible hominin status of each of them in turn, but the announcement of a new fossil hominin species, Australopithecus ramidus , in heralded a new chapter in paleoanthropology White et al.
The specimens were found at the locality of Aramis, in the Middle Awash region of Ethiopia. The dating of the deposits is very accurate at 4. Although the remains were originally named Australopithecus ramidus , the following year White and colleagues published a correction assigning the species to a new genus, Ardipithecus White et al.
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. For example, octopodes sometime walk bipedally in order to camouflage themselves from predators 1. The octopus piles 6 of its 8 limbs on top of its head, assuming the shape of a drifting plant, and then uses the 2 remaining limbs to quite literally walk away.
As for quadrupeds animals that move on four limbs , it is not uncommon to see antelope standing on their 2 hind limbs while supporting themselves on their forelimbs when reaching for food in high branches. Chimpanzees have been documented walking on 2 legs in order to carry things with their hands.
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