[edit] Criticisms
Several theoretical problems have been found with the AAH, and some claims made by the AAH have been challenged as having explanations aside from a period of aquatic adaptation.
[8]
[edit] Theoretical considerations
The AAH has been criticized for containing multiple inconsistencies, and lacking evidence from the
fossil record to support its claims.
[8][27] It is also described as lacking
parsimony despite purporting to be a simple theory uniting many of the unique anatomical features of humans.
[8]
Though describing the theory as plausible,
Henry Gee went on to criticize it for being untestable, as most of the evolutionary adaptations described by Morgan would not have fossilized. Gee also stated that while purely aquatic mammals such as whales show strong skeletal evidence of adaptation to water, humans and human fossils lack such adaptations; that there are many hypothetical and equally plausible scenarios explaining the unique characteristics of human adaptation without involving an aquatic phase of evolution; and that proponents are basing arguments about past adaptations on present physiology, when humans are not significantly aquatic.
[28] There is ultimately only
circumstantial evidence to suggest, and no solid evidence to support the AAH.
[29][30] ScienceBlogs author Greg Laden has described the AAH as a "human evolution
theory of everything" that attempts to explain all anatomical and physiological features of humans, and is correct in some areas only by chance. Laden also states that the AAH was proposed when knowledge of human evolutionary history was unclear, while more recent research has found that many human traits have emerged at different times over millions of years, rather than simultaneously due to a single evolutionary pressure.
[9]
[edit] Habitat
Morgan presented the AAH as an alternative to the "savanna model", which uses very vague descriptive statements portraying protohumans as moving out from forested environments and into a hot dry
savanna. However, this idea has been called a caricature of the actual environments in which protohumans are thought to have evolved, and presents a
false dichotomy as more recent theories propose a tree or forest-based habitat providing the driving forces for adaptation,
[29] and a
straw man of the actual theories and arguments used in the study of paleoanthropology. Morgan further criticized scientists for admitting they were uncertain regarding the reasons for the development of hairlessness, bipedalism, brain size and speech. This ignores the fact that science legitimately admits ignorance when it is unclear and that a lack of "final answers" does not legitimate a competing theory by default.
[8]
The belief that wading into shallow water would help proto-humans avoid dry-land predation discounts the risks presented by aquatic animals such as
crocodiles and
hippopotamuses that present a current risk to Africans living near bodies of water,
[31] and that protohumans lacked the fangs, claws or size to defend themselves from these threats.
[32] In addition, humans lack
immune system defences to protect against
waterborne parasites.
[32]
[edit] Anatomical and physiological claims
- Hairlessness - Most aquatic mammals that are comparably sized to humans are not hairless, but have dense, insulating fur and swim very well, with fatty layers beneath the skin.[31] Aquatic mammals do not vary greatly in their body hair, while humans do.[9] Hairless skin is also only an advantage for fully-aquatic mammals that dive, swim quickly or migrate long distances such as whales and dolphins,[32] and only appears and is an advantage for extremely large aquatic mammals who would overheat with large amounts of body hair, who are fully-aquatic and have evolved as an aquatic species for millions of years. The loss of body hair is also explainable through a lower parasite load, and maintenance through sexual selection.[33] Furthermore, while shaving human swimmers to eliminate the little body hair that remains results in a minor decrease in drag,[34][35] this cannot be extrapolated to a beneficial effect of loss of a full coat of fur, which has been shown to have superior drag reduction ability.[36] While relative hairlessness and hair direction is cited as an adaptation to swimming and diving, there is no evidence of similar skeletal or soft tissue adaptations that are expected to accompany such adaptations.[8]
- Breath control - The position, evolutionary timing of changes, and size of the nerve openings in the vertebra suggest that breath control in humans improved because of the increased complexity and use of speech rather than an aquatic phase of evolution.[37] In addition, breath control is thought to be preceded by bipedalism, which frees the muscles around the upper torso from locomotion and allows breathing rates to occur independent of locomotion. Voluntary speech is thought to be a sufficient evolutionary pressure to explain breath control, independent of other explanations. The vocalizations of dolphins and other aquatic species are not thought to be comparable to humans. In addition, certain birds have speech and breath control comparable to humans, without a phase of aquatic adaptation.[8]
