One of the most obvious difference between human beings and the other great apes (in addition to naked bodies and speech) is that Homo sapiens habitually gets about on two legs. But bipedalism is far from being an efficient system of locomotion. As Elaine Morgan pointed out: "The incentive to walk upright must have been imperative because it is a very costly option. Some of the costs are: takes a long time for infants to acquire (hence costly for females); is slower than running on all fours and consumes more energy; direct cause of disorders such as lumbago, slipped disc, hernia, varicose veins, obstetric complications; exposes vulnerable vital organs to frontal attack especially in males, and above all it means that damage to one limb which causes other animals to proceed on three legs until it heals, incurs a degree of crippling that would normally prove fatal in the wild. That alone would have taken some powerful advantage to outweigh the costs." (AAT Group, October 2011)
So why did our ancestors find it necessary to start walking about on two legs instead of four? The answer to this question has so far managed to evade the paleoanthopologists. At least, there is no consensus. Until now, the main theories suggested that walking on two legs freed our hands for tools, or carrying infants, or exposed less of our body to the sun when we lost our fur; but chimps use tools and they still prefer to walk on four limbs, or three if they are holding a tool, and they avoid the heat of the midday sun by staying in the shade.
"Students of anthropology today are consequently taught a very tentative picture about why only we, Homo sapiens, are obligate bipeds. This lack of consensus is indicative that, so far, either insufficient unequivocal evidence has been provided in favor of any particular idea on the matter to make it the generally accepted one, or that perhaps one of the ideas has been misunderstood." [1]
The orthograde ape
Homo may be the only habitually bipedal ape, but we are not unique among the apes as far as the alignment of the spine is concerned. If we compare all apes to monkeys, we can see that apes have adapted to swinging beneath branches, rather than running through them, and this can be seen in Morotopithecus even twenty million years ago.
"Apes differ from monkeys in having a below-branch locomotion, with larger and broader bodies and thoraxes, very long arms that can easily be extended above the head, and tail loss. Whereas most mammals and monkeys predominantly move pronogradely (with horizontal spine and trunk), the remarkably humanlike lumbar vertebra of Morotopithecus suggests that by about 20 Ma the early apes were already orthograde (with a generally vertical spine)." [2]
In other words, whether climbing in the branches, hanging from the branches or moving around on the ground on their back legs and fore-knuckles, apes already have an advantage that most quadrupeds don't have to standing erect. But still, they only do it only on very rare occasions and certainly have not developed a habitual bipedal form of locomotion as their main means of getting from A to B. There must have been another reason which forced some apes to stand erect which outweighed all the previously mentioned disadvantages and was probably not even caused by a need for getting from A to B.
"According to the palaeo-environmental data, the fossils of Mio-Pliocene apes typically lay in coastal and swamp forest sediments around the Tethys Sea (the ancient Mediterranean Sea). The Miocene (23.0 to 5.3 Ma) and the Pliocene (5.3 to 2.6 Ma) epochs were generally hotter and wetter than the Pleistocene Ice Ages (2.6 to 0.01 Ma). Recently, the highest population densities of orangutans as well as gorillas have been discovered in extremely hot and wet swamp forests." [3]
If we accept (as most paleoanthropologists now do) that forest dwelling apes did not immediately come down from the trees and start walking upright on the savannah, and if we take into account the paleo-environment that existed during the Miocene and Pliocene epochs, then we have to assume that there was a transitional stage in which an already orthograde ape with long arms and a bigger body, found himself living in hot, swampy, forested wetlands, at least during seasonal wet periods of for a major part of the year. A favourite food of many lowland apes even today is found in swamps which they frequently wade into in order to obtain them.
Since all great apes can make and use tools, and most fossil great apes had thick enamel, the ancestral great ape diet in flooded forests might have included durophagy of hard-shelled foods (e.g., palm nuts or molluscs). Locomotor requirements for flooded forest dwelling could arguably have included a bigger body with vertical climbing abilities, including with arms overhead and arm-hanging. Lowland gorillas employ an orthograde posture and locomotion when they climb, wade through shallow swamps, and sit and feed in shallow water." [4]
The wading hominid
All apes automatically wade if they enter a body of water (from the shore / bank), unlike many pronograde animals (hippos, dogs, etc.) which enter on all fours. It makes sense that an ape knuckle-walking on land can go a little further and keep his head above water for longer if he straightens his spine and stands on two legs.
