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Theories on the Origin of Bipedalism

The postural feeding hypothesis has been recently supported by Dr. Kevin Hunt, a professor at Indiana University. This hypothesis asserts that chimpanzees were only bipedal when they ate. While on the ground, they would reach up for fruit hanging from small trees. While in trees, bipedalism was utilized by grabbing for an overhead branch. These bipedal movements may have evolved into regular habits because they were so convenient in obtaining food. Also, Hunt hypothesises that these movements coevolved with chimpanzee arm-hanging, as this movement was very effective and efficient in harvesting food. When analyzing fossil anatomy, Australopithecus afarensis has very similar features of the hand and shoulder to the chimpanzee, which indicates hanging arms. Also, the Australopithecus hip and hind limb very clearly indicates bipedalism, but these fossils also indicate very inefficient locomotive movement when compared to humans. For this reason, Hunt argues that bipedalism evolved more as a terrestrial feeding posture than as a walking posture. As Hunt says, "A bipedal postural feeding adaptation may have been a preadaptation for the fully realized locomotor bipedalism apparent in Homo erectus."

Another theory on the origin of bipedalism is the behavioral model presented by C. Owen Lovejoy, known as "male provisioning." Lovejoy theorizes that the evolution of bipedalism was linked to monogamy. In the face of long inter-birth intervals and low reproductive rates typical of the apes, early hominids engaged in pair-bonding that enabled greater parental effort aimed at the rearing of offspring. Lovejoy proposes that male provisioning of food would improve the survivorship of the offspring and increase the pair's reproductive rate. Thus, the male would leave his mate and offspring to search for food and return carrying the food in his arms walking on his legs. This model is supported by the reduction ("feminization") of the male canine teeth in early hominids such as Sahelanthropus tchadensis and Ardipithecus ramidus, which along with low body size dimorphism in Ardipithecus and Australopithecus, suggests a reduction in inter-male antagonistic behavior in early hominids. In addition, this model is supported by a number of modern human traits associated with concealed ovulation (permanently enlarged breasts, lack of sexual swelling) and low sperm competition (moderate sized testes, low sperm mid-piece volume) that argues against recent adaptation to a polygynous reproductive system.

However, this model has generated some controversy, as others have argued that early bipedal hominids were instead polygynous. Among most monogamous primates, males and females are about the same size. That is sexual dimorphism is minimal, and other studies have suggested that Australopithecus afarensis males were nearly twice the weight of females (as well as a great deal taller). However, Lovejoy's model posits that the larger range a provisioning male would have to cover (to avoid competing with the female for resources she could attain herself) would select for increased male body size to limit predation risk. Furthermore, as the species became more bipedal, specialized feet would prevent the infant from conveniently clinging to the mother - hampering the mother's freedom and thus make her and her offspring more dependent on resources collected by others. Modern monogamous primates such as gibbons tend to be also territorial, but fossil evidence indicates that Australopithecus afarensis lived in large groups. However, while both gibbons and hominids have reduced canine sexual dimorphism, female gibbons enlarge ('masculinize') their canines so they can actively share in the defense of their home territory. Instead, the reduction of the male hominid canine is consistent with reduced inter-male aggression in a group living primate.

 

Earliest Humans

Homo habilis

Homo habilis

Figure 22. Skull of Homo habillis, with circle representing the size of the brain.
"Skull of Homo habilis" by User:Editor at Large, Wikimedia Commons is in the Public Domain

 

Homo habilis ("handy man") is a species of the genus Homo which lived from approximately 2.3 to 1.4 million years ago at the beginning of the Pleistocene period. The discovery and description of this species is credited to both Mary and Louis Leakey, who found fossils in Tanzania, East Africa, between 1962 and 1964. Homo habilis was the earliest known species of the genus Homo until May 2010, when H. gautengensis was discovered, a species believed to be even older than H. habilis. H. habilis was short and had disproportionately long arms compared to modern humans, with a less protruding face than the australopithecines from which it is thought to have descended. H. habilis had a cranial capacity less than half the size of modern humans. Despite the apelike morphology, H. habilis remains are often accompanied by primitive stone tools.

