Savannah Theory

Supports that bipedalism was more efficient compared to quadripedalism for long distance terrestrial locomotion. This type of locomotion was favored by the scarcity of resources caused by the drying of African forests in the Miocene. In this case evolution to bipedalism is based on the principles of behavioral ecology of extant primates. Every day travel or movement is not only subject to resource scarcity but also heavily dependent on the group size brought about by intergroup scramble competition (Hunt, 26).  The drying of the Miocene had triggered the evolution of more energetically efficient locomotion patterns or smaller group sizes. The latter strategys implication is that smaller groups are less favored in comparison to larger groups as far as intergroup competition is concerned (smaller groups are supplanted from limiting resources). Therefore, the earliest hominoids (human beings) must have adopted bipedalism to favor them over small group sizes, an energetic response mechanism to match the increasing resource scarcity. As an alternative, humans would have maintained their quadripedal pattern with slight evolution to fission-fusion grouping in order to minimize their daily travel as well as their energetic costs.

This strategy must have been adopted as a result of inherent differences in feeding ecology between the two closely related hominoids, humans and chimpanzees. This was the potential factor that resulted in speciation process which led to modern humans.  This environmental change, expansion of open grassland and the disappearance of forest was the underlying cause of bipedalism. Due to increasing resource scarcity the following consequences were faced by humans (1) there was a reduction in food density within closed forest, (2) the distance between forest patches increased and (3) there was extensive increase in savanna habitat.
The fact that humans are primates meant that they are subject food competition utilizing both scramble and contest competitions. In such competitions, humans would gain access to food via overt (the threat of aggression). In scramble competition, food access relies on the numbers of others nearby that consume the same food thereby depleting its availability (Stanley, 634). An increase in the number of conspecifics in resource sharing implies a reduction in food availability for each individual. In group living primates i.e humans, contest competition often occur both between and within groups, this competition can be expressed as the degree of aggression among groups. On the other hand, intra group competition is highly dependent on strong dominance hierarchies.   

Owing to the fact that success in aggressive encounters is highly dependent on the group size of the competing species, intergroup contest competition acted as the selective force affecting group size. Furthermore, intergroup scramble completion between humans and other closely related species (chimpanzees) depended on the home range size coupled with changes in group size (Isaac, 29). Humans had long home ranges compared to other primates. The home range size is determined by the abundance or the density food. With the increase in group size and the expansion of home ranges into other groups, the number of animals sharing resources in such overlaps increases and hence the availability of food to other groups decreases. These patterns required that humans adopted bipedalism so as to be advantageous over other competing species such as chimpanzees.
As the forests became drier, they were thinned from within causing the death of more species and thus reducing at their peripheries. In the absence of forest reduction, availability of resources reduced with regional drying and this required that groups of given sizes travelled farther to harvest the same quantity of food.  Humans, as frugivorous hominoids living in groups, acquired two strategies available to match the increasing food scarcity (Diamond, 34). Faster locomotion, they adopted bipedalism thus gaining advantage over smaller groups in this intergroup competition for food. Bipedalism meant that humans increased their day length even without changing their group sizes in this era of food limitation. Under such circumstances, humans were favored by selection since they had minimized their energetic costs of daily movement while at the same time maintaining their larger groups without unfair competitions.
The habitual bipedalism was efficient in energy conservation while travelling terrestrially over long ranges. Increased efficiency of bipedal locomotion not only enabled greater daily travels relative to group sizes but also facilitated the evolution of larger body sizes. By living in small groups, humans would thrive on smaller patches which were less available to larger groups (Tanner, 15). The use of bipedalism mechanism enabled humans to travel alone or when favored by food distribution, in small foraging parties thus minimizing potential costs incurred by humans in solitary living. The new posture introduced by bipedalism mechanism in humans made terrestrial foraging at higher levels of vegetation more efficient but less efficient at the ground levels. Due to the fact that initial location on an adaptive landscape resulted in divergent trajectories, similar but defined niches predisposed humans to solve their problems differently.  This means that if humans fed on resources which could be acquired by smaller groups such as chimpanzees, they became less likely to suffer the costs of living alone. In such case, the response to the drying of the Miocene would be to reduce their group size other than adopt bipedalism.
In addition to the bipedal stance, humans would gain wider view of their surrounding and early spotting of enemies during their food gathering expeditions. In the terrestrial travels bipedalism would aid faster cooling while in the open thus increasing their chances of survival and adaptability more than their quadripedal food competitors who would suffer the intense heat due to their distance from the ground (Gutirrez, 47). Bipedalism increased the distance of the human body from the ground surface and thus reduced the amount of radiation from the ground. In a bipedal stance, only the shoulders and head are exposed to direct sunlight which is a slightly smaller are exposed compared to other primates in the quadripedal stance. This adaptation was very vital since it offered an impulsive for both transition to the savannah and an immediate reward (food gains) for covering the ever increasing expansion in areas of the changing environment.
Because of effective cooling effects, humans would have a competitive advantage over other primates within the competing cycles since in the bipedal stance they spend less energy in comparison to the quadripedals. Reduced energy loss also meant that humans became less vulnerable to enemies during the food hunt since they would easily escape. Nevertheless, this posture increased subsistence capabilities of humans at the height of food limitations by providing much freedom for hands in accommodating other activities s such as making tools for effective food hunting among competitors (Stanley, 642). In the savannah, humans needed to be more vigilant over tall grass in order to adapt in its sultry condition, it thus offered a means of adjusting to these conditions through standing upright and thus less exposure to sunlight.
Therefore, bipedalism was developed by humans so as to provide safety in the terrestrial forest floor and on the more open terrain. In the human species, males were charged with the responsibility of long range hunting for food as the females were restricted to short range hunting and caring for infants. This called for the males to free their hands for carrying food on a wider foraging range. In this sense, the females would spend less energy caring for the young while the males covered long distances gathering food (Tanner, 12). This increased the survival of the infants and the general human species in the food competition environment.
 In the bipedal stance, human eyes face forward giving them orbital convergence which improves the depth of perception in the terrestrial environment. This gave humans an added advantage when reaching and grasping for food objects in this environment. This improved vision not only enabled humans to find small fruits and capture insects but also the ability to aim at small, hard to spot branches during mid-leaps through trees.
As far as food competition was concerned, the bipedalism stance introduced squat feeding which enabled humans to lessen the weight bearing abilities of the arms, under such situations the quadripedal mechanism would be less efficient than upright walking.  The squat feeding mechanism pre-adapted humans for bipedalism and hence solving many of the problems associated with quadripedal stance with regard to efficiency. The adoption of bipedalism helped in merging the terrestrial environmental changes of widening food patches as well as human consumption of meat which served as an impetus for bipedalism (Isaac, 29).
With declining rainfall and increased seasonality levels, which led to clear forests instead of denser ones, this changed food distribution and called for increased efficiency associated with bipedalism of moving around the greater distances between food points. Efficient bipedalism would allow humans to spend more time in search for food than their cousin apes in the quadripedal mode. This meant that humans would not suffer greater inter group food scramble competitions since they would not be required to travel each day compared to smaller ape groups while maintaining the same per capita food intake. It is important to note that bipedalism was developed by humans depending on their social group structure and the competition strategies which required more efficient means of interactions (Jablonski, 24). Bipedalism was inevitable.
The advantage of the savannah theory is that it is based on solid evidence especially that gathered from fossils and may well explain why humans adopted bipedalism as a means of locomotion. The study and dating of fossil bones, the associated fossil remains are enough to establish what environment humans survived in. Furthermore, conditions that facilitate fossilizations of the remains of dead animals are good along the sea shore and poor on the open grasslands. Up to date, there exists no evidence on the coastal plains which suggest mans existence along these areas. The only evidence that has been documented comes from the savannah hence affirming that man surely existed in the savannah.