Algis Kuliukas, who has conducted a fair amount of research on bipedal wading has noted that there are many strong arguments in its favour which appear to meet key Darwinian evolutionary models:
Increases Survival No other published model can offer such a clear-cut and decisive theoretical survival advantage for bipedalism over quadrupedalism. In waist to chest deep water bipedalism places the head well above the surface allowing the individual to breathe.
Is not Teleological As all great apes have been observed switching to bipedalism in waist deep water, it cannot be claimed that this model is anthropomorphic. It proposes a very Darwinian scenario that would encourage bipedalism from the outset and continues to do so even in modern human forms.
Improves Food Acquisition Four of the published wading models specify food procurement as the motivation for increased wading and shallow water niches are known to be very productive in terms of biomass.
Explains why other Great Apes are not Bipedal Wading models suggest that the most important factor in human-ape locomotor divergence was the level of exposure to moving through water. This is a very simple and clear-cut reason to explain why some great apes become more bipedal than others.
Provides Plausible Precursor to both Human Bipedalism and Knuckle-Walking It can be argued that bipedal wading, as a pattern of locomotion, is an ideal precursor to both human striding bipedalism and chimp/gorilla knuckle-walking.
Provides Extended Explanatory Power Most of the published wading models explain bipedalism as just one of a suite of unique human traits that resulted from greater selection from moving through water. [5]
Bonobos, chimpanzees, gorillas and orang-utans have all been observed wading and occasionally even swimming in water. Lowland gorillas often spend up to two hours a day in the water, feeding on aquatic herbaceous vegetation. Even probosicis monkeys, although more distantly related to us, often wade in deep water; capucin monkeys occasionally swim to off-shore islands and have been seen eating mangrove oysters, while macaques dive under water to collect food but rarely stay in the water. [6] However, the question remains, why then do extant wading apes not continue to walk on two legs when they leave the water? Regardless of the obvious advantage of standing on two legs to wade in water, they all generally return to four legs, or knuckle-walking when back on terra firma (except for proboscis monkeys which have been observed continuing to walk bipedally on land). We could argue that their bodies are not streamlined in the way that ours are, and that their shorter legs and longer arms make walking on four limbs more efficient. At some point since the split from Pan and Homo, our ancestors developed much longer, thinner legs and a much straighter spine. The wading argument certainly supports longer and thinner legs (eg: the flamingo) but it is not in itself the answer adopted by every animal that enters the water.
Diving homo
Marc Verhaegen claims that bipedalism came about as the result of Homo ancestors swimming, diving or foraging on the bottom of rivers, lakes or coastal shallows. This would result in a more permanent straightening of the spine. When we swim or dive, our head, spine and feet tend to align. As we have seen, all apes have orthograde (straight) postures, so it would have been more natural for our ancestors to assume this posture when diving. Bipedalism is therefore explained as an ancestral feature of the hominoid clade while the erect posture is explained by the linear build required for foraging underwater. So the imperative wasn't so much to walk erect, but to be in one line (streamlined) while foraging underwater. It is because our ancestors were well adapted to forage underwater that they evolved an aligned body, and it is because of our shared ancestry with the other apes, including gibbons, that we were able to walk bipedally with this aligned body on land.
