Bipedalism offers numerous advantages in water, including permitting deeper wading, improved balance and reduced strain on the back, hips and knees as well as improved blood circulation.(Niemitz, 2002; Verhaegen, 1987; Niemitz, 2010) However, the disadvantages cited for bipedalism within the AAH are often the result of comparing humans to medium, terrestrial quadrupeds, but the evolution of humans from ape ancestors never included a period of quadrupedal locomotion. Instead, human evolution features mainly brachiation, suspension and climbing as the primary method of transportation, with a gradual increase in bipedal locomotion over time. In addition, the elongated lower limbs of humans, which is explained as improving swimming speeds, appears only after the evolution of the Homo genus(Langdon, 1997) and biomechanical analysis indicates humans are far to poor swimmers to have derived from an ape ancestor that swam.(Preuschoft & Preuschoft, 1991)
Morgan claimed the relatively hairless skin of humans was due to comparable adaptations in aquatic mammals and land-dwelling mammals that have aquatic ancestors as well as those that currently spend much of their time in wet conditions and what body hair humans do have follows the flow of water over the body.(Morgan, 1982; Morgan, 1997) However, humans vary strongly in the amount and distribution of body hair(Laden, 2009) and comparably-sized mammals adapted to semi-aquatic lifestyles actually have dense, insulating fur.(Vanstrum, 2003) Hairlessness is only an advantage for aquatic mammals that dive, swim quickly or migrate long distances such as whales and dolphins, who show considerable skeletal and cardiovascular adaptations over millions of years to an aquatic lifestyle.(Jablonski, 2008; Langdon, 1997) MAINSTREAM EXPLANATION FOR REDUCED HAIR
The human larynx is situated in the throat rather than the nasal cavity, a feature shared by some aquatic animals who use it to close off the trachea while diving and facilitates taking large breaths of air upon surfacing.(Morgan, 1997) Humans also have a considerable amount of control over their breathing, which is an involuntary reflex for most terrestrial mammals.(Niemitz, 2002; Morgan, 1997) However, a descended larnyx is seen in numerous nonaquatic animals, and breath control was thought to be preceded by bipedalism, which frees up the muscles of the upper torso from locomotion and allows breathing independent of limb position. Both of these adaptations are though to derive from improvements in vocalization and the evolution of the ability to speak(Langdon, 1997; MacLarnon & Hewitt, 1999) and the human larynx is shaped differently from that of aquatic animals, predisposing humans to choking.(Langdon, 1997)
Morgan (1997) and several more recent authors (REFERENCES) have suggested that the encephalization of the human brain was a response to increased consumption of fatty acids found in fish. Critics have pointed out that landlocked humans without access to fish develop normal brains(Langdon, 1997) and these nutritional requirements are easily met with a land-based diet.(Carlson & Kingston 2007)