The Aquatic Ape Theory (AAT)
This suggests that before humans emerged as apes on the savannah, it had spent some time in water. The peculiar layer of fat on the human skin is only found in water mammals but not in the other apes, man must have been aquatic in the past. The shore based environment had contributed to adoption of bipedalism by humans. Man escaped the competition in the forest to the sea shores in search for food, urchins and shell fish present in the shallow waters of the coast. Commencing his paddling in the shallow waters, man ventured into the deeper waters, rooting at the bottom with his head out of the water surface and his weight being supported by water, man learnt how to bipedal in the water.
Humans were far from killer apes with the role of the females far from that of passive breeders. This required both the females and males to jointly harvest the sea paddling along the shoreline gathering urchins and shellfish. The human society was not composed of alpha males or hierarchies of violence and dominion but an effort of communal gathering. In such a case humans had to develop brains for aquatic life, marine nutrients inn their dietary compositions necessary for the growth of brain for aquatic animals. The sticking in this aquatic life was made possible through the restriction that, had humans moved away from water then this change in diet would be hugely detrimental to the development of their brain. This ensured that humans remained in water and exploited the aquatic environment.
Humans retreated into water for the sake of food safety and food security because their reduced numbers resulting from starvation in the savannah forest. The newly acquired aquatic environment provided support for bipedalism and brain food.  Bipedalism must have been acquired by humans so as to provide the buoyant properties of water which triggered incremental evolution from quadripedalism.
 The advantage of this theory is that it is based on sound evidence and may greatly explain the bipedalism nature of humans.  Firstly, humans can effectively control their breathing patterns like many aquatic and semi-aquatic creatures. Furthermore, there are other anecdotal facts, our fat surplus, the ability of infants to hold their breath as well as swim from birth and the perpendicular nostrils (Isaac, 29). The face to face sex pattern is seen in dolphins all show evidence of the previous aquatic life of humans. Moreover, some of the aquatic animals such as whales are capable of going bipedal, at least temporarily such as when they emerge out of the water surface. 

This means that humans must have adopted this and enhanced it for permanent bipedalism.  Again, our hairlessness is as a result of traveling longer distances in water corresponding from the need of dissipating heat more effectively out of the water surface. The presence of sebaceous glands depicts previous aquatic life in humans. Aquatic animals possess rudimentary sebaceous glands, while they become active in humans during puberty as better scent receptors. This is evident in the seals which use these glands for waterproofing the glands are secreted by keratinized skin which is similar to the human skin. Humans are efficient swimmers possessing shapes suitable for rapid travel through water only that swimming is learnt with the new borns not capable of effectively propelling through water as well as lifting their faces for breath (Hunt, 26. The nose shape of humans is another evidence of human existence in water.  The human nose is easily comparable to that of other aquatic animals and may perform the function of preventing water entry during swimming and that the muscles around the nose are well a developed to aid in communication while in water just as in other water animals such as the goose.
Genera    lly,  the Aquatic Ape Theory (AAT) is well accounted for to posit the previous aquatic life of humans with humans possessing though not all the features that enable aquatic life such as surplus skin fats seen in the whales. This theory is evident-based.


Marc Verhaegen комментирует...

For a recent review of the so-called "aquatic ape theory" (a misnomer: it's about Homo who during the Pleistocene (Ice Ages) followed the African & Eurasian coasts & rivers, beach-coming, diving & wading bipedally for littoral, shallow aquatic & waterside foods), see "The aquatic ape evolves: common misconceptions and unproven assumptions about the so-called Aquatic Ape Hypothesis" Hum.Evol.28:237-266, 2013, google researchGate marc verhaegen.

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