"In the fossil record, all uniquely Homo features appear after ~ 2 Ma, e.g., extreme brain size, external nose, very long leg bones, heavy skeleton (pachyostosis). When during the Ice Ages coastal forests shrank and sea levels dropped, our aquarboreal omnivorous ancestors were pre-adapted to colonize the vast tidal flats and deltas, embayed lagoons, coral reefs and near-shore islands on the now-submerged continental shelves along the Indian Ocean. They reduced their climbing skills and might have beach-combed for palm nuts, birds’ eggs and stranded animals, and waded and dived for shell and crayfish. We have more efficient diving and breath-holding skills than non-human primates – a prerequisite for developing voluntary sound production and speech. At the shore we might have acquired our large brain (DHA), refined dexterity and tool use, external nose, fat and furless skin, linear swimming-build with head-spine-legs in one line, and possibly long wading-legs." [7]
There is enough fossil evidence to suggest that by 2 million years ago, our Homo ancestors were living and moving via the coasts through parts of Asia and Eurasia. Verhaegen believes that H. erectus was predominantly semi-aquatic, a frequent swimmer and efficient slow diver, and that wading may have come about later, as part of a move back to a more terrestrial existence, as a result of the cooling global temperatures and receding oceans at the end of the pleistocene. He disputes that wading came first or was the predominant reason for our becoming habitual bipeds on land.
"Some scenarios think the MAP (most aquatic phase) adaptations were rather limited, and mostly included bipedal wading rather than diving. This is contradicted by, e.g., the abundant semi-aquatic features of H. erectus, the rich sessile foods at the sea bottom, the high energetic costs of wading compared to swimming, the remarkable slow-diving skills of humans, and our linear body (head-spine-legs in one line): wading as well as running birds typically have horizontal spines, but the non-wading, like humans, have vertical spines on land. H. sapiens, however, more than H. erectus, resembles wading-birds in having relatively very long (although less slender) legs and especially tibias, suggesting terrestrialization via a wading phase. Arguably the invention of reed boats, dugouts, harpoons and/or nets could have catalyzed the shift from diving (archaic Homo) to wading (sapiens)." [8]
Bipedalism in African Fossil relatives
Discoveries in paleontology of fossils apes that demonstrate bipedal walking are the main reasons why these species have been thought to be ancestors of humans, eg: "Lucy" (Australopithecus afarensis). But other features such as brain size and shape, dental morphology, shoulder blades shape and position, and curved phalanges seem to align them more closely with extant African apes such as Chimpanzee and Gorilla, for which no fossils have officiall been found.
Could it be that Lucy is instead an ancestor of Chimpanzees, who was at one time bipedal (due to parallel evolution of wading and living in mangrove forests) that later reverted to a knuckle walking locomotion?
References:
1. Kuliukas, Algis V. Was Man More Aquatic in the Past? Chapter 3: A Wading Component in the Origin of Hominin Bipedalism. Bentham ebooks, p.36 2. Verhaegen, M., Munro, S., Puech, P-F., Vaneechoutte, M. Was Man More Aquatic in the Past? Chapter 4: Early Hominoids: Orthograde Aquarboreals in Flooded Forests? Bentham ebooks, p.67 3. Ibid. 4. Ibid. 5. Kuliukas, Algis V. Ibid. p.49-50 6. Kuliukas, Algis V. Ibid. 7. Verhaegen et al. Ibid. p.77 8. Verhaegen et al. Ibid. p.78
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Homo may be the only habitually bipedal ape, but we are not unique among the apes as far as the alignment of the spine is concerned. If we compare all apes to monkeys, we can see that apes have adapted to swinging beneath branches, rather than running through them, and this can be seen in Morotopithecus even twenty million years ago. 
Increases Survival
Bonobos, chimpanzees, gorillas and orang-utans have all been observed wading and occasionally even swimming in water. Lowland gorillas often spend up to two hours a day in the water, feeding on aquatic herbaceous vegetation. Even probosicis monkeys, although more distantly related to us, often wade in deep water; capucin monkeys occasionally swim to off-shore islands and have been seen eating mangrove oysters, while macaques dive under water to collect food but rarely stay in the water. [6] However, the question remains, why then do extant wading apes not continue to walk on two legs when they leave the water? Regardless of the obvious advantage of standing on two legs to wade in water, they all generally return to four legs, or knuckle-walking when back on terra firma (except for proboscis monkeys which have been observed continuing to walk bipedally on land). We could argue that their bodies are not streamlined in the way that ours are, and that their shorter legs and longer arms make walking on four limbs more efficient. At some point since the split from Pan and Homo, our ancestors developed much longer, thinner legs and a much straighter spine. The wading argument certainly supports longer and thinner legs (eg: the flamingo) but it is not in itself the answer adopted by every animal that enters the water. 