Homo habilis has often been thought to be the ancestor of the more gracile and sophisticated Homo ergaster, which in turn gave rise to the more human-appearing species, Homo erectus. Debates continue over whether H. habilis is a direct human ancestor, and whether all of the known fossils are properly attributed to the species. However, in 2007, new findings suggest that the two species coexisted and may be separate lineages from a common ancestor, instead of H. erectus descending from H. habilis.

Compared to australopithecines, H. habilis's brain capacity of 363 and 600 cm³ was on average 50 percent larger than australopithecines, but considerably smaller than the 1,350 to 1,450 cm³ range of modern Homo sapiens. These hominins were smaller than humans, on average standing no more than 1.3 m (4 feet 3 inches) tall. The small size and rather primitive attributes have led some experts (Richard Leakey among them) to propose excluding H. habilis from the genus Homo, and renaming it as "Australopithecus habilis."

 

Homo ergaster

Homo ergaste

Figure 23. Skull of Homo ergaster, with circle representening the size of the brain.
"Skull of Homo ergaster" by User:Editor at Large, Wikimedia Commons is in the Public Domain

 

Homo ergaster is an extinct chronospecies of Homo that lived in eastern and southern Africa during the early Pleistocene era, about 2.5-1.7 million years ago. There is still disagreement on the subject of the classification, ancestry, and progeny of H. ergaster, but it is now widely accepted to be the direct ancestor of later hominids such as Homo heidelbergensis, Homo sapiens, and Homo neanderthalensis, rather than Asian Homo erectus. It is one of the earliest members of the genus Homo, possibly descended from, or sharing a common ancestor with, Homo habilis. H. ergaster lived in arid habitats and had long legs adapted for long-distance walking. Other features include short straight fingers, indicating they were not tree climbers, and smaller teeth than Australopiths suggesting different foods were eaten or food was prepared by mashing or heating.

 Homo ergaster used more diverse and sophisticated stone tools than its predecessor, Homo habilis. H. ergaster refined the inherited Oldowan technology, developing the first Acheulean bifacial axes; while the use of Acheulean tools began ca. 1.6 million years ago, the line of H. erectus diverged some 200,000 years before the general innovation of Acheulean technology.

Sexual dimorphism in H. ergaster is greatly reduced from its australopithecine ancestors (around 20 percent), but still greater than dimorphism in modern humans. This diminished dimorphism is speculated to be a sign of reduced competition for mates between males, which may also correspond to the more modern social practices of ergaster. Not only was H. ergaster like modern humans in body, but also more similar in organization and sociality than any earlier species.

 

Homo erectus

Homo erectus

Figure 24. Skull of Homo erectus, with circle representening the size of the brain.
"Skull of Homo erectus" by User:Editor at Large, Wikimedia Commons is in the Public Domain

 

Homo erectus (from the Latin ērigere, "to put up, set upright") is an extinct species of hominid that originated in Africa — and spread as far as China and Java — from the end of the Pliocene epoch to the later Pleistocene, about 1.8 to 1.3 million years ago. There is still disagreement on the subject of the classification, ancestry, and progeny of H. erectus, with two major alternative hypotheses: erectus may be another name for Homo ergaster, and therefore the direct ancestor of later hominids such as Homo heidelbergensis, Homo neanderthalensis, and Homo sapiens; or it may be an Asian species distinct from African ergaster. Fossilized remains 1.8 to 1 million years old have been found in Africa, Europe, Indonesia, Vietnam, and China.

 H. erectus had a cranial capacity greater than that of Homo habilis: the earliest remains show a cranial capacity of 850 cm³, while the latest Javan specimens measure up to 1,100 cm³, overlapping that of H. sapiens. The frontal bone is less sloped, with the dental arcade smaller than the australopithecines'; and the face is more orthognatic (less protrusive) than either the australopithecines' or H. habilis's, with large brow ridges and less prominent zygomata (cheekbones). These early hominins stood about 1.79 m (5 feet 10 1/2 inches), and were more robust than modern humans.

 


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