- Diving reflex - the mammalian diving reflex is more easily explained as a reaction to hypothermia than underwater swimming.[38]
- Diet - a broad terrestrial diet would ensure sufficient access to required essential fatty acids without a high consumption of seafood[39] and the "best" fats found in fish are from cold water fish that did not occupy the same costal environments as humans. In addition, the requirements of these fats are very minimal, with no evidence that extra fats would result in an evolutionary pressure towards a larger brain. Humans without access to shoreline foods also develop normal brains.[40][8]
- Body fat - the subcutaneous fat distribution in humans is more similar to a domesticated animal than an aquatic one, and is nearly identical to that of other primates. The subcutaneous fat of aquatic mammals and humans also seems to serve different uses - it forms the streamlined shape of seals, while in humans it is used for sexual selection.[41][42] In addition, the distribution of fat and blood vessels allows for improved thermoregulation, as hot blood from the body can bypass the fat to radiate heat through the skin.[8]
- Bipedalism - the disadvantages cited for bipedalism within the AAH are often the result of comparing humans to medium, terrestrial quadrupeds, but human evolution 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.[8] Brachiating apes are more commonly bipedal than are ground-dwelling ones, and of aquatic species only birds travel bipedally on land. Also, the human fossil record demonstrates a gradual adaptation from tree-dwelling to bipedalism rather than an abrupt transition to an aquatic environment.[42]
- Descended larynx - the human larynx is not shaped like the larynxes of aquatic animals; it forms and descends as an infant begins to speak, making it easier to aspirate water and drown. Additionally, a descended larynx is not unique to aquatic animals, and permanently or temporarily descended larynxes are seen in dogs, pigs, goats, monkeys,[43] big cats,[44] deer,[45] and young chimps.[46] Mainstream anthropology explain the descended larynx as an adaptation to improve vocalizations by increasing the number of pronounceable vowels and improving the ability of humans to control their speech.[8]
- Nose shape - the shape of the human nose is extremely variable within the species, and believed to be related to climactic adaptations and the warming and moistening of air before it enters the respiratory tract, not to prevent water entry while swimming. In addition, the muscles surrounding the nose show no evidence of having been previously more developed, but are part of a complex of muscles that are specially developed in humans to show emotion and aid in communication.[8]
- Interdigital webbing - syndactylism, the presence of webbing between the fingers, is a birth defect, not an adaptation, that is found in both humans and apes.[42][8]
- Sebaceous gland - many aquatic animals have rudimentary or no sebaceous glands. In humans, sebaceous glands become active during puberty with men having far more than women while women have much better scent receptors. This suggests the glands are sexually dimorphic for sexual selection rather than waterproofing. In seals that use sebaceous glands for waterproofing, the glands are active from birth and are secreted by hard, keratinized skin that is very different from human skin.[47] [8]
- Swimming - humans are inefficient swimmers, with shapes that are not well suited to rapid travel through water.[48] Swimming is also a learned trait, and though babies are able to propel themselves inefficiently through water, they are unable to lift their faces to breathe.[42] Human eyes do not see well underwater, and human skin is not waterproof (as demonstrated by the "pruning" of the fingertips when submerged for long periods).[38]
Generally the evidence provided for the AAH is equally well accounted for by land-based adaptations without needing to posit an aquatic phase of human development. In addition, the AAH is contradictory in several places; the AAH theorizes humans developed some unique skin features due to adaptation to water, but other features emerged after leaving the habitat, and the specialization that is hypothesized for an aquatic life are uneven, with humans lacking many truly specialized features of aquatic species (such as head shape, repositioned nostrils and streamlining of the body). Parallels made by proponents of the AAH between humans and the
proboscis monkey, which shows mainly behavioral adaptations to a water-based habitat, contradicts any claims of anatomical evidence for the theory.
[8